Pilot Hole Calculator
Pilot and clearance sizes from your own screw measurements, and why skipping the clearance hole stops the joint closing.
Measure your own screw with callipers rather than trusting a gauge number — screw dimensions vary between manufacturers and standards, and two minutes with callipers beats any chart. Nothing is uploaded.
There are two holes, and they are different sizes
The pilot goes into the piece the threads are meant to bite, and matches the
root diameter — the shank between the threads — so the threads have material to cut
into. For this screw that is 2.90 mm in hardwood and
2.61 in softwood, which will not compress the same way.
The clearance hole goes through the piece being pulled down, and has to pass
the threads entirely — so it matches the outer diameter,
4.20 mm. Using the pilot size for both is the common
mistake, and it is worth understanding why it fails rather than just being told it does.
The clearance hole is what closes the joint
If the threads bite the top piece as well as the bottom one, the screw is threaded into both boards at once — and turning it cannot draw them together. It holds them where they are, or jacks them apart if there was already a gap. The joint feels tight because the screw is tight; the gap stays because nothing was ever pulling.
| Hole in the top piece | Thread still biting it | Joint closes? |
|---|---|---|
| none | 0.65 mm | no |
| 2.9 mm (pilot size) | 0.65 mm | no |
| 3.2 mm | 0.50 mm | no |
| 3.5 mm | 0.35 mm | no |
| 4.0 mm | 0.10 mm | no |
| 4.2 mm (clearance) | 0.00 mm | yes |
| 4.5 mm | 0.00 mm | yes |
Notice the second row: drilling the top piece to pilot size leaves exactly as much thread biting as drilling nothing at all — the full 0.65 mm — because the pilot passes the shank and not the threads. Drilling both holes the same size is no better than drilling one. Partway helps partway: at 3.5 mm there is still 0.35 mm of thread in the wrong board. Only at the full clearance diameter does the bite reach zero, and going wider than that changes nothing further.
How to use
- Measure the screw across the threads and at the shank.
- Say whether it is going into hardwood or softwood.
- Read the pilot and clearance sizes.
- Check how much thread a given top-piece hole still leaves biting.
Frequently asked questions
Why are there two different holes?
They do different jobs. The pilot, in the piece the threads are meant to bite, matches the ROOT diameter so the threads have material to cut into. The clearance hole, through the piece being pulled down, matches the OUTER diameter so the threads pass through it entirely.
What happens if I skip the clearance hole?
The threads bite the top piece as well as the bottom one, so the screw is threaded into both boards at once and turning it cannot draw them together. It holds them where they are, or jacks them apart if there was a gap. The joint feels tight because the screw is tight — the gap stays because nothing was ever pulling.
Can I just drill both holes the pilot size?
That is no better than drilling one. A pilot-sized hole passes the shank but not the threads, so it leaves exactly as much thread biting the top piece as no hole at all. Only at the full clearance diameter does the bite reach zero.
Does a partway hole help partway?
Yes, proportionally — on a screw with 0.65 mm of thread depth, a 3.5 mm hole against a 4.2 mm clearance still leaves 0.35 mm biting. Better than nothing and not the same as right, which the table on the page shows directly.
Why does hardwood need a bigger pilot?
Because the fibres will not compress. In softwood an undersized pilot lets the threads compact the wood around them, which holds well; in hardwood the same hole splits the piece or snaps the screw. The same screw takes two different bits depending only on what it is going into.
Should I use a chart for my screw size?
Callipers beat any chart. Screw dimensions vary between manufacturers and standards, and a gauge number does not pin down the root diameter — which is the measurement the pilot actually depends on. Two minutes of measuring removes the guesswork.
What about pocket screws and self-drilling screws?
Pocket-hole jigs cut the clearance hole as part of the pocket, which is why they pull tight without a second operation. Self-drilling screws with a cutting nib do something similar in thin stock. Both are solving the same problem this page describes.
Does this send anything anywhere?
No. Every figure is computed in your browser, and nothing is uploaded or stored.
🔒 This tool runs entirely in your browser. Nothing you enter is uploaded, logged, or stored.