Thread Size Calculator
Pick the metric thread size and whether it is coarse or fine pitch, and the material you are tapping. The calculator gives the tap drill size, the minor diameter, clearance holes and how deep to thread.
- Tap drill size
- Clearance holes
- Coarse or fine
Coarse pitch is the default for most bolts. Fine pitch is used where there is vibration or thin walls.
Every metric thread is a diameter and a pitch. An M8 thread is 8 mm across with a 1.25 mm coarse pitch. Drill the diameter less the pitch, 6.8 mm, and the tap cuts about 75% of a full thread, which is strong and easy to tap.
Metric tap drill chart
Coarse pitch. Search for a thread.
| Thread | Pitch (mm) | Tap drill (mm) | Clearance (mm) |
|---|---|---|---|
| M3 | 0.5 | 2.5 | 3.4 |
| M4 | 0.7 | 3.3 | 4.5 |
| M5 | 0.8 | 4.2 | 5.5 |
| M6 | 1 | 5 | 6.6 |
| M8 | 1.25 | 6.8 | 9 |
| M10 | 1.5 | 8.5 | 11 |
| M12 | 1.75 | 10.2 | 13.5 |
| M16 | 2 | 14 | 17.5 |
| M20 | 2.5 | 17.5 | 22 |
Clearance holes are medium fit from ISO 273.
Diameter and tap drill
Coarse pitch. Slide the diameter.
The bolt is drawn with its thread profile. The pitch is the distance from one crest to the next.
How to tap a hole
Drill, then tap. Step through it.
Find the pitch
From the thread.
Use cutting fluid and back the tap off half a turn every turn to break the chip.
Coarse, fine or extra fine
Pitches. Compare them.
Standard bolts.
Closer threads.
Very close.
Coarse threads are less likely to cross-thread and strip in soft materials.
Before tapping
Tapping checks. Tick each item.
Tick each item as you go.
Blind holes need a bottoming tap to cut full threads near the bottom.
What you get
Thread data. Each tile explains one result.
Diameter − pitch.
Standard size.
Root of thread.
Close and medium.
Thread depth.
Thread engagement of one diameter in steel, or two in aluminium, makes the bolt break before the thread strips.
Frequently Asked Questions
No question matches that search.
How do I work out a tap drill size?
Subtract the pitch from the thread diameter. For M8 × 1.25, drill 6.8 mm.
What is the pitch of a common metric bolt?
An M6 bolt has a 1 mm coarse pitch and a 0.75 mm fine pitch. An M8 has 1.25 mm coarse.
What size clearance hole for a bolt?
Slightly bigger than the bolt, such as 6.6 mm for M6 or 9 mm for M8.
How deep should a tapped hole be?
At least one diameter of thread in steel, and about two in aluminium or plastic.
What is the difference between coarse and fine threads?
Fine threads have more turns per millimetre, so they hold better against vibration but are slower to fit.
What is the minor diameter of a thread?
The smallest diameter, at the root of the thread.
Why does tap drill size matter?
Too small and the tap can break. Too large and the thread is weak.
The right hole every time
Pick the thread, drill the diameter less the pitch and tap with care. Use the clearance hole for the part the bolt passes through.
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