Drill Bit Sizing: Inner Diameter (ID) vs. Outer Diameter (OD) – A Comprehensive Guide with Brand and Model Breakdowns

1. Twist Drill Bits: The Most Common Type, Sized by Outer Diameter

1.1 Bosch: Professional-Grade Twist Drill Bits for Precision and Durability

• Sizing Basis: Explicitly based on outer diameter, with the set including bits ranging from 1mm to 13mm in 0.5mm increments (e.g., 1mm, 1.5mm, 2mm, …, 13mm). Each bit is laser-etched with its OD for easy identification.
• Material: HSS-R (High-Speed Steel-Refined), a premium alloy containing 0.8% carbon, 4% chromium, 1% molybdenum, and 0.2% vanadium. This composition enhances hardness (up to 65 HRC on the Rockwell scale) and heat resistance, allowing the bits to drill through metals (steel, cast iron, copper) without overheating.
• Design Features: The bits feature a 135° split point tip, which eliminates the need for a center punch by self-centering on the workpiece—reducing slippage and ensuring precise hole placement. The helical flutes (with a 30° helix angle) are optimized for efficient chip evacuation: the 30° angle balances cutting speed and chip removal, making the bits suitable for both ferrous (iron-based) and non-ferrous (aluminum, brass) metals. Additionally, the bits are coated with TiN (Titanium Nitride), a golden-colored coating that reduces friction, resists wear, and extends tool life by up to 300% compared to uncoated HSS bits.
• Applications: Ideal for mechanical engineering (drilling holes in engine components), electrical work (drilling for wire conduits in metal junction boxes), and DIY projects (installing shelf brackets on metal studs). The 1mm bit is used for precision tasks like drilling holes in circuit boards, while the 13mm bit is suitable for larger applications like mounting heavy-duty hooks on steel beams.
• Compatibility: Works with corded/cordless 手电钻 (12V–24V), bench drills (500W+), and pillar drills. The bits feature a cylindrical shank (6mm diameter for bits up to 6mm OD, 10mm diameter for bits 6mm–13mm OD) that fits standard 3-jaw chucks.
• Pricing and User Feedback: The set retails for approximately $18–$22, making it a mid-range professional option. Users consistently praise its durability: a mechanical technician reviewing the set noted, “I’ve drilled over 50 holes in 10mm thick 45# steel with the 8mm bit, and it still maintains its sharpness—no sign of dulling.” Another user highlighted the self-centering tip: “No more marking holes with a punch; the 135° tip grabs the metal instantly, even on smooth surfaces.”

1.2 DeWalt: Industrial-Grade Twist Bits for Heavy-Duty Use

• Sizing Basis: Outer diameter is the primary sizing metric, with the set including 13 bits: 1.5mm, 2mm, 2.5mm, 3mm, 3.5mm, 4mm, 4.5mm, 5mm, 5.5mm, 6mm, 7mm, 8mm, and 10mm. Each bit has a bold OD marking on the shank for quick identification, even in low-light conditions (e.g., under a vehicle or in a dark workshop).
• Material: HSS (High-Speed Steel) with a proprietary “Extreme 3” heat treatment process that increases toughness and wear resistance. The alloy includes 1.05% carbon, 5% chromium, 1.5% molybdenum, and 0.4% vanadium, resulting in a hardness of 63–64 HRC—ideal for drilling through mild steel, cast iron, aluminum, and plastic.
• Design Features: The bits have a 118° point angle, a common choice for general-purpose drilling (the 118° angle balances cutting efficiency and tip strength, making it suitable for both soft and hard materials). The flutes are parabolic in shape, which increases chip capacity—critical for high-volume drilling, as it prevents clogging and overheating. The shanks are hexagonal (hex shank) rather than cylindrical, which provides a tighter grip in drill chucks and reduces slippage (even when drilling at high torque). This is a key advantage for construction workers, who often use impact drills that generate high rotational force.
• Applications: Perfect for construction (drilling holes in steel framing for drywall screws), automotive repair (drilling through engine blocks for oil lines), and maintenance (drilling through metal pipes for valve installations). The 10mm bit is frequently used to drill holes for M8 bolts (a common fastener in industrial equipment), while the 3mm bit is used for precision tasks like drilling holes in aluminum sheets for electrical terminals.
• Compatibility: Works with cordless impact drills (18V–54V), corded drills (750W+), and bench drills. The hex shank fits both 3-jaw chucks and quick-change chucks (common in DeWalt’s own drills), allowing for fast bit changes—saving time in high-pressure work environments.
• Pricing and User Feedback: The set costs around $25–$30, positioning it as a premium industrial option. A construction foreman reviewed the set, stating, “We use these bits daily on steel studs—drilling 20–30 holes per day—and they last 2–3 times longer than generic HSS bits. The hex shank never slips, even when the drill is set to high torque.” A mechanic added, “The parabolic flutes keep chips out, so I don’t have to stop and clear debris every few holes—saves me at least an hour a day.”

