Calipers

What it is

Throwing a set of six identical mugs is a memory problem. The eye is a poor judge of a 2 mm difference in rim diameter, and a set that is slightly off looks wrong in a way that is hard to name. Calipers replace the eye with a physical reference.

The classic pottery caliper is a hinged arm with two curved jaws: set it against a finished piece, then hold it against the next pot as you throw and stop when the clay meets the jaws. Inside calipers measure the opening of a form; outside calipers measure wall diameter. Modern versions add a locking screw or a digital readout.

Calipers also measure wall thickness through a small hole or across a rim, which is how you confirm a pot is not heavier on one side. For production work, a simple caliper plus a height stick turns throwing a set from guesswork into a repeatable process.

How to use it well

• Throw one reference pot, let it firm, then set the calipers from it rather than from a number.
• Measure at the widest point of the body, not the rim, since rims move most during throwing.
• Check wall thickness at four points around a rim to catch off-centre walls early.
• Note the setting in your log alongside the clay weight — that pair is what makes a form repeatable.

Safety notes

• Pointed jaws are sharp — store closed and locked.
• Digital calipers dislike kiln rooms; keep them away from damp and dust.

Compatible materials

Clay bodies

• All bodies

Glazes & surface

• Not applicable — measuring tool

Where it sits in the process

Forming on the wheel, from centring to shaping.

Throwing  Wheel & Shaping  Entry tier

Browse every tool in this stage from the Wheel & Shaping category.