Pottery Calculator
Calculate clay shrinkage, convert between wet and finished sizes, plan plaster molds, and estimate clay weight for your pottery projects.
Clay Shrinkage Percentage
Use a test bar to measure how much your clay body shrinks from wet to fired. You can work in mm, cm, or inches – just keep units consistent.
How the pottery calculator works
Clay shrinks as it dries and again during firing. To hit accurate finished sizes, you need to know your clay body’s shrinkage and apply it consistently. This pottery calculator combines three tools:
- Shrinkage % – find the exact shrinkage of your clay from a test bar.
- Size converter – convert between wet and finished sizes using that shrinkage.
- Clay batch estimator – approximate how much clay you need for a run of similar pieces.
1. Clay shrinkage formula
Linear shrinkage %
\[ \text{Shrinkage \%} = \frac{L_\text{wet} - L_\text{fired}}{L_\text{wet}} \times 100 \]
where:
- \(L_\text{wet}\) = length of the test bar when freshly made
- \(L_\text{fired}\) = length of the same bar after firing
The calculator uses this formula in the Shrinkage % tab. You can use any unit (mm, cm, inches) as long as both measurements use the same unit.
2. Wet size vs. finished size
Once you know your shrinkage percentage \(s\), you can move between wet and finished sizes with these formulas:
From wet to finished size
\[ L_\text{finished} = L_\text{wet} \times (1 - \frac{s}{100}) \]
From finished to wet size
\[ L_\text{wet} = \frac{L_\text{finished}}{1 - \frac{s}{100}} \]
The Size Converter tab implements both directions. Enter shrinkage % and either the wet or finished size; the other is calculated automatically.
3. Estimating clay weight for a batch
For the batch estimator, we approximate your piece as a hollow cylinder (like a mug or bowl) with uniform wall thickness. The volume of clay is roughly:
Clay volume (simplified)
\[ V_\text{clay} \approx \pi \cdot r_\text{outer}^2 \cdot h - \pi \cdot (r_\text{outer} - t)^2 \cdot h \]
which simplifies to:
\[ V_\text{clay} \approx \pi \cdot h \cdot (2 r_\text{outer} t - t^2) \]
where:
- \(r_\text{outer}\) = outer radius
- \(h\) = height
- \(t\) = wall thickness
The calculator converts your finished dimensions to wet dimensions using the shrinkage %, then estimates volume and multiplies by clay density to get weight. Finally, it multiplies by the number of pieces to give a batch total.
Practical tips for accurate pottery calculations
Run a proper shrinkage test
- Roll a slab and cut a straight bar at least 10–15 cm (4–6 in) long.
- Mark a precise gauge length (e.g. 100 mm or 4 in) with a needle tool and ruler.
- Dry and fire the bar to your usual firing schedule.
- Measure the fired gauge length with calipers or a ruler.
- Enter wet and fired lengths into the Shrinkage % tab.
Typical shrinkage ranges
- Earthenware: ~5–10%
- Stoneware: ~10–15%
- Porcelain: ~14–18% (sometimes more)
Always test your own clay body and firing schedule – even small changes can affect shrinkage.
Using the calculator for plaster molds & slip casting
For slip-cast ware, the same formulas apply. Use the shrinkage % for your casting slip and calculate the required mold cavity size from your desired finished size. This is especially helpful for lids, sets, and parts that must fit together.
FAQ
Can I mix metric and imperial units?
You can use either system, but within a single calculation you must keep units consistent. For example, if your wet length is in mm, the fired length must also be in mm. The percentage result is unitless.
Is the clay batch estimate exact?
No – it’s an engineering-style approximation. Real pots have curves, handles, and trimming. The estimate is usually close enough for planning clay purchases and wedging batches, but you should still keep a small safety margin.
Does shrinkage change with firing temperature?
Yes. Higher firing temperatures generally increase shrinkage up to a point. If you change clay body, firing temperature, or firing schedule significantly, run a new shrinkage test and update the calculator inputs.