Clearance Hole Calculator
Pick the right drill bit for any inch screw or bolt. Choose the size, thread, fit, head style and material, and get the clearance drill, tap drill and thread percentage, minimum thread engagement, hardwood and softwood pilot holes, and the countersink or counterbore size and depth — with full drill charts and a downloadable PDF.
| Size | Close | Normal | Loose |
|---|---|---|---|
| Pick a size to see the chart | |||
| Size | Tap drill | Hardwood pilot | Softwood pilot |
|---|---|---|---|
| Pick a size to see the chart | |||
How to use the clearance hole calculator
Four quick steps turn a screw or bolt size into every drill bit you need — clearance, tap or pilot, and countersink or counterbore — with full drill charts and a downloadable report.
Pick the fastener
Choose the size from #2 to 3/4″ and the thread series — UNC (coarse) or UNF (fine).
Choose fit & head
Pick a close, normal or loose fit, and a flat 82° head, socket cap or hex / pan head.
Set the material
Choose steel, aluminum, plastic, hardwood or softwood and enter the part thickness.
Read results & save a PDF
Get the clearance, tap or pilot drill, thread engagement and head recess — then download a report.
How clearance and tap holes are sized
Clearance holes come from common close / normal / loose practice (ASME B18.2.8), tap drills give about 75% thread, and the head recess is worked out from the head size. Here is the exact math the calculator runs.
Worked example — 1/4-20 UNC flat-head screw, normal fit, tapped into 1/2″ aluminum
Clearance: H drill = .266" (6.76 mm) · Play: .266 − .250 = .016" (about 1/64")
Tap drill: #7 = .201" · Thread: 20 × (.250 − .201) ÷ .01299 ≈ 75% · Engagement: .250 × 1.5 = 3/8"
Countersink: .507 + 1/64 ≈ .523" (33/64") · Depth: (.523 − .266) ÷ 2 ÷ tan 41° ≈ .148"
Tap the aluminum with a #7 drill at least 3/8″ deep, drill the top part with an H bit, and countersink to about 33/64″ so the head sits flush.
Clearance, tap and pilot drill charts
Handy lookups for clearance holes by fit, tap and pilot drills by size, and countersink and counterbore sizes for common screws.
| Size | Close | Normal | Loose |
|---|---|---|---|
| #2 | #43 · .089 | #38 · .1015 | #32 · .116 |
| #4 | #32 · .116 | #30 · .1285 | #27 · .144 |
| #6 | #27 · .144 | #25 · .1495 | #18 · .1695 |
| #8 | #18 · .1695 | #16 · .177 | #9 · .196 |
| #10 | #9 · .196 | #7 · .201 | #2 · .221 |
| #12 | #2 · .221 | #1 · .228 | D · .246 |
| 1/4″ | F · .257 | H · .266 | 9/32″ · .281 |
| 5/16″ | P · .323 | Q · .332 | 11/32″ · .344 |
| 3/8″ | W · .386 | X · .397 | 13/32″ · .406 |
| 7/16″ | 29/64″ · .453 | 15/32″ · .469 | 31/64″ · .484 |
| 1/2″ | 33/64″ · .516 | 17/32″ · .531 | 9/16″ · .563 |
| 5/8″ | 41/64″ · .641 | 21/32″ · .656 | 11/16″ · .688 |
| 3/4″ | 49/64″ · .766 | 25/32″ · .781 | 13/16″ · .813 |
Normal fit is the everyday choice. Use close for accurate location and loose when several holes have to line up.
| Size | UNC tap drill | UNF tap drill | Hardwood / softwood pilot |
|---|---|---|---|
| #2 | #2-56: #50 | #2-64: #50 | 1/16″ / 3/64″ |
| #4 | #4-40: #43 | #4-48: #42 | 5/64″ / 1/16″ |
| #6 | #6-32: #36 | #6-40: #33 | 3/32″ / 5/64″ |
| #8 | #8-32: #29 | #8-36: #29 | 7/64″ / 3/32″ |
| #10 | #10-24: #25 | #10-32: #21 | 1/8″ / 7/64″ |
| #12 | #12-24: #16 | #12-28: #14 | 9/64″ / 1/8″ |
| 1/4″ | 1/4-20: #7 | 1/4-28: #3 | 3/16″ / 5/32″ |
| 5/16″ | 5/16-18: F | 5/16-24: I | 1/4″ / 3/16″ |
| 3/8″ | 3/8-16: 5/16″ | 3/8-24: Q | 5/16″ / 1/4″ |
| 7/16″ | 7/16-14: U | 7/16-20: 25/64″ | through-bolt |
| 1/2″ | 1/2-13: 27/64″ | 1/2-20: 29/64″ | 3/8″ / 5/16″ |
| 5/8″ | 5/8-11: 17/32″ | 5/8-18: 37/64″ | through-bolt |
| 3/4″ | 3/4-10: 21/32″ | 3/4-16: 11/16″ | through-bolt |
Tap drills give about 75% thread. Pilot holes are typical for wood screws (#2–#12) and lag screws (1/4″–1/2″); use a through-bolt for larger sizes in wood.
