Clay Shrinkage Calculator
Plan clay shrinkage before you build: predict the fired size from greenware dimensions, reverse-plan the greenware size you must throw to hit a target final size, compute the shrinkage rate from two measurements, and compound per-stage shrinkage (wet → bone-dry → bisque → glaze-fired). For example, a 25 cm greenware vase in a clay that shrinks 12% fires to 22 cm — or in reverse, a 20 cm fired plate from a clay that shrinks 11.5% must be built at 22.5989 cm.
🔒 100% private — everything runs in your browser, nothing is uploadedShrinkage rate from two measurements
Measure the same dimension twice — for example a test bar marked 10 in wet that measures 9 in fired has shrunk 10%.
Predict fired size from greenware
Enter your greenware dimensions and the shrinkage % — the tool computes the fired size for each dimension: final = green × (1 − shrinkage).
Reverse: greenware size to build
You know the fired size you want (a lid that must fit, a plate that must be exactly 20 cm). Enter it with your shrinkage % and get the greenware size to throw: green = target ÷ (1 − shrinkage).
Multi-stage shrinkage breakdown
Shrinkage happens in stages and compounds — it is NOT the sum of the stage percentages. Enter the wet dimension and each stage's rate.
Frequently asked questions
What is clay shrinkage and why does it happen?
Clay shrinks in two main phases. First, water evaporates as the piece dries from wet to bone-dry, pulling particles closer together. Then, firing burns off chemically bound water and fuses particles together (sintering and vitrification), making the body denser and smaller. That is why a piece can lose over 10% of its wet size by the time it comes out of the glaze firing.
How much does clay shrink? Typical shrinkage rates by clay type
As illustrative general ranges only (always check your clay manufacturer's spec sheet): earthenware about 5-8%, stoneware about 10-12%, porcelain about 12-15%, and raku about 4-7%. The real number depends on the specific clay body, the firing temperature (cone), grog content, and how the piece was formed. For exact work, fire a marked test bar and use the Rate tab above.
Why is vertical shrinkage different from horizontal shrinkage?
Gravity and how the piece is supported: tall walls slump slightly under their own weight while drying and firing, and wheel-thrown clay has particles aligned by the throwing process, which makes one direction shrink differently from the other. Pieces also settle into the kiln shelf. For precision fits, measure your own vertical and horizontal rates separately with test bars, then run the Reverse mode once per direction.
Can I just add the stage shrinkages together, like 6% + 4% + 3% = 13%?
No - shrinkage compounds, so adding stage percentages overstates the total. Each stage shrinks a dimension that already shrank in the previous stage. For example, 6% drying + 4% bisque + 3% glaze-fire gives 12.4672% total, not 13%. A 20 cm wet piece would be 18.8 cm bone-dry, 18.048 cm bisque, and 17.5066 cm fired. Use the Stage Breakdown tab to compound them correctly.
How do I measure my clay's actual shrinkage rate?
Make a test bar: roll a small slab, mark exactly 10 cm (or 4 in) between two lines while it is wet, then let it dry and fire it with your normal schedule. Measure the fired distance and compute (wet - fired) / wet x 100. For example, 10 cm wet and 9 cm fired means 10% shrinkage. For the most accurate planning, mark and measure after each stage: wet to bone-dry, bone-dry to bisque, and bisque to glaze-fired.
What shrinkage percentage should I use to make a lid fit a jar?
Use your own measured rate for your clay and your kiln schedule - not a generic table. Generic ranges can be off by several percent, and a 2% error on a 12 cm lid is about 2.4 mm, enough to ruin the fit. In Reverse mode, enter the target fired dimension and your measured rate to get the greenware size to build. Fire a test lid and test jar together first.
Why did my piece crack during drying or firing?
Cracking is usually caused by uneven shrinkage: one part of the piece dries or heats faster than another, for example a thin rim versus a thick base, or the bottom versus the top. Dry slowly and evenly (loosely cover thin parts), avoid drafts and direct heat, and keep wall thickness as consistent as you can. This tool assumes uniform shrinkage - if a piece cracks, the cause is the process, not the math.
Should I use different shrinkage rates for bisque and glaze firing?
Yes - measure them separately. Drying (wet to bone-dry) is mostly free-water loss; bisque firing drives off chemically bound water and pre-sinters the body; the glaze firing produces the most vitrification and the biggest density change. The Stage Breakdown tab lets you enter a separate percentage for each stage and compounds them into the correct total automatically.
Free tool by NRTechStudio