How a shrink ruler works
Each unit on a shrink ruler is 1 ÷ (1 − shrinkage) real units long. At 12%, 1 ÷ 0.88 = 1.136. Measure a wet piece with it and you read its fired size.
- Test bar: shrinkage = (wet − fired) ÷ wet. 100 mm firing to 88 mm is 12%.
- Drying and firing: total = 1 − (1 − drying) × (1 − firing).
- Throw-to size = fired size ÷ (1 − shrinkage); capacity = fired capacity ÷ (1 − shrinkage)³.
Example: a 12 oz mug from 12% clay
You want a mug 4 in tall that holds 12 oz after firing. 4 ÷ 0.88 = 4.55, so throw it 4 9/16 in tall. 12 ÷ 0.88³ = 17.6, so it should hold about 18 oz when thrown. Open this example.
Questions
How do I measure my clay's shrinkage?
Roll clay about 1 cm (3/8 in) thick and cut a bar about 11 by 2.5 cm. While it's wet, press two marks exactly 100 mm apart and leave it until dry. After firing, measure between the marks with the same ruler or calipers and type both under Test bar. Marks far apart make a small reading error matter less.
Can I add drying and firing shrinkage together?
No. Firing shrinkage is measured from the dry length, which is already smaller. Digitalfire's example: 6.25% drying and 6.66% firing make 12.5% total, not 12.9%. Drying + firing combines them for you.
How do I use a shrink ruler?
Measure the freshly made, wet piece with it: the number you read is its size after the last firing. For a mug 4 in tall when fired, throw until the shrink ruler reads 4. Measure early, as some shrinkage happens before leather hard.
How do I know the printout is the right size?
The square at the top left of each sheet must measure exactly 1 in (or 25 mm). If not, open More settings in the print window, set Scale to 100% or Actual size (not Fit to page or Fit to printable area) and print again. From a phone, save a PDF and print it at 100%.
Why does my mug hold so much less after firing?
Capacity shrinks in all three directions. At 12%, 0.88 × 0.88 × 0.88 is about 0.68, so a mug that holds 16 oz when thrown holds about 11 oz after firing.
Does it work for metal clay?
Yes. Type the shrinkage from your clay's instructions, or measure a test bar. Figures differ by brand and firing (Art Clay World gives 8 to 10% for Art Clay Silver). For rings, follow the maker's sizing advice: this tool doesn't predict ring sizes.
Sources: Digitalfire Reference Library, Drying Shrinkage, Firing Shrinkage and the Shrinkage and Absorption Test; Microsoft, Print in Microsoft Edge; Art Clay World, Frequently Asked Questions.