Limits of size for the bolt and the nut, the tap drill, and the measurement over wires. Unified inch (UNC, UNF, UNEF) and metric. Nothing to sign up for.
Made by Pylvos, job tracking for small machine shops. This calculator, a tap drill chart, speeds and feeds, bar yield and more are built into the app, next to your jobs, prints and certs.
Try the live demo →Unified inch threads are calculated from the formulas in ASME B1.1, for classes 1A, 2A and 3A (outside) and 1B, 2B and 3B (inside), with the standard's own rounding steps. Every class of every UNC, UNF and UNEF size listed here was checked against a published limits table, all 300 rows. For the few sizes where the standard's table differs from its own formulas by a tenth (figures carried over from older editions), the calculator gives the published figure, because that is what gauges are made to.
Metric threads use the ISO 965-1 fundamental deviations and tolerance grades for 6g and 6h (outside) and 6H (inside). The tap drills are the sizes the charts list.
A diameter and pitch typed in by hand ("Other diameter and pitch") is worked out from the same formulas. A published table can differ from that by 0.0001 in on a limit.
The size over three wires is M = E + 3W − 0.866025 P, where E is the pitch diameter, W the wire size and P the pitch. The best wire, 0.57735 P, touches the thread at the pitch line, but any wire from 0.505 P to 1.010 P works. The calculator uses the wire in a standard set nearest the best size, or the one you type in, so the number matches the wires on your bench. The small lead-angle correction is left out; it only matters on coarse, small-diameter threads.
For gauged or drawing-controlled threads, check against your copy of the standard or your gauges. The minor diameter of a rounded-root (UNR) thread is a reference figure and can differ from a table by 0.0001.
A #7 drill (0.201 in) gives about 75% thread, the usual choice. In tough material or a deep hole, a #5 or #4 gives less thread and taps easier, with little loss of strength. Pick any thread above to see its tap drill, and whether that drill is inside the class's minor diameter limits.
A 2A outside thread has an allowance: it is made slightly under the basic size, which leaves room for plating and makes assembly easy. A 3A thread has no allowance and a tighter pitch diameter tolerance (three quarters of 2A's), for precision work. 1A is looser still. The inside thread classes 1B, 2B and 3B work the same way, without an allowance.
Unified Coarse, Fine and Extra Fine: the same thread form with fewer or more threads per inch for a given diameter. UNR is an outside thread with a rounded root, which most rolled threads and bolts are; its limits are the same as UN apart from the minor diameter.
The usual 75% tap drills are chosen for easy tapping, and on some threads, often in class 3B, they fall outside the tighter minor diameter limits. Where the minor diameter is inspected, drill under and ream or bore to size. The calculator says so when it happens.
Pylvos is job tracking for small machine shops: a board everyone can see, prints and certs on every job, and these calculators built in. Runs on a PC you already have. Free for up to 3 people.