1.3 Makita: Precision Twist Drill Bits for Fine Work

• Sizing Basis: Outer diameter is clearly marked, with the set including 21 bits ranging from 0.5mm to 10mm in 0.5mm increments. The 0.5mm bit is one of the smallest common twist drill bits, catering to micro-drilling needs.
• Material: High-quality HSS with a focus on ductility (to prevent breakage in small diameters) and sharpness. The alloy is formulated with 0.9% carbon, 4.5% chromium, and 1.2% molybdenum, ensuring a balance of hardness (62 HRC) and flexibility—critical for thin bits that are prone to snapping.
• Design Features: The bits have a 90° point angle, which is shallower than the standard 118° or 135° angles. This shallow angle is ideal for drilling soft materials like wood, plastic, and aluminum, as it reduces the risk of splitting or cracking the workpiece. The flutes are narrow and closely spaced, which improves chip control for small-diameter holes (preventing chips from jamming the flute and breaking the bit). The shanks are cylindrical with a polished finish, which reduces friction in the chuck and ensures smooth rotation—essential for micro-drilling, where even minor wobble can ruin the hole.
• Applications: Used in electronics (drilling holes in printed circuit boards for component leads), jewelry making (drilling holes in silver or copper sheets), and woodworking (drilling pilot holes for small screws in hardwoods like oak or maple). The 0.5mm bit is used to drill holes for 0.4mm enameled wire in electrical transformers, while the 5mm bit is used to drill pilot holes for 4mm wood screws in furniture assembly.
• Compatibility: Designed for low-torque drills, such as cordless micro-drills (6V–12V) and precision bench drills. The small shanks (3mm diameter for bits up to 3mm OD) require a chuck with a minimum capacity of 0.5mm to ensure a secure fit.
• Pricing and User Feedback: The set retails for $12–$15, making it an affordable option for precision work. An electronics technician commented, “I’ve used the 0.5mm bit to drill over 100 holes in PCB boards, and it hasn’t broken once—unlike generic bits that snap after 10–15 holes.” A jewelry maker added, “The 90° point angle drills cleanly through copper without burring the edges, which saves me time on finishing.”

