Thread Pitch Calculator
Tap drill from thread geometry — metric and unified, major minus pitch, with the engagement percentage and ISO tooth depth priced in.
Thread Pitch Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- 60-degree profiles (ISO M and unified)
- Drill sizes rounded to chart convention
In short: An M10×1.5 thread takes an 8.5 mm tap drill — major 10 mm minus pitch 1.5 mm, the 75%-engagement habit that every tap chart compresses into one column. The same rule in imperial lands a 1/4-20 UNC on a #7 drill: 0.250 in major minus 0.050 in pitch. The ISO tooth itself is 1.299 mm tall (0.866×P) with a minor diameter of 8.160 mm — the V that the drill leaves the tap to finish.
Formula
drill = major − P ··· H = 0.866×P · minor = d − 1.2269×P
A tap removes only the crests the drill left: the 75% rule drills major minus pitch so the tap threads rather than fabricates. The ISO profile hangs everything on P — tooth height 0.866P, depth of thread 1.2269P, the constants of a 60-degree V truncated at an eighth. Smaller pitch means shallower teeth and a bigger drill: fine threads cut easier and clamp finer; coarse threads forgive dirt and damage.
Worked Example
- Enter the major diameter and pitch.
- Read the tap drill — major minus pitch.
- Check the tooth geometry the thread will own.
- Pick the nearest stocked drill within tolerance.
Defaults: M10×1.5 → 8.5 mm drill, H 1.299000 mm, minor 8.160 mm. The coarse check: M10×1.75 → 8.2 mm drill (the classic chart value for a 75% cut). The finer the pitch, the bigger the drill and the easier the tap.
Strengths & Limits Of This Model
Where this engine is strong
- Tooth geometry priced beside the drill
- Engagement rule stated, not implied
Where it stops
- No thread-gauge identification
- No NPT or buttress profiles
Practical Use Cases
Machine shop work
drill before the tap
Repair threads
identify pitch from gauge
Teaching
the geometry inside tap charts
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Thread Pitch Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
Why does the 75% rule subtract the pitch?
Because the drill's job is to leave exactly the metal the tap's V will shape into flanks: at 75% engagement the remaining groove depth equals about three-quarters of the full tooth height. Major minus pitch lands the drill size in that band for the standard metric and unified profiles, which is why every tap chart's 75%-column is this subtraction wearing a chart.
Why 75% engagement and not 100%?
The last quarter of engagement multiplies torque far faster than it multiplies strength — the tap is pushing nearly full-width chips through a closing V, and taps break exactly there. Full engagement buys a few percent of strip strength for a large cut-risk; below roughly 60% the threads themselves start to strip. 75% is the industry's standing compromise, and the reason drill sizes feel 'one size up' from a naive guess.
How do I measure an unknown thread's pitch?
Metric: crest-to-crest distance in millimetres — a pitch gauge or a ruler over ten crests divided by ten. Unified: threads per inch, converted by pitch = 25.4/TPI. Counting crests over a longer run beats a single gap measurement every time. Once pitch and major diameter are known, the page prices the drill directly.
What do H = 0.866P and the 1.2269 constant mean?
They are the 60-degree V's geometry in pitch units: a full sharp V would be 1.5493P tall (P over tan 30, doubled and halved into constants), the ISO thread truncates the crest by P/8 and the root by P/4, leaving height 0.866P and a depth of thread 1.2269P between major and minor. The minor diameter — d minus 1.2269P — is what a gauge feels at the bottom of the groove.
Does the rule work for imperial threads too?
The subtraction generalises — drill = major minus pitch with pitch = 1/TPI inches — which is how 1/4-20 lands on a #7 drill (0.250 − 0.050 = 0.201 in, and #7 is 0.201). Unified coarse and fine series both follow it; the constants differ slightly because the Whitworth-influenced profiles round differently, but the chart agreement is close enough to drill from.
Which pitch — coarse or fine?
Coarse (larger pitch) cuts easier, tolerates dirt and plating, and strips before the bolt breaks — the default for machine structures. Fine engages more turns per length and resists vibration loosening — the automotive and aerospace habit where fine adjustment and fatigue matter. The page prices both: entering the fine pitch raises the drill size, the tap's work shrinks with it.
What if the material is soft — aluminium or brass?
Softer, gummier chips jam at high engagement, so shops often run 65–70% in aluminium (slightly larger drill) and accept the strength margin since the base material strips before steel threads do. Cast iron, being brittle, cuts happily at full engagement. The 75% rule is a steel-biased starting point; the material adjusts it.
Why is my tap binding even with the right drill?
Because drill size is only one of four variables — alignment, chip evacuation, lubrication and the tap's own wear share the blame. A slightly oversize or burred drill hole, a tap started crooked, or chips packing in a blind hole all raise torque the same way an undersize drill does. Break chips with quarter turns in blind holes and never blame the chart before checking the spindle's square.