Drill & Tap Size Chart

Find the correct drill bit size for tapping metric and imperial threads. Includes tap drill and clearance hole sizes.

Reference
Thread Pitch (mm) Tap Drill (mm) Clearance Drill (mm)
Size TPI Tap Drill
Size TPI Tap Drill

Tap drill sizes assume approximately 75% thread engagement. Clearance drills provide a close fit.

How to Use

  1. 1
    Select the Thread Standard and Screw Size

    Choose between metric (ISO) or unified (UNC/UNF) thread standards and enter the nominal bolt or screw diameter; the thread pitch or threads-per-inch is auto-populated from the standard coarse thread series.

  2. 2
    Set the Desired Thread Engagement and Material

    Specify the material being tapped (steel, aluminum, cast iron, or plastic) and the desired thread engagement length as a percentage of nominal diameter; the calculator recommends 75% thread engagement for steel and 100% for aluminum.

  3. 3
    Read Drill Size, Tap Drill Diameter, and Recommended Feed Rate

    The required tap drill diameter (and nearest standard drill size), theoretical thread percentage, and recommended cutting speed and tap geometry for the workpiece material are displayed.

About

Drilling and tapping is among the most common machining operations in metalworking, and selecting the correct tap drill size — the drill that produces the pilot hole before threading — is critical to achieving strong, undamaged threads without breaking taps or stripping workpieces. The relationship between drill size, thread engagement percentage, and strip strength must be understood to make informed decisions across different material combinations.

The AlloyFYI Drill Tap Chart covers all standard metric ISO coarse and fine thread series (M1–M72), unified coarse (UNC) and fine (UNF) series (No. 0 through 1½ in.), and pipe threads (NPT, BSPP, BSPT). It integrates material-specific guidance for cutting speeds, tap geometry recommendations, and cutting fluid selection derived from tooling manufacturer data consistent with ASME B1.1 (UN threads) and ISO 965 (metric threads) standards. The tool eliminates the need for multiple reference tables and ensures that drill size, thread specification, and material considerations are evaluated together.

FAQ

What is the standard tap drill size formula and why is 75% thread common?
The standard tap drill diameter formula for metric threads is: drill diameter = major diameter − pitch. For example, M10×1.5 tap drill = 10 − 1.5 = 8.5 mm, giving approximately 75% thread engagement. Full 100% thread engagement requires a drill diameter equal to the minor diameter of the thread but provides only marginally higher strip strength while dramatically increasing tapping torque and tap breakage risk. Research has demonstrated that 75% engagement produces 90–95% of the theoretical strip strength for steel, while reducing torque requirements by 25–30% and tap life improves significantly.
How does material hardness affect drill and tap selection?
Drilling and tapping harder materials requires higher-quality tooling, reduced cutting speeds, and appropriate cutting fluids. For mild steel (up to 250 HBW), HSS (high-speed steel) drills and spiral-point plug taps work reliably with cutting oil. Steels above 35 HRC require carbide or TiN-coated cobalt drills and spiral-flute taps, which provide better chip evacuation in blind holes. Aluminum requires razor-sharp drills with polished flutes to prevent built-up edge (BUE) and uses rake angles and geometries different from steel tooling. Cast iron is abrasive and prefers carbide drills; it produces discontinuous chips that require a different tap geometry than ductile materials.
What is the difference between spiral-point (gun) taps and spiral-flute taps?
Spiral-point (gun) taps have a modified end with a forward-cutting point that pushes chips ahead of the tap in the direction of travel. They are designed for through holes where chips can exit the hole. Spiral-flute taps have helical flutes that curl chips backward and out of the hole, making them the correct choice for blind (non-through) holes where there is no exit path for chips. Using a gun tap in a blind hole will pack chips in the bottom of the hole, increasing torque dramatically and risking tap breakage or incomplete thread formation. For production tapping, form (roll) taps are preferred where the material is ductile (aluminum, mild steel) because they displace rather than cut material, producing no chips and superior thread surface finish.
How much thread engagement is required to develop full bolt tensile strength?
For a threaded hole in steel with a strength equivalent to the bolt grade, the minimum thread engagement to develop the full tensile strength of the bolt is approximately 0.5×D for Grade 8.8 bolts and approximately 0.7×D for Grade 12.9 bolts, where D is the nominal bolt diameter. For weaker materials (aluminum, cast iron, or when the hole material is weaker than the bolt), engagement must increase to compensate: for 6061-T6 aluminum receiving a Grade 8.8 bolt, engagement of at least 1.5×D is typically required. Heli-Coil or equivalent thread inserts are used in aluminum to restore full bolt strength with shorter engagement lengths.
What is the correct procedure for drilling and tapping in stainless steel?
Stainless steel work-hardens rapidly under the heat and pressure of cutting operations, creating a hard-to-cut surface layer if the cutting tool dwells or rubs without cutting. Drilling requires sharp cobalt or TiN-coated HSS drills, firm consistent feed (2–3 mm/rev), and generous application of cutting fluid (sulfur-based or emulsion recommended for austenitic grades). The drill must be advanced without pausing. Tapping requires spiral-flute taps with titanium nitride or titanium aluminum nitride coating, lower cutting speed (5–10 m/min vs. 20–30 m/min for mild steel), and cutting fluid. A dull tap or hesitation during threading will immediately work-harden the bore and seize the tap.
All tools