2. Masonry/Impact Drill Bits: OD-Centric Sizing for Construction and Renovation

2.1 DeWalt Extreme 2 Masonry Drill Bit (Model: DWA562)

• Sizing Basis: Outer diameter is prominently displayed on the bit’s shank (e.g., “8mm” for the 8mm model). The OD directly corresponds to the hole size, ensuring compatibility with standard masonry fasteners: a 6mm bit fits 6mm expansion anchors, a 10mm bit fits 10mm sleeve anchors, etc.
• Material: The cutting tip is made of tungsten carbide (WC), a super-hard material (92 HRC) that can withstand the abrasion of concrete and stone. The body is made of high-carbon steel (0.8% carbon) with a heat-treated core for strength, preventing bending or breaking when drilling into reinforced concrete (which contains steel rebar).
• Design Features: The bit has a “4-flute” design, with four carbide-tipped cutting edges that distribute pressure evenly—reducing vibration and improving drilling speed. The flutes are wide and deep, allowing for efficient removal of concrete dust (a major issue in masonry drilling, as dust can clog flutes and slow down the process). The shank is a “SDS-Plus” shank, a standard for impact drills that features grooves to lock the bit into the drill’s chuck—preventing slippage even when drilling at high impact force. Additionally, the bit has a “chisel tip” (a sharpened edge at the top of the carbide tip) that helps break through hard concrete surfaces, reducing the initial force required to start the hole.
• Applications: Used in construction (drilling holes in concrete foundations for rebar placement), renovation (drilling holes in brick walls for electrical outlets), and home improvement (mounting shelves, mirrors, or TV brackets on concrete or stone walls). The 8mm model is the most versatile, fitting most standard wall anchors, while the 16mm model is used for larger applications like installing pipe brackets for 15mm copper pipes.
• Compatibility: Designed for SDS-Plus impact drills (18V–36V) and rotary hammers (common in construction). The SDS-Plus shank is compatible with most major drill brands (DeWalt, Bosch, Makita) and allows for tool-free bit changes—critical for contractors who need to switch bits quickly on the job.
• Pricing and User Feedback: A single 8mm DWA562 bit costs $8–$10, while larger sizes (16mm) cost $15–$18. A contractor reviewing the bit said, “I drill through concrete walls every day, and the DWA562 lasts 3–4 times longer than cheaper masonry bits. The 4-flute design cuts through concrete like butter, even with rebar.” A homeowner added, “I used the 8mm bit to mount a TV bracket on a concrete wall—it took 2 minutes per hole, and the hole was perfectly straight, no wobble.”

2.2 Bosch Professional SDS-Plus Masonry Drill Bit (Model: 2608680375)

• Sizing Basis: Outer diameter is laser-etched on the shank, with each size corresponding to a specific fastener or fixture. For example, the 10mm model is designed for 10mm expansion bolts used in heavy-duty applications (e.g., mounting a washing machine to a concrete floor), while the 5mm model is for small anchors (e.g., hanging lightweight pictures on brick).
• Material: The cutting tip is made of “Carbide 4” (Bosch’s proprietary tungsten carbide alloy), which contains 94% tungsten carbide and 6% cobalt. This alloy is harder and more impact-resistant than standard carbide, making it suitable for drilling through reinforced concrete (where the bit may hit steel rebar). The body is made of chrome-molybdenum steel (Cr-Mo), which is stronger and more corrosion-resistant than plain carbon steel—extending the bit’s life in damp environments (e.g., basement renovations).
• Design Features: The bit features a “Century Tip” (a rounded carbide tip with a sharp center point) that self-centers on the masonry surface, reducing slippage and ensuring accurate hole placement. The flutes are “V-shaped,” which improves dust evacuation by channeling dust directly out of the hole—reducing heat buildup and preventing the bit from overheating. The SDS-Plus shank has a “double-groove” design that provides a tighter lock in the drill chuck, even when drilling at high impact energy (up to 3.2 J, common in professional rotary hammers). Additionally, the bit has a “wear indicator” (a colored band on the shank) that fades when the carbide tip is worn down, alerting the user to replace the bit before it becomes ineffective.
• Applications: Ideal for industrial construction (drilling holes in concrete columns for steel beams), commercial renovation (drilling through stone walls for HVAC ductwork), and residential projects (installing outdoor lighting fixtures on concrete patios). The 20mm model is used for large applications like drilling holes for 19mm PVC pipes (common in plumbing), while the 6mm model is for small electrical conduits.
• Compatibility: Works with all SDS-Plus impact drills and rotary hammers (12V–48V). The bit is compatible with Bosch’s own GBH series rotary hammers, as well as models from DeWalt, Makita, and Milwaukee.
• Pricing and User Feedback: A 10mm 2608680375 bit costs $10–$12, and a 20mm bit costs $25–$30. An industrial construction worker said, “We drill through reinforced concrete with rebar every day, and this bit doesn’t chip or dull—even when hitting rebar at full speed. The wear indicator is a game-changer; we no longer waste time using worn bits.” A plumber added, “The 20mm bit drills clean holes for PVC pipe—no need to sand the edges, which saves me time on installation.”

