Tap Drill Size Guide: How to Calculate the Right Hole
Every tapped thread starts with a drilled hole, and the size of that hole decides everything that follows. Drill it a fraction too small and the tap fights you the whole way in, often snapping off deep in the part. Drill it too big and the threads come out shallow and weak. This guide explains what tap drill size really means, the simple formula behind the charts, and why almost every reference lands on 75% thread engagement.
What tap drill size actually controls
When you tap a hole, the tap does not remove all the material inside the thread. It cuts grooves into the wall of an already-drilled hole. The drill sets the starting diameter of that hole, called the minor diameter, and the tap cuts inward from there up toward the major diameter.
The gap between the drilled hole and the major diameter is how much thread actually gets formed. A smaller drill leaves more material for the tap to cut, so you get a taller, deeper thread. A larger drill leaves less, so the thread is shorter. That depth, expressed as a percentage of a theoretical full thread, is called thread engagement.
The tap drill formula
Charts make it look complicated, but the standard 75% metric case is about as simple as engineering formulas get. For a metric thread, the 75% tap drill is just the major diameter minus the pitch.
So an M10 × 1.5 needs a 10 − 1.5 = 8.5 mm drill. An M8 × 1.25 needs 8 − 1.25 = 6.75 mm, and an M6 × 1 needs 6 − 1 = 5 mm. Check these against the charts on this site and they line up exactly, because those numbers come from the same relationship.
If you want a different engagement level, the full formula adds a factor for the thread geometry:
Here %engagement is written as a decimal, and the 1.299 comes from the geometry of the 60° thread. (Solve the same relationship for engagement instead and its reciprocal, 76.98, appears: %engagement = 76.98 × (major − drill) ÷ pitch.) Plug in 0.75 and 0.75 × 1.299 = 0.974, so it reduces to almost exactly "major minus pitch," which is why the shortcut works. For 65% on that same M10 you get 10 − (0.65 × 1.299 × 1.5) = 8.73 mm, which matches the engagement chart on the M10 page exactly.
Why 75% is the magic number
It is tempting to think a fuller thread is always stronger, so why not tap to 100%? The answer is in the strength-versus-effort curve, and it is steeply diminishing.
| Engagement | Approx. holding strength | Tapping effort |
|---|---|---|
| 50% | ~90% | Very easy |
| 65% | ~93% | Easy |
| 75% | ~95% | Moderate |
| 100% | 100% | Very hard, high break risk |
The jump from 75% to 100% engagement adds only about 5% more holding strength, but it roughly doubles the cutting torque. That extra torque is exactly what snaps taps, especially in small diameters and tough materials. For the sake of a few percent of strength that a properly sized bolt never needs, you take on a large increase in broken-tap risk. That is a bad trade, and it is why 75% became the default.
Rule of thumb: use 75% for general work in steel and aluminium. Drop to 60–65% for hard or gummy materials, deep blind holes, or small taps under M4, where reducing torque matters more than the last sliver of strength.
When to go lower than 75%
Lower engagement is not a compromise so much as the right call in specific situations:
- Hard materials like stainless steel or titanium fight the tap. A larger drill (60–65%) cuts torque and protects the tap.
- Small taps (M3, M4 and below) are fragile. Even a slightly oversized hole dramatically lowers the chance of snapping one off.
- Deep or blind holes pack chips and raise friction. Less engagement gives the chips more room and eases the cut.
- Soft or thin material strips easily regardless, so the extra strength of a full thread is wasted — go easier on the tap instead.
Common tap drill mistakes
The single most common error is reaching for the nearest drill without checking. A tap chart is quick, and being off by half a millimetre changes engagement dramatically at small sizes. The second is forgetting that the chart value is a target, not a law: if you only have a drill 0.1 mm off, the slightly larger one is almost always the safer choice, because it lowers torque and tap-breakage risk at a tiny cost in strength.
For exact numbers on any size, the drill and tap chart lists both 75% and 65% drills for every thread, and each individual thread page shows the full engagement range from 50% to 85%.
The bottom line
Tap drill size is the lever that sets thread engagement, and engagement is a balance between strength and how hard the tap has to work. For most jobs, subtract the pitch from the major diameter, tap to 75%, and move on. When the material is hard, the tap is small, or the hole is deep, size up a little and trade a few percent of strength for a much lower chance of a broken tap buried in your workpiece.
Frequently asked questions
How do I calculate tap drill size?
For a metric thread, the standard 75% tap drill is simply the major diameter minus the pitch — an M10 × 1.5 needs a 8.5 mm drill. For other engagement levels, use tap drill = major diameter − (%engagement × 1.299 × pitch) with the engagement written as a decimal (0.75 for 75%), or just read it off a tap drill chart.
Why is 75% thread engagement the standard?
75% gives about 95% of the full holding strength while cutting far less material. Going to 100% adds only a few percent of strength but roughly doubles tapping torque and the risk of breaking the tap, so 75% is the practical default.
What happens if the tap drill hole is too small?
The tap has to cut a higher percentage of thread, which sharply raises cutting torque. That makes tapping harder, gives poorer threads, and is the leading cause of broken taps in small sizes and hard materials.