How To Measure A Screw Without A Caliper

Lay the screw on a millimetre ruler and read the widest point of the thread — crest to crest, not down in the valleys. Metric ruler graduations are 1 mm apart and you can interpolate to about half of that, which is enough to separate M5 from M6 but not M3.5 from M4.

Read it twice, rotating the screw 90 degrees between readings. A thread that has been driven into steel usually has one flattened crest, and averaging two readings keeps that from costing you a size.

Pitch is the one measurement you can take accurately with no tools at all, because it averages. Line the first crest up with a graduation, count how many crests fall within 10 mm, and divide. Ten crests in 10 mm is a 1.0 mm pitch; eight crests is 1.25 mm.

For an inch thread, count the crests in half an inch and double the result to get threads per inch. Counting a single gap is hopeless — counting ten divides your error by ten.

A known nut is a go/no-go gauge. If the screw threads in freely by hand for several turns, it matches that nut in both diameter and pitch, and you have an exact answer rather than an estimate.

Stop at the first sign of resistance. A near-miss cross-system pair — an M8 in a 5/16-18 nut — will take a turn or two and then bind, and forcing it recuts the nut and destroys your reference.

Drill bits are ground to close tolerance and marked, which makes a drill index a set of known diameters. Find the bit whose shank just matches the screw thread and read the size off it.

The same trick sizes a hole: the bit that just slides through a nut gives you the nut bore, and the tap drill rule — major diameter minus pitch — turns that back into a thread size.

A phone screen can act as a ruler, but only after it has been told how big its own pixels are. Calibrating against a bank card, which is 85.60 mm wide by international standard, gives the screen a real-world scale, after which an on-screen caliper can be laid against the fastener directly.

This is what the Screw Identifier app does with its caliper and gauge board: the calibration turns screen pixels into millimetres, and the millimetres are then matched against the standard size tables. It is the same measurement you would take with a ruler, with the interpolation done for you.