Tap Drill Size Calculator
Find the right tap drill size for metric, UNC and UNF threads, at any thread percentage, with the nearest standard drill and the thread it gives.
See the full Thread Size and Tap Drill Chart
New: please check your results.
For guidance only. This is not a substitute for a qualified engineer, and important designs should be checked by one.
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- Drill to use
- 5.0 mm
- Thread it gives
- 77 %
- Exact size for 75%
- 5.026 mm
- Pitch
- 1 mm
Nearby drill sizes
| Drill | Size (mm) | Thread % |
|---|---|---|
| 4.8 mm | 4.8 | 92.4 |
| 4.9 mm | 4.9 | 84.7 |
| 5.0 mm ✓ | 5 | 77 |
| 5.1 mm | 5.1 | 69.3 |
| 5.2 mm | 5.2 | 61.6 |
What it does
Find the drill to use before tapping a thread. Choose a metric, UNC or UNF thread and the thread engagement you want, and see the exact drill diameter, the nearest standard drill you can buy, and the percentage of thread it will cut. A table shows the drills either side, so you can go slightly bigger for hard metals or smaller for soft ones.
How to use it
- Thread type
Metric coarse is the everyday metric thread (an M6 bolt is 1 mm pitch). UNC and UNF are the American inch threads, still common on machinery and imported equipment.
- Thread size
The Metric coarse size, as written on the bolt or drawing.
- Thread size
The Metric fine size, as written on the bolt or drawing.
- Thread size
The UNC size, as written on the bolt or drawing.
- Thread size
The UNF size, as written on the bolt or drawing.
- Thread engagement
How deep the thread is cut. 75% is standard and is what tap drill charts use. Go down to 60 to 65% for hard or tough metals such as stainless steel, or for deep holes, to make tapping easier; a full thread is barely stronger.
- Drill sizes
Which drill set you have. Match the thread uses mm drills for metric threads and inch drills for UNC and UNF. Metric drills come in 0.1 mm steps; inch drills include fractional, number (#1 to #56) and letter (A to Z) sizes.
Common uses
- Drilling and tapping holes for bolts and machine screws.
- Choosing a drill when the chart size isn't in your set.
- Tapping hard materials like stainless steel with less risk of breaking the tap.
- Laying out the holes on a flange or plate. Try the Bolt Circle Calculator (PCD)
- Tightening the bolts to the right torque. Try the Bolt Torque Calculator
- Setting the drill's speed and feed.
The formula
Drill = major diameter − (1.299 × pitch × thread % ÷ 100)Thread % with a given drill = (major diameter − drill) ÷ (1.299 × pitch) × 100Pitch (UNC/UNF) = 1 ÷ threads per inchWhere:
- 1.299 × pitch
- the depth of a 100% thread, on both sides of the hole
- major diameter
- the nominal size: 6 mm for M6, 0.25 in for 1/4-20
At 75% this reproduces the usual tap drill charts. For metric threads it's very close to the simple rule: drill = diameter − pitch (M6 × 1 → 5 mm). Form (roll) taps need a bigger hole; use the tap maker's figure.
Worked example
An M6 (1 mm pitch) thread at 75%:
- Thread depth: 1.299 × 1 mm pitch × 75 ÷ 100 = 0.974 mm
- Exact drill: 6 mm − 0.974 mm = 5.026 mm
- Nearest standard drill: 5.0 mm, giving 77 % thread
Questions people ask
What size drill for an M6 tap?
5.0 mm, which gives about 77% thread. For M8 it's 6.8 mm, for M10 8.5 mm and for M12 10.2 mm (10.3 mm is closer to 75%). For metric coarse threads, the rule of thumb is diameter minus pitch.
Why not 100% thread?
A full-depth thread is only a few percent stronger than a 75% thread, but takes much more force to tap and breaks far more taps. 60 to 75% is plenty for most work; the bolt will snap before the thread strips if the hole is deep enough (at least 1 to 1.5 times the diameter).
What are number and letter drills?
Inch drill sizes that fill the gaps between the 1/64 in fractions: #1 to #80 run from 0.228 in down to 0.0135 in, and A to Z from 0.234 in up to 0.413 in. Tap drill charts for UNC and UNF threads often use them, such as a #7 (0.201 in) for 1/4-20.
Last checked 1 November 2026 · Last rev. 31 October 2026 · 65fe