Tap Drill Size Chart

About Tap Drill Size Chart

The Tap Drill Size Chart tells you exactly which drill to use before tapping a thread, for both metric (ISO) and imperial (UNC/UNF) taps. Instead of a fixed one-size table, it lets you dial in the percentage of thread engagement and instantly shows the recommended drill from three different drill sets — metric, number/letter gauge, and fractional — together with the actual % of thread each drill produces. The interactive cross-section shows what that percentage physically means: how deep the tap's teeth bite into the wall of the drilled hole.

What is a tap drill size?

A tap cannot cut threads into a solid block — it needs a pre-drilled hole, called the tap drill. If the hole is too small, the tap has to remove too much material: torque rises steeply and small taps snap. If the hole is too big, the thread crests are cut away and the thread strips under load. The tap drill size is the compromise between those two failures, and the "% of thread" is how machinists express that compromise.

For cutting taps the standard formulas (Machinery's Handbook) are:

$$\text{Metric:}\quad d = D - \frac{\%T \times P}{76.98}$$ $$\text{Inch:}\quad d = D - \frac{0.01299 \times \%T}{n}$$

where \(D\) is the major diameter, \(P\) the pitch in mm, \(n\) the threads per inch, and \(\%T\) the desired percentage of thread. For roll-form (fluteless) taps, which displace material instead of cutting it, the constants become 147.06 (metric) and 0.0068 (inch) — that is why form-tap holes are always larger.

How to use this tool

1. Thread size — pick your thread from the list (metric coarse/fine, UNC, UNF) or choose a custom option and enter the major diameter plus pitch or TPI.
2. Tap type — choose Cutting tap for ordinary taps, or Roll-form tap for thread-forming taps.
3. Thread engagement % — drag the slider. 75% is the general-purpose standard; use 60–65% in hard materials, and 55–70% for form taps. The cross-section preview updates live.
4. Click Find Tap Drill Size. You get the best stock drill, close alternatives one size up and down, cross-references from the other drill systems, and the exact math.

Choosing the right % of thread

A thread's strength grows very slowly above 75% engagement — a 100% thread is only about 5% stronger than a 75% thread, but requires roughly three times the tapping torque. That extra torque is what breaks taps, which is why nobody taps at 100%.

Metric coarse thread tap drill chart

Cutting-tap values follow the classic "major diameter minus pitch" rule (≈75% thread); roll-form values are ≈65% thread, rounded to the nearest 0.05 mm stock drill.

Metric fine thread tap drill chart

UNC (Unified coarse) tap drill chart

Standard ≈75% thread tap drills for cutting taps.

UNF (Unified fine) tap drill chart

Pipe thread tap drill sizes (NPT and BSP)

Pipe threads are not sized like bolts: the name is the nominal pipe size, not a diameter, so a 1/4 NPT hole is far bigger than 1/4 inch. Tapered threads (NPT, BSPT) seal on the taper, so the drill is chosen to let the tap cut the full taper, not by % of thread. These are the drills listed on standard charts for drilling without a taper reamer.

BSPP (G) is the parallel British pipe thread used across Europe, the UK, India and Australia; it seals with a washer or O-ring, not on the thread. If you ream the hole with a taper pipe reamer first, NPT drills can be one or two sizes smaller — check the tap maker's chart.

Clearance holes: the hole the bolt passes through

The tap drill is for the part you thread. The other part, the one the bolt passes through, needs a clearance hole slightly larger than the bolt. Pick a fit after the tool shows your tap drill: close fit when parts must line up precisely, normal fit for general work, and loose fit when holes are drilled separately and have to line up by hand.

Cutting taps vs roll-form taps

A cutting tap removes material with fluted cutting edges — chips must evacuate through the flutes. A roll-form tap (also called a fluteless or thread-forming tap) has no cutting edges at all: it cold-forms the thread by displacing material, which flows out of the roots and up into the crests. Because the displaced metal moves inward, the starting hole for a form tap is always larger than for a cutting tap of the same thread. Form taps produce stronger, work-hardened threads with no chips (ideal for blind holes and CNC production), but they only work in ductile materials — aluminum, brass, copper, mild and stainless steel — and need good lubrication. Never use them in cast iron or brittle plastics.

Tips for accurate tapping

• Drilled holes usually run slightly oversize — in critical work, drill and then measure before tapping, or step up in two drill sizes.
• Chamfer the hole to the thread's major diameter before tapping; it helps the tap start straight and prevents a raised burr.
• Use a tapping fluid matched to the material; in aluminum even kerosene or WD-40 beats tapping dry.
• For blind holes, drill deeper than the required thread depth — a taper or plug tap cannot cut full threads to the very bottom.
• Back the tap off a quarter turn regularly (cutting taps only) to break chips; form taps run continuously.

FAQ

Q: What size drill do I use for a tap?
A: Use a drill slightly larger than the thread's minor diameter, so the tap only forms part of the full thread height. For the common 75% engagement: metric threads follow the "diameter minus pitch" rule (M6 × 1.0 → 5.0 mm), and inch threads use \(d = D - 0.01299 \times 75 / n\) (1/4-20 → #7, i.e. 0.201″).

Q: What does percentage of thread mean?
A: It is how much of the full theoretical thread height is actually formed in the hole. A smaller drill leaves more material for the tap to cut — a higher, stronger percentage of thread that takes much more torque to tap. The cross-section diagram above shows it directly: at 75% the thread crests are truncated flats; at 100% they would be sharp.

Q: Is 100% thread stronger than 75% thread?
A: Barely — about 5% stronger, while demanding roughly three times the tapping torque. That trade-off is so bad that 75% is the universal standard, and 50–65% is preferred in hard materials.

Q: What is the tap drill size formula?
A: Metric cutting taps: \(d = D - \%T \times P / 76.98\). Inch cutting taps: \(d = D - 0.01299 \times \%T / n\). Roll-form taps use 147.06 and 0.0068 instead, giving larger holes.

Q: What drill size do I need for an M6 or a 1/4-20 tap?
A: M6 × 1.0 cutting tap → 5.0 mm drill (≈77% thread). 1/4-20 UNC cutting tap → #7 drill, 0.201″ (≈75% thread). With roll-form taps: ≈5.55–5.6 mm for M6 and a #1 drill (0.228″) for 1/4-20.

Q: Why is the drill for a roll-form tap larger than for a cutting tap?
A: Because a form tap removes nothing — it squeezes the material. The metal displaced from the thread roots flows inward to build up the crests, so the hole must start larger. Form taps like 55–70% thread, ductile materials, and plenty of lubricant.

Q: What size drill do I use for a 1/4 NPT or 1/2 NPT tap?
A: 1/4 NPT takes a 7/16″ drill (11.1 mm) and 1/2 NPT a 23/32″ drill (18.3 mm). 1/8 NPT takes an R drill (0.339″). Pipe sizes are nominal names, not diameters, so the drill is much bigger than the name suggests. The full NPT and BSP chart is above.

Q: What size clearance hole do I need for an M6 or a 1/4 inch bolt?
A: For M6, drill 6.4 mm for a close fit or 6.6 mm for a normal fit. For a 1/4 inch bolt, an F drill (0.257″) gives a close fit and an H drill (0.266″) a free fit. The tool shows the clearance hole for your thread under the tap drill result.

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