3. Core Drill Bits: The Exception – Sizing by Inner Diameter

3.1 Milwaukee Core Drill Bit (Model: 48-20-5260)

• Sizing Basis: Inner diameter is clearly marked on the bit’s housing (e.g., “50mm ID” for the 50mm model). The ID is the critical measurement here: a 50mm ID bit creates a hole that can accommodate objects up to 50mm in OD (e.g., 50mm PVC pipes, 48mm steel conduits). The outer diameter of the bit itself is slightly larger (e.g., 55mm for the 50mm ID model) to account for the thickness of the bit’s cutting wall.
• Material: The cutting edge is made of “Max-Life Carbide” (Milwaukee’s proprietary tungsten carbide blend), which is engineered for high abrasion resistance—essential for drilling through concrete and stone. The bit’s body is made of 16-gauge steel (thick enough to withstand impact but lightweight enough for easy handling) with a powder-coated finish to prevent rust and corrosion.
• Design Features: The bit has a “segmented cutting edge” (multiple carbide segments spaced around the inner circumference) that distributes cutting force evenly, reducing vibration and improving drilling speed. The body features large “dust slots” (rectangular openings along the side) that allow concrete dust to escape, preventing clogging and overheating. The bit is compatible with Milwaukee’s “Core Drill Arbor” (Model: 48-20-5200), which connects the bit to the drill and includes a water port for wet drilling (wet drilling reduces dust and extends bit life by cooling the carbide edge). Additionally, the bit has a “centering pin” (a small metal pin in the center) that helps align the bit with the marked hole location, ensuring accuracy.
• Applications: Used in plumbing (drilling holes in concrete walls for PVC drain pipes), electrical work (drilling through brick walls for large steel conduits), and HVAC (drilling through concrete floors for ductwork). The 100mm ID model is commonly used to drill holes for 100mm round vents in commercial buildings, while the 25mm ID model is for small electrical conduits (e.g., 20mm OD aluminum conduit).
• Compatibility: Requires a core drill arbor (sold separately) to connect to a rotary hammer or core drill machine (1500W+). The arbor features a SDS-Max shank (compatible with large rotary hammers) or a threaded shank (compatible with core drill machines).
• Pricing and User Feedback: A 50mm ID 48-20-5260 bit costs $40–$45, and a 150mm ID bit costs $120–$130. A plumber reviewed the bit, stating, “I use the 50mm ID bit to drill holes for PVC drain pipes in concrete walls—it cuts through 10cm thick concrete in 5 minutes, and the dust slots keep it from clogging.” An electrician added, “The centering pin ensures the hole is perfectly aligned, which is crucial for running conduit—no more off-center holes that require patching.”

