Throwing Weight Calculator
How much clay to centre for a form, and why a pot twice the size takes under four times the clay rather than eight.
This is wet clay off the wheel, before trimming — a foot ring comes off again and the weight you centre is more than the weight that survives. Wet density varies by body and by how much water is in it, so weigh a ball you know and adjust the figure. Nothing is uploaded.
A pot twice the size does not take eight times the clay
The instinct is that doubling a pot means eight times the material, because volume goes with the cube. That would be right if the walls doubled too — and they do not. A potter throwing a larger form keeps the wall about the same thickness. So the clay is a shell, and a shell follows the surface: doubling this mug takes it from 411 g to 1545 g — 3.75 times, not 8.
| Scale | Walls held constant | If the walls scaled too | Against the cube | Against the square |
|---|---|---|---|---|
| 0.5× | 116 g (0.28×) | 51 g (0.13×) | 226% | 113% |
| 1× | 411 g (1.00×) | 411 g (1.00×) | 100% | 100% |
| 1.5× | 888 g (2.16×) | 1389 g (3.38×) | 64% | 96% |
| 2× | 1545 g (3.75×) | 3292 g (8.00×) | 47% | 94% |
| 3× | 3402 g (8.27×) | 11109 g (27.00×) | 31% | 92% |
Scaling the walls as well gives exactly the cube — 8.00 at double, 27.00 at triple — which is the check that the two cases are modelled honestly rather than fudged. Held constant, the figure sits at 92% to 96% of the square across this range: roughly the square is the useful summary, and exactly the square is not true. The wall's own thickness contributes a term that does not grow with the pot, which is what keeps it just underneath.
The base is a bigger share than it looks
On this mug the base is 29% of the clay, because it is a
solid disc across the full outside diameter while the wall is only a thin ring. That share
rises as a form gets shorter and wider — on a plate almost all of the clay is base, which is
why plates take far more than their height suggests and why trimming one removes so much.
It also explains a practical thing about centring weights: the difference between a
0.6 cm wall and a 1 cm wall on the same form is real but modest, while
the difference between a 0.9 cm base and a 2 cm base is large. Thick bases are
where the wasted clay usually is, and they are also what cracks in the drying.
How to use
- Enter the interior diameter and height.
- Add your usual wall and base thickness.
- Read the clay to centre, split into wall and base.
- Check what a larger version would take.
Frequently asked questions
Does a pot twice the size need eight times the clay?
No, closer to four. Volume goes with the cube only if the walls thicken too, and they do not — a larger form is thrown at about the same wall thickness. The clay is a shell, and a shell follows the surface. Doubling the mug on this page takes it from 411 g to 1,545 g, which is 3.75 times.
So does clay go with the square?
Roughly, and not exactly. The wall own thickness contributes a term that does not grow with the pot, so the figure sits at 92% to 96% of the square across ordinary sizes and drifts further under as the form gets larger. Roughly the square is the useful summary; exactly the square is not true.
Why does the base matter so much?
Because it is a solid disc across the full outside diameter while the wall is only a thin ring. On a mug it is around a fifth of the clay; on a plate almost all of it. That is why plates take far more clay than their height suggests, and why trimming one removes so much.
Is this the weight I centre or the weight I end up with?
The weight you centre, before trimming. A foot ring comes off again, and anything you lose to the slurry bucket is on top of that. Treat it as a starting ball rather than a finished weight.
What clay density should I use?
Around 2.0 g per cubic centimetre for wet throwing clay is a working figure, but it varies by body and by how much water is in it. Weigh a ball you have already thrown something from and adjust — that calibrates the tool to your clay in one step.
Why do my pots come out heavier than this?
Usually the base. A base left at twice the wall thickness is easy to do and hard to see, and it is where most of the extra weight in a beginner pot lives. It is also what cracks in the drying, so the correction is worth making twice over.
Can I use this for a bowl or a plate?
It models a straight-sided form, so a bowl with a strong flare will take somewhat more than it says. The scaling relationship still holds — that part is about the shell, not the profile.
Does this send anything anywhere?
No. Every figure is computed in your browser, and nothing is uploaded or stored.
🔒 This tool runs entirely in your browser. Nothing you enter is uploaded, logged, or stored.