| Size | 82° countersink | Countersink depth | Counterbore × depth |
|---|---|---|---|
| #6 | 19/64″ (.295″) | .083″ | 9/32″ × .169″ |
| #8 | 11/32″ (.348″) | .098″ | 5/16″ × .195″ |
| #10 | 13/32″ (.401″) | .115″ | 3/8″ × .221″ |
| #12 | 29/64″ (.454″) | .130″ | 13/32″ × .247″ |
| 1/4″ | 33/64″ (.523″) | .148″ | 7/16″ × .281″ |
| 5/16″ | 21/32″ (.651″) | .183″ | 17/32″ × .344″ |
| 3/8″ | 25/32″ (.778″) | .219″ | 5/8″ × .406″ |
Countersink depth is measured from the surface to where the cone meets the clearance hole. Counterbores are for socket cap screws.
What the clearance hole calculator gives you
One fastener choice gives you a complete drilling plan for both parts of the joint.
Drilling at a glance
Who uses a clearance hole calculator
From building furniture to machining brackets, the same calculator picks the right bits.
Joining boards with wood screws and wants pilot and shank holes that pull tight without splitting.
- Pick hardwood or softwood
- Drill shank in the top board
- Countersink flat heads
Tapping steel or aluminum and needs the tap drill, thread percentage and minimum thread depth.
- Choose UNC or UNF
- Check engagement depth
- Counterbore socket caps
Bolting parts together for a project and just wants to know which bit fits the bolt.
- Use normal fit
- Read decimal and mm
- Test on scrap first
Tips for drilling clean, accurate holes
The right bit is half the job — these habits make the joint fit and hold.
Clearance hole calculator FAQ
Quick answers about clearance holes, tap drills, pilot holes, countersinks and thread engagement.
A clearance hole is drilled slightly larger than the screw or bolt so it slides through without touching the threads. The screw then threads into the part below, or into a nut.
For a 1/4 inch bolt, a normal clearance hole is an H drill at .266 inch, which leaves about 1/64 inch of total play.
Use an F drill (.257 inch) for a close fit, an H drill (.266 inch) for a normal fit, or a 9/32 inch drill (.281 inch) for a loose fit.
Normal fit suits most work. Choose close when holes must line up precisely, and loose when several holes have to line up or parts are hand-drilled.
A clearance hole is bigger than the screw so it passes through freely. A tap drill is smaller than the screw so a tap can cut threads into the hole.
For a 1/4-20 screw, the clearance hole is an H drill (.266 inch) and the tap drill is a #7 (.201 inch).
The standard tap drills in the chart give about 75% thread — for example #7 for 1/4-20, F for 5/16-18 and 5/16 inch for 3/8-16.
Thread percentage is TPI × (major diameter − tap drill) ÷ 0.01299. For 1/4-20 with a #7 drill that is 20 × (.250 − .201) ÷ .01299, or about 75%.
For full strength, thread at least 1 × the screw diameter in steel, 1.5 × in aluminum and 2 × in plastic. A 1/4 inch screw needs at least 3/8 inch of thread in aluminum.
If the part is thinner than that, use a nut, a threaded insert or a rivet nut instead.
It depends on the screw size and the wood. A #8 wood screw takes a 7/64 inch pilot in hardwood and a 3/32 inch pilot in softwood, with an 11/64 inch shank clearance hole in the top board.
The shank clearance lets the screw pull the boards tight, and the pilot keeps the wood from splitting. Drill the pilot about as deep as the screw is long.
US flat head screws use an 82° countersink. The calculator sizes it at the head diameter plus 1/64 inch — about 11/32 inch for a #8 and 33/64 inch for a 1/4 inch screw.
Depth is (countersink diameter − clearance hole) ÷ 2 ÷ tan 41°, about .148 inch for a 1/4 inch flat head in a normal-fit hole.
Use the standard counterbore drill for the size and cut it about 1/32 inch deeper than the head height so the head sits just below the surface.
A 1/4 inch socket cap screw takes a 7/16 inch counterbore about .281 inch deep; a #10 takes 3/8 inch about .221 inch deep.
Typical drill sizes — test on scrap first. This calculator works with US inch fasteners and uses common close / normal / loose clearance practice (ASME B18.2.8), standard 75% tap drills and typical wood-screw and lag-screw pilot sizes. Wood density, drill wander and the fastener maker’s specifications can change the best size, so always drill a test hole first. Wear eye protection when drilling, and follow the manufacturer’s specs for structural or load-bearing joints.