Work out the percentage of full thread a tap drill produces. Enter the drill diameter, the thread's major diameter and the pitch — 75% engagement is the usual target.
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Request a Proposal →Published starting values are not measurements. They are recommendations, and they differ because each manufacturer optimises for something different — tool life, surface finish, chip control, or spindle power. Two suppliers can quote the same ISO grade and give you cutting speeds 30% apart, and each figure can be right for that supplier’s own geometry, edge preparation and coating.
That is why the number this calculator returns is a starting point, not a setting. Treat the catalogue figure as the middle of a band, then correct it on the first part: if the chips come off blue or the spindle load sits above roughly 80%, come down; if the finish is smeared and the chips are thin and stringy, come up. Write down what actually worked — that shop-specific number beats any published value.
Thread percentage measures how much of the full theoretical thread depth is engaged after tapping. Higher percentages give stronger threads but require more tapping torque.
65-75% is standard for most applications. 50-60% is acceptable for low-stress fasteners and reduces tapping torque significantly. Over 80% rarely adds strength (the bolt fails before the thread strips above ~65%) but dramatically increases tap breakage risk.
Yes, but with diminishing returns. The difference in pull-out strength between 50% and 75% engagement is about 10-15%. Going from 75% to 100% adds less than 5% strength but doubles tapping torque and tap wear.
Use a micrometer or caliper on the drill, not the nominal size. A worn 6.8 mm drill might actually cut 6.9 mm, changing your thread percentage. For critical threads, measure the drilled hole with a pin gauge before tapping.
Enter your tap drill diameter, the thread major diameter, and pitch. The calculator shows the resulting thread engagement percentage and suggests an alternative drill size for the standard 75% target.
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SETTING THREAD PERCENTAGE? The percentage drives both torque and tap breakage, and it belongs in the purchase specification. See what a tool inquiry needs to include.
Thread percentage is set by the tap's class of fit, not by the operator. Send the thread standard, the class and the material.