Calipers measure the one dimension in pottery that has to be right rather than approximately right: the fit between a lid and the gallery it sits in. Everything else on a thrown pot can be judged by eye, and a lid cannot, because clay shrinks and a millimetre out is a lid that rattles or does not go on.
Why a lid needs measuring
The gallery is thrown first and the lid afterwards, often on a different day, and both shrink as they dry and again in the firing. Calipers transfer the measurement from one to the other so that the two shrink by the same proportion from the same starting relationship. Judging it by eye works occasionally and fails often, which is why every production potter owns a pair.
Types, and which to buy
Simple double-ended calipers transfer an inside measurement to an outside one directly and cost very little. Sprung calipers hold their setting more firmly and are quicker in use, and are easier to knock out of true. For a hobby potter the simple wooden or plastic double-ended pair does everything needed and survives being dropped.
Using them well
Measure the gallery while the pot is still on the wheel and note the setting, then throw the lid to the same figure. Measure across the widest points, keep the calipers square to the pot, and remember to allow for the flange thickness on a lid that sits inside rather than over. Write the setting down if the lid is being thrown later.
Both spellings, deliberately
Callipers with two Ls is standard British spelling and calipers with one is American, and both are searched heavily in the UK because tools, videos and packaging come from both places. This site carries both rather than choosing, in the same way it carries modelling and modeling, because a buyer types what they saw.
Questions people ask about calipers
Do I need calipers for pottery?
For lidded pots, yes. It is the one measurement that has to be accurate because clay shrinks and a lid either fits or does not.
Calipers or callipers?
The same tool. Callipers is the usual British spelling; both are widely searched in the UK.
Which type should I buy?
Simple double-ended calipers do everything a hobby potter needs, cost very little and survive being dropped.