3.2 Lenox Core Drill Bit (Model: 1778968)

• Sizing Basis: Inner diameter is the primary sizing metric, with each model labeled by its ID (e.g., “32mm ID”). The ID is chosen to match common pipe and conduit sizes: a 32mm ID bit fits 32mm OD plastic pipes (used in residential plumbing) and 30mm OD steel pipes (used in light commercial applications). The bit’s outer diameter is approximately 3mm larger than its ID (e.g., 35mm OD for the 32mm ID model) to accommodate the cutting wall.
• Material: The cutting edge is made of “Bi-Metal Carbide” (a combination of high-speed steel and tungsten carbide), which provides both the sharpness of HSS (for cutting metal) and the abrasion resistance of carbide (for cutting masonry). This makes the bit unique—it can drill through both concrete and thin steel (e.g., steel studs behind drywall) without switching bits. The body is made of high-strength aluminum alloy, which is lightweight and corrosion-resistant.
• Design Features: The bit has a “continuous cutting edge” (a single carbide strip around the inner circumference) that provides a smooth, clean cut—ideal for applications where hole quality is critical (e.g., installing decorative pipe fixtures). The body has “spiral dust grooves” (helical slots along the side) that improve dust evacuation and reduce heat buildup. The bit includes a “universal arbor” (compatible with both 3-jaw chucks and SDS-Plus drills), eliminating the need for separate arbors. Additionally, the bit has a “depth stop” (a adjustable metal ring) that allows the user to set the drilling depth—preventing over-drilling (a common issue when drilling through walls).
• Applications: Used in residential renovation (drilling holes in concrete/brick walls for plumbing pipes), electrical work (drilling through steel studs and drywall for conduit), and DIY projects (installing outdoor faucets with copper pipes). The 16mm ID model is used for small electrical conduits (e.g., 15mm OD PVC conduit), while the 100mm ID model is for large vents or exhaust pipes.
• Compatibility: Works with corded drills (1000W+), impact drills (SDS-Plus), and core drill machines. The universal arbor fits standard 3-jaw chucks (13mm+ capacity) and SDS-Plus chucks, making it versatile for different tools.
• Pricing and User Feedback: A 32mm ID 1778968 bit costs $30–$35, and a 100mm ID bit costs $80–$85. A DIY enthusiast commented, “I used this bit to drill through a brick wall and a steel stud for a 32mm PVC pipe—it worked perfectly on both materials, no need to switch bits. The depth stop kept me from drilling through the other side of the wall.” A contractor added, “The universal arbor is a huge plus; I can use this bit with my corded drill and my impact drill, depending on the job.”

4. Step Drill Bits: OD-Based Sizing for Versatile Hole-Making

4.1 Irwin Unibit Step Drill Bit (Model: 10231)

• Sizing Basis: Outer diameter is labeled by the maximum step size (e.g., “20mm Max OD” for the 10231). The maximum OD tells the user the largest hole the bit can drill, while the intermediate steps provide smaller sizes. Each step is clearly marked with its OD (e.g., “10mm” on the 10mm step) for easy identification during use.
• Material: Made of high-speed steel (HSS) with a “Titanium Nitride (TiN)” coating. The HSS provides sharpness and durability for drilling through metal (aluminum, steel, brass) and plastic, while the TiN coating reduces friction and wear—extending the bit’s life.
• Design Features: The bit has a “tri-flute” design (three helical flutes) that improves chip evacuation and reduces vibration, making it suitable for high-speed drilling (up to 3000 RPM). The steps are “self-starting” (each step has a sharpened edge that initiates the hole without the need for a center punch), which saves time and improves accuracy. The shank is 3/8” (9.5mm) in diameter and has a hexagonal shape (hex shank) that prevents slippage in the drill chuck—even when drilling at high torque. Additionally, the bit has a “flat top” (no pointed tip), which allows it to drill through thin materials (e.g., sheet metal) without piercing all the way through if a smaller step is used.
• Applications: Used in metalworking (drilling holes in aluminum sheets for electrical enclosures), automotive repair (drilling holes in steel bumpers for fog light mounts), and DIY projects (drilling holes in plastic bins for drainage). The 6mm step is used for small screws in sheet metal, while the 20mm step is for larger holes (e.g., installing grommets in metal panels).
• Compatibility: Works with corded/cordless drills (12V–24V) with a 3-jaw chuck (minimum capacity 3/8”). The hex shank fits quick-change chucks, allowing for fast bit changes.
• Pricing and User Feedback: The 10231 retails for $25–$30. A metalworker reviewed the bit, stating, “I use this bit to drill multiple hole sizes in aluminum sheets—no more switching between 5 different bits. The TiN coating keeps it sharp, even after drilling 100+ holes.” A DIYer added, “The self-starting steps are great; I don’t have to mark holes with a punch—just line up the bit and drill.”

