Quick Answer: