Free tools
Thread Pitch Calculator
Short answer
Measure the major diameter across the thread crests, then the pitch from one crest to the next (or count threads per inch), enter both, and the calculator returns every metric and Unified inch size that fits — and warns you when the two systems sit too close to separate by diameter alone.
Across the thread crests, in millimetres. Enter inches as a decimal and switch the unit below the field.
Optional, but it is what separates two sizes of equal diameter.
Matching sizes
Enter a diameter to see the sizes that fit.
Measure the diameter across the crests, not the valleys
The number in a screw designation is the major diameter: the widest point of the thread, measured crest to crest. An M6 bolt measures very close to 6.0 mm across the crests, and about 4.9 mm across the valleys, so measuring the wrong feature makes an M6 look like an M5.
Measure on the plain shank if the screw has one, and take the reading in two places at 90 degrees. A thread that has been driven into metal often has a flattened crest on one side.
Pitch is crest-to-crest in metric, threads-per-inch in imperial
Metric threads are specified by pitch: the distance from one crest to the next, in millimetres. M6 x 1 means the crests are 1.0 mm apart. Unified inch threads are specified the other way round, as threads per inch, so 1/4-20 has twenty crests in every inch.
The two are the same measurement in different units. Twenty threads per inch is a pitch of 25.4 / 20 = 1.27 mm. The calculator accepts either and converts internally.
Pitch is what actually decides the answer
Diameter alone almost never settles it. Every size has a coarse and usually a fine variant that share the same diameter — M8 x 1.25 and M8 x 1 are both 8 mm — and several metric sizes land within a tenth of a millimetre of an inch size.
This is why a wrong screw feels almost right for the first turn or two and then binds: the diameters matched and the pitches did not. Once it binds, forcing it recuts the hole.
When two answers come back, that is the honest result
Measuring a diameter by hand resolves sizes about a millimetre apart, which is enough to separate M5 from M6 but not enough to separate M3.5 from M4 with confidence. Where the reading falls between two sizes the calculator returns both, nearest first, rather than snapping to one and being silently wrong.
To break the tie, thread the fastener into a known nut, or compare the pitch against a size you already have. Two candidates with different pitches are easy to separate; two with the same pitch differ only in diameter and need a better measurement.
Frequently asked questions
What is thread pitch?
The distance from one thread crest to the next, measured along the screw axis. Metric fasteners state it in millimetres (M6 x 1 has 1 mm between crests). Inch fasteners state the reciprocal instead, as threads per inch.
How do I measure thread pitch without a pitch gauge?
Lay the screw against a ruler, line the first crest up with a graduation, and count how many crests fall in 10 mm. Ten crests in 10 mm is a 1.0 mm pitch. For inch threads, count the crests in half an inch and double it. Counting over a span instead of a single crest keeps the error small.
Is an M8 bolt the same as a 5/16 inch bolt?
No. M8 is 8.00 mm and 5/16 inch is 7.94 mm, which is 0.06 mm apart and indistinguishable by any hand measurement — but M8 coarse has a 1.25 mm pitch and 5/16-18 UNC works out at 1.41 mm. They will start in the same hole and jam a few turns later.
The diameter matches but the pitch does not. What now?
Trust the pitch. Diameter tells you the size family; pitch tells you the standard. A pitch that sits between the coarse and fine value for a size usually means the measurement spanned a damaged crest — remeasure over ten threads rather than one.
Other tools
Would rather just take a photo?
These charts are the same tables the app matches against. Screw Identifier does the measuring for you: photograph the fastener next to a ruler, or measure it on the screen against a calibrated on-screen caliper.
Get the app