The tap drill size for a standard M8 × 1.25 thread is 6.8 mm. Drill a 6.8 mm hole, then tap it with an M8 tap. Look the size up on the M8 tap drill calculator. That page is the tool. This note is why the number is 6.8 mm.
If you want to know why it is not 8 mm, how to work it out for any metric thread, and how to avoid broken taps, read on.
Quick answer: M8 tap drill sizes
| Thread | Pitch | Tap type | Drill size |
|---|---|---|---|
| M8 × 1.25 (coarse) | 1.25 mm | Cut tap | 6.8 mm |
| M8 × 1.0 (fine) | 1.0 mm | Cut tap | 7.0 mm |
| M8 × 1.25 (coarse) | 1.25 mm | Roll (form) tap | about 7.4 mm |
Roll tap drill sizes vary between makers, so check the chart that came with your tap.
Why the tap drill is smaller than the thread size
An M8 thread has a major diameter of 8 mm. That is the outside diameter of an M8 bolt and the widest point of the thread inside the hole.
The tap does not drill that 8 mm. It cuts a thread groove into the wall of a hole that already exists. For that to work, the hole must start out smaller than 8 mm. The material left between the hole and the 8 mm line is what the tap cuts into, and it becomes the thread.
Drill the hole at the full 8 mm and there is nothing left to cut. The bolt would slide in loose with almost no grip. That is why the drill for a tapped hole is always smaller than the thread size.
How to calculate the tap drill size
For metric threads, a simple rule gives the answer:
Tap drill size = major diameter − pitch
Pitch is the distance from one thread crest to the next. For M8 × 1.25, the pitch is 1.25 mm.
- Major diameter: 8 mm
- Pitch: 1.25 mm
- 8 − 1.25 = 6.75 mm
A 6.75 mm drill is not a common stock size, so round to the nearest standard drill: 6.8 mm. That leaves roughly 75% thread engagement, which is the usual target for general work.
The same rule works for other metric threads. For M6 × 1.0 it gives 5.0 mm. For M10 × 1.5 it gives 8.5 mm. Treat it as a reliable starting point, and check your tap maker's chart for critical work. The tap drill size calculator uses that 75% rule for metric and UNC threads.
Tap drill sizes for common metric coarse threads
| Thread | Pitch | Tap drill size |
|---|---|---|
| M3 | 0.5 mm | 2.5 mm |
| M4 | 0.7 mm | 3.3 mm |
| M5 | 0.8 mm | 4.2 mm |
| M6 | 1.0 mm | 5.0 mm |
| M8 | 1.25 mm | 6.8 mm |
| M10 | 1.5 mm | 8.5 mm |
| M12 | 1.75 mm | 10.2 mm |
These sizes are for cut taps in a standard hole. If you work with several thread sizes, keep this table near the machine.
Cut taps vs roll taps: the drill size is different
A cut tap removes material as it cuts. It produces chips, and the formula above applies.
A roll tap, also called a form tap, does not cut. It pushes the material aside and squeezes it into the thread shape, so there are no chips. Because the material has to flow somewhere, the pre-drilled hole must be larger than for a cut tap. For M8 × 1.25, roll tap charts commonly list about 7.4 mm.
Mixing these up causes real problems:
- A roll tap in a cut-tap hole (6.8 mm) needs very high torque and often breaks.
- A cut tap in a roll-tap hole (7.4 mm) gives weak, shallow threads.
Roll taps work best in ductile materials such as aluminium, mild steel and brass. They are not suited to brittle materials such as cast iron.
What thread percentage should you aim for?
Thread percentage describes how much of the full thread depth is actually formed in the hole. Many beginners assume 100% is the goal, but it is not.
Around 75% is the usual target for general work. Going toward 100% adds very little strength, yet the tap needs much more torque to cut the extra material. That extra torque is one of the main reasons taps snap.
In tough or work-hardening materials such as stainless steel, many shops use slightly lower engagement to cut tapping torque. Follow your tap maker's recommendation for these materials.
A 7.0 mm drill in M8 × 1.25 gives roughly 60% engagement. It is easier to tap, but the thread is weaker, so keep it for cases where strength is not critical.
Common mistakes that break taps
- Drilling to 8 mm. There is no material left for the thread.
- Mixing up coarse and fine. M8 × 1.25 needs 6.8 mm. M8 × 1.0 needs 7.0 mm. Check the pitch before you pick the drill.
- Using a dull or badly ground drill. It can cut an oversize hole, which leaves a weak thread.
- Skipping cutting fluid. Tapping without lubrication raises torque and wears the tap quickly.
- Not drilling deep enough in blind holes. The tap bottoms out and the chips pack at the bottom. Drill extra depth beyond the thread length you need.
- Tapping at an angle. Keep the tap square to the hole. A guide or a drill press helps for hand tapping.
- Forcing the tap. Turn it forward, then back off a little to break the chips. If it gets tight, stop and clear the hole.
Tap drill size vs clearance hole size
A tap drill makes the hole that will carry the thread. A clearance hole is a different thing: it lets a bolt pass through freely. For M8, clearance holes are typically 8.4 mm (close fit) or 9 mm (medium fit). Do not use these sizes for tapping. The M8 page lists both the tap drill and the clearance hole.
Skip the chart: use a tap drill size calculator
If you do not want to hunt for a chart every time, use the free tap drill size calculator on AlwaysReach. It covers metric and UNC threads, needs no sign-up, and works on a phone at the machine.
FAQ
What is the tap drill size for M8 x 1.25?
6.8 mm with a cut tap.
What is the tap drill size for M8 x 1.0?
7.0 mm. This is the fine pitch version, so the pitch is smaller and the drill is larger.
Can I use a 7 mm drill for a standard M8 thread?
You can, but the thread will be shallower (roughly 60% engagement) and weaker. Use 6.8 mm for normal work.
Why do taps break so often?
The most common causes are a drill that is too small, no lubrication, packed chips in blind holes, misalignment and a worn tap.
Is the drill size the same for steel and aluminium?
The standard cut tap size of 6.8 mm is used for both. For tough materials such as stainless steel, check your tap maker's advice.
Summary
For a standard M8 × 1.25 thread, use a 6.8 mm drill with a cut tap. The rule is major diameter minus pitch, rounded to the nearest standard drill. Use a larger hole for roll taps, check the pitch before drilling, and use cutting fluid. For critical work, always confirm against your tap manufacturer's chart. Where the other charts on this site come from is in how the charts are kept.