Thread & Tap-Drill Calculator
Pick a thread, drag the engagement slider, and watch the joint's cross-section redraw. Theoretical tap drill plus the stock drills you actually own — metric and number/letter — with the engagement each one really delivers.
Major Ø 10.000 mm · pitch 1.500 mm
Formula: drill = major − (engagement% × pitch) / 76.98 — the standard cut-tap sizing rule, in millimetres. Inch threads are converted (pitch = 25.4 / TPI) and shown both ways.
The joint, up close
schematic 60° V — not to scale8.54
Theoretical, mm
0.3362″
8.5 mm
Stock metric drill
8.50 mm → 77% engagement
R
Inch alternative
8.61 mm → 71% engagement
The tap drill is a strength-vs-torque dial
Every tap chart is quietly making a decision for you: how much of the thread form to actually cut. Drill big and the tap turns easily but the nut threads are stubby; drill small and the threads are tall but the tap is carving nearly the full V in one pass. The asymmetry is what makes 75% the folklore answer — past it, joint strength has mostly plateaued (the bolt starts breaking before threads strip), while tapping torque keeps climbing steeply. In tough materials, in deep holes, on the last operation of an expensive part, that trade is everything.
The cross-section drawing is the part I wish every chart included: engagement stops being an abstract percentage the moment you can see the internal thread growing toward the sharp root. It's the same idea as the scallop drawing in the surface finish calculator — geometry first, numbers second. And if the hole itself is the hard part, tapping torque and tool life are exactly the kind of cutting-tool problem I wrote about in why tool geometry choices matter.
Frequently asked questions
What does % thread engagement actually mean?+
It's how far the internal thread reaches from the major diameter toward the theoretical sharp root of the thread form, as a percentage. The tap drill sets it: a smaller drill leaves more material for the tap to cut, so the nut's threads are taller and engage more of the bolt's flank. The cross-section drawing on this page shows exactly that — drag the slider and watch the internal thread grow toward the sharp-V root.
Why is 100% engagement a myth?+
Because strength plateaus while torque explodes. Test data going back to the U.S. Bureau of Standards shows that above roughly 60–75% engagement, the joint's strength barely increases — the failure mode shifts to the bolt breaking rather than threads stripping — while tapping torque roughly doubles from 75% to 100%. So a 100% hole buys you almost no strength and a dramatically higher chance of snapping the tap in the part. That's why every handbook chart is built around ~75%.
What formula does this calculator use?+
The standard cut-tap rule: drill = major − (engagement% × pitch) / 76.98, everything in millimetres. It's the metric form of the classic inch formula drill = major − (0.0130 × %) / TPI. Sanity anchors: M10×1.5 at 77% gives 8.50 mm, and 1/4-20 at 75% gives 0.201 inches — the #7 drill, exactly what the chart on your shop wall says.
Does this work for form (roll) taps?+
No — these sizes are for cut taps only. A forming tap doesn't remove material; it displaces it, and the displaced metal flows up to form the crest. That means the hole must start significantly larger than a cut-tap hole for the same thread (roughly major − pitch/2 at 65–70%, but it varies by material and manufacturer). Use the tap maker's chart for form taps; using a cut-tap drill size with a form tap is a broken-tap story waiting to happen.
What are number and letter drills, and why do the decimals matter?+
They're the American wire-gauge drill series: #1 (0.228") down to #60 (0.040"), then letters A (0.234") to Z (0.413") above that, filling the gaps between fractional sizes. Tap charts lean on them because a tap drill rarely lands on a clean fraction — 1/4-20 wants 0.201", which is exactly #7. This calculator returns both the nearest number/letter/fractional drill and the nearest metric drill, with the engagement each one actually delivers, so you can grab whichever cabinet is closer.
Snapping taps or stripping threads in production?
Cutting tools, parameters, and process — that's my day job.