Tap and clearance drill sizes
For the machine screws that hold shop fixtures together. The tap drill goes in the piece that holds the thread and the clearance drill in the piece the screw passes through — getting that backwards is why a first threaded joint never pulls tight.
= 0.75"
Drills for 1/4-20
Tap drill
#7
0.2010" — leaves 75% thread
Clearance
17/64"
through the piece the screw passes through
Thread this deep
1/2"
So drill this deep
5/8"
Computed size
0.2000"
Where to buy this
- 1/4-20 tapHome DepotLowe'sAmazon
- #7 tap drillHome DepotLowe'sAmazon
- 17/64" clearance drillHome DepotLowe'sAmazon
A threaded insert is usually the better answer than tapping wood, and its hole is bored to the insert maker’s own spec rather than to the sizes here.
Plain search links, nothing sponsored. Prices and stock are the shop's, and a local lumberyard will usually beat both on solid stock.
Tap the far piece, drill the near one
The tap drill goes in the piece that holds the thread. The clearance drill goes in the piece the screw passes through — and it has to, because tapping both means the near threads fight the far ones and the joint never pulls tight. It is the single most common way a first threaded joint goes wrong.
Depth
3/4" is enough for 1/2" of thread — 2 diameters — plus the chip room a plug tap needs under it.
Two diameters of thread, rather than the one and a half a machinist would use in steel: aluminium, hardwood and plywood all need more thread to reach the same holding, and in wood it is the threads that fail rather than the screw.
Every size on the index
- #4-40clearance 1/8" · 80% thread
- #44
- #6-32clearance 9/64" · 78% thread
- #36
- #8-32clearance 11/64" · 69% thread
- #29
- #10-24clearance 13/64" · 75% thread
- #25
- #10-32clearance 13/64" · 76% thread
- #21
- 1/4-20clearance 17/64" · 75% thread
- #7
- 1/4-28clearance 17/64" · 80% thread
- #3
- 5/16-18clearance 21/64" · 77% thread
- F
- 3/8-16clearance 25/64" · 77% thread
- 5/16"
- 1/2-13clearance 33/64" · 78% thread
- 27/64"
Major diameter less one pitch, snapped to the nearest drill that exists. That reproduces the published chart at every size here but the smallest, where this lands on a #44 at about 80% thread and the chart prefers the gentler #43 — which is why the percentage is shown rather than left implied.
About tapping a thread
- What size hole do I drill to tap 1/4-20?
- 0.201", which is a #7 drill. The rule is the major diameter less one pitch — 0.250 − 1/20 = 0.200 — and the nearest thing in the drill index is the #7 the published chart names. That leaves about 75% of a full thread, which is the standard target.
- Why 75% of a thread and not 100%?
- Because the last 25% is nearly all risk. Going from a 75% to a 100% thread adds only a few percent to the strength of the joint and multiplies the torque needed to turn the tap, and a tap snapped off in a finished jig is not a recoverable situation — hardened steel does not drill out. The percentage is shown next to every answer so you can see where a substituted drill puts you; over about 80% is where taps start breaking.
- Why is the tap drill a number drill but the clearance hole a fraction?
- Because they need different precision. A tap drill has to hit a thousandths-level target, and the number and letter series exist precisely to fill the gaps between the sixty-fourths. A #8-32 works out at 0.1328", which is not a fraction at all: the nearest sixty-fourth is 1/8", eight thousandths undersize, and that is a 96% thread — a hole that snaps taps. The drill the chart names is a #29 at 0.136", which leaves 69%. A clearance hole has no such target and only has to let the screw through, so it is the next sixty-fourth up, which is what the chart's free-fit column says.
- Should I tap wood?
- For a jig that comes apart a few times, yes, and coarse thread in dense hardwood holds better than people expect. For anything that gets adjusted repeatedly, no — the threads are wood fibres and they wear. A threaded insert is the right answer there, and the hole for one is bored to the insert maker's spec rather than calculated here, because the outside diameter and thread form vary by product.