Machining Speeds & Feeds Card
One page. Speeds, chip load and drill feed, imperial and metric.
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BuyTap Drill Size Calculator
SAE and metric tap drill sizes with adjustable thread engagement
Select thread system, size, and tap type, then set thread percentage from 50–100% to get the closest fractional, number, and letter drill for your hole.
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Complete Machining Speeds & Feeds Guide
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Quote shop work and check operations against your spindle's top speed.
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Tap and threading shop picks
Drill bits, cutting fluid, and measurement tools that support clean holes and reliable thread cutting.
Irwin Cobalt M35 Drill Bit Set (29 pc)
- Wide fractional, number, and letter coverage
- Split-point tips reduce walking on tap holes
- Strong choice for steel and stainless prep
Bosch CO14B M42 Cobalt Drill Set
- Heat-resistant cobalt for tough alloys
- Three-flat shanks reduce chuck slip
- Useful when tap drills run undersize
WD-40 Specialist Cutting Oil
- Reduces friction during tapping
- Helps prevent galling in stainless
- Extends tap life on deep holes
Starrett Digital Caliper
- Verify hole diameter before tapping
- Check thread major diameter quickly
- Catch oversize drills before tap breakage
Klein Screwdriver Set
- Fixture and cover removal during setup
- Supports repeatable assembly workflow
- Pairs with torque specs after tapping
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How to use this tap drill calculator
Threading holes requires precision. Drill too small and you will break the tap. Drill too large and the threads will be weak and strip under load. This calculator removes guesswork for SAE and metric machine threads.
Unlike static tap charts that only show 75% engagement, you can adjust thread percentage to match your material and tap life goals.
Workflow
- Select your thread system (SAE or Metric)
- Choose thread size from the dropdown
- Pick cut tap or form tap (different hole sizes)
- Adjust the thread percentage slider (50–100%)
- Calculate to get decimal size and closest drill name
What is thread percentage?
Thread percentage is how much of the thread profile you are actually cutting. At 100% thread the valleys are sharp points. A slightly larger hole flattens those valleys — that is less than 100% engagement.
Going from 75% to 100% thread only increases strength by about 5%, but it can triple the torque required to tap the hole. That extra torque breaks taps and wears them out faster.
Recommendations by material
- Aluminum, brass, soft materials: 60–70% thread — easier tapping, adequate strength
- Steel, iron: 70–75% balances strength and tap life
- Stainless, hard materials: 65–70% reduces tap breakage
- Critical structural joints: 75–80% for maximum reliability
- Avoid 100% thread: minimal strength gain, maximum tap wear
Cut taps vs form taps
Cut taps remove material with flutes and create chips. SAE formula: Drill Size = Major Diameter − (0.01299 × Thread % ÷ TPI). Metric at 75%: Drill Size = Major Diameter − Pitch.
Form taps cold-form threads by displacing metal — no chips, but they need a larger hole. SAE form tap constant: 0.0068 instead of 0.01299.
When to use form taps
- Ductile materials (aluminum, brass, low-carbon steel)
- Blind holes where chip evacuation is difficult
- Production runs where tap life matters
When not to use form taps
- Brittle materials (cast iron, hardened steel)
- Very hard materials (above ~35 HRC)
- Abrasive materials that wear taps quickly
Quick reference: common tap drill sizes (75%)
SAE coarse (UNC)
| Thread Size | Tap Drill | Decimal |
|---|---|---|
| 1/4"-20 | #7 | 0.2010" |
| 5/16"-18 | F | 0.2570" |
| 3/8"-16 | 5/16" | 0.3125" |
| 7/16"-14 | U | 0.3680" |
| 1/2"-13 | 27/64" | 0.4219" |
| 9/16"-12 | 31/64" | 0.4844" |
| 5/8"-11 | 17/32" | 0.5312" |
| 3/4"-10 | 21/32" | 0.6562" |
Metric coarse
| Thread Size | Tap Drill (mm) | Tap Drill (inch) |
|---|---|---|
| M3 × 0.5 | 2.5mm | #39 (0.0995") |
| M4 × 0.7 | 3.3mm | #30 (0.1285") |
| M5 × 0.8 | 4.2mm | #19 (0.1660") |
| M6 × 1.0 | 5.0mm | #8 (0.1990") |
| M8 × 1.25 | 6.8mm | 17/64" (0.2656") |
| M10 × 1.5 | 8.5mm | Q (0.3320") |
| M12 × 1.75 | 10.2mm | 13/32" (0.4062") |
| M14 × 2.0 | 12.0mm | 15/32" (0.4688") |
Use the calculator above to adjust for thread percentages other than 75% or to see the closest match in your drill index.
Pipe threads (NPT): Tapered pipe threads use different drill sizes than this calculator. Always follow the tap manufacturer's recommendation for NPT work. See our tap drill size chart guide for reference tables.
FAQ and related calculators
Why not always use 75% thread?
75% is a conservative shop standard. Many production jobs run 60–70% for dramatically better tap life with negligible practical strength loss in most materials.
What is the difference between cut and form tap drill sizes?
Form taps displace metal instead of cutting it, so they need a larger hole. This calculator applies the correct constant for each tap type.
Can I use fractional drills when the calculator shows a number drill?
Yes, if you are within a few thousandths for general work. For precision or critical joints, use the exact decimal size or the named drill the calculator recommends.
Related tools
After you size the tap drill, confirm pitch series and engagement depth — wrong pitch still strips a perfect % thread. Diagnose assembly failures with Thread Pitch & Engagement Troubleshooting and look up TPI/metric pitch on the Thread Pitch Chart.