4.2 DeWalt Step Drill Bit (Model: DWA1620)

• Sizing Basis: Outer diameter is marked by the maximum step size (“25mm Max OD”) and each intermediate step (e.g., “12mm” on the 12mm step). The 1/2” shank is larger than standard step bits, providing more stability for drilling through thick metal (up to 6mm thick steel).
• Material: Made of “High-Speed Steel with Cobalt (HSS-Co)” (5% cobalt content), which increases heat resistance and hardness—ideal for drilling through high-strength metals like stainless steel and alloy steel. The bit has a “TiAlN (Titanium Aluminum Nitride)” coating, which is more heat-resistant than TiN (withstands temperatures up to 800°C) and provides better wear resistance.
• Design Features: The bit has a “4-flute” design that improves chip evacuation and reduces heat buildup—critical for drilling through hard metals. The steps are “tapered” (each step has a slight angle), which reduces the force required to start the hole and prevents the bit from “grabbing” the material (a common issue with thick metal). The shank is 1/2” (12.7mm) in diameter and has a hexagonal shape with a “power groove” (a small notch that locks into the drill chuck for extra grip). Additionally, the bit has a “depth marker” (a series of lines on the shank) that shows the drilling depth for each step, helping the user avoid over-drilling.
• Applications: Used in industrial metalworking (drilling holes in stainless steel panels for machinery), automotive manufacturing (drilling holes in alloy steel frames for bolts), and construction (drilling holes in galvanized steel sheets for roofing). The 25mm step is used for large holes (e.g., installing electrical boxes in steel framing), while the 8mm step is for small fasteners (e.g., M6 bolts).
• Compatibility: Works with heavy-duty corded drills (1000W+) and bench drills with a 1/2” chuck. The hex shank with power groove fits DeWalt’s “Max Grip” chucks, which provide maximum torque transfer.
• Pricing and User Feedback: The DWA1620 costs $35–$40. An industrial metalworker said, “I drill through 5mm thick stainless steel every day, and this bit lasts 2–3 times longer than HSS-only step bits. The TiAlN coating keeps it cool, even at high speeds.” A roofer added, “The tapered steps drill cleanly through galvanized steel without leaving burrs—saves me time on finishing the holes.”

5. Industry Standards and Sizing Consistency

• For twist drill bits, masonry bits, and step bits, the primary sizing dimension is the outer diameter (OD), measured across the cutting edges.
• For core drill bits, the primary sizing dimension is the inner diameter (ID), measured across the hollow opening.
• All sizes must be clearly marked on the drill bit (e.g., laser-etched, stamped) for easy identification.

6. How to Choose the Right Drill Bit: Sizing, Material, and Brand

1. Determine the hole’s purpose: If the hole is for a fastener (bolt, screw) or component that fits into the hole, choose a bit sized by OD (twist, masonry, step bit). If the hole is for an object that passes through (pipe, conduit), choose a core bit sized by ID.
2. Measure the required size: For OD-based bits, measure the OD of the fastener (e.g., a 6mm bolt requires a 6mm OD bit). For ID-based bits, measure the OD of the object passing through (e.g., a 50mm PVC pipe requires a 50mm ID core bit).
3. Select the right material: Choose HSS or HSS-Co bits for metal, carbide-tipped bits for masonry/concrete, and HSS bits for wood/plastic.
4. Choose a reputable brand: Brands like Bosch, DeWalt, Milwaukee, and Makita are known for consistent sizing, durability, and performance—avoid generic brands, which may have inconsistent sizes (e.g., a “6mm” generic bit may actually have a 5.8mm or 6.2mm OD, leading to ill-fitting fasteners).

7. Conclusion