The tap drill for a 1/4-20 thread is a #7 drill (0.201 in, 5.1 mm). It gives about 75% thread, the standard target for steel and aluminium. If you only have fractional drills, 13/64 in (0.2031 in) is the usual substitute and gives about 72% thread.
1/4-20 UNC is the most common inch thread in a workshop: jig knobs, T-track bolts, threaded inserts, tripod mounts and machine parts all use it. Below are the drill choices and the thread each one gives, the clearance hole for the bolt itself, and how deep to drill a blind hole. For any other thread, use the tap drill size calculator.
1/4-20 tap drill chart
| Drill | Size | Thread | Use it for |
|---|---|---|---|
| #8 | 0.1990 in (5.05 mm) | 79% | Soft aluminium, brass, plastics: a fuller thread |
| #7 | 0.2010 in (5.11 mm) | 75% | The standard choice: steel, aluminium, most metals |
| 5.1 mm | 0.2008 in (5.10 mm) | 76% | Metric drill set: same result as #7 |
| 13/64″ | 0.2031 in (5.16 mm) | 72% | Fractional set: fine in mild steel and aluminium |
| #6 | 0.2040 in (5.18 mm) | 71% | Tougher steels, long engagement |
| #5 | 0.2055 in (5.22 mm) | 69% | Stainless and hard steel: easier tapping |
| #3 | 0.2130 in (5.41 mm) | 57% | Very hard material only (57%); for 1/4-28 it is the standard drill |
Why not just use the biggest thread? Above about 75% the thread gets only slightly stronger while the force needed to turn the tap rises steeply, and 1/4 in taps break easily. Below about 60% the thread starts to lose strength, so the bigger drills are for hard material where tapping is the problem.
Clearance hole for a 1/4-20 bolt
If the bolt only passes through a part and threads into another, the first part gets a clearance hole, not a tap drill:
- Close fit: F drill, 0.257 in (accurate location, holes must line up)
- Normal fit: 17/64 in, 0.2656 in (general use)
- Loose fit: 9/32 in, 0.2812 in (easy assembly, room to adjust)
The clearance hole size calculator has the same three fits for every inch and metric size.
How deep to drill and tap a 1/4-20 blind hole
A common rule is to engage about 1 x the diameter in steel and 2 x in aluminium: 1/4 in or 1/2 in of full thread. A plug tap has 3 to 5 tapered threads at its tip, so the hole must be deeper than the thread you need:
| Part material | Full thread | Tap reaches (plug tap) | Drill depth, full diameter | Including the drill point |
|---|---|---|---|---|
| Steel | 1/4 in | 0.50 in | 0.60 in | 0.66 in |
| Aluminium | 1/2 in | 0.75 in | 0.85 in | 0.91 in |
Work out other threads, tap types and engagement lengths with the tapped hole depth calculator.
Tips for tapping 1/4-20
- Drill square to the surface: a drill press or a guide block makes the tap start straight.
- Use cutting fluid in steel and aluminium. In cast iron and brass, tap dry.
- Turn the tap a half turn forward and a quarter turn back to break the chip, and back it out to clear chips in blind holes.
- In wood, use a threaded insert rather than tapping the wood: a 1/4-20 insert needs its own, larger pilot hole, given by the insert maker.
- Chamfer the hole slightly with a countersink so the first thread does not raise a burr.
Frequently asked questions
What drill bit do you use for a 1/4-20 tap?
A #7 drill, 0.201 in. It gives about 75% thread, the standard target. If you only have fractional drills, use 13/64 in (about 72%).
Is 13/64 the same as a #7 drill?
Nearly. 13/64 in is 0.2031 in and a #7 is 0.201 in, two thousandths larger. The thread is slightly shallower (72% instead of 75%) but strong enough for most work.
What size hole for a 1/4-20 bolt to pass through?
That is a clearance hole, not a tap drill: 17/64 in for a normal fit, an F drill (0.257 in) for a close fit or 9/32 in for a loose fit.
What is the tap drill for 1/4-28?
A #3 drill (0.213 in), about 80% thread. 7/32 in gives about 67%. The finer 28 TPI thread is shallower, so it needs a bigger drill than 1/4-20.
Can I use a 5 mm drill for a 1/4-20 tap?
It is too small: 5 mm is 0.197 in, about 82% thread, which makes the tap hard to turn and easy to break. Use 5.1 mm or 5.2 mm.
Is 1/4-20 the same as M6?
No. M6 x 1 is 6 mm (0.236 in) with a 1 mm pitch; 1/4-20 is 6.35 mm with a 1.27 mm pitch. They do not interchange. M6 uses a 5 mm tap drill.
Sources: ASME B1.1 (Unified inch screw threads) for the thread size; ASME B94.11M for drill diameters; the clearance holes are from a published close, normal and loose fit chart (see the sources on the clearance hole calculator); thread engagement rule from Nord-Lock’s “Thread engagement in a tapped hole”; tap chamfer lengths as listed for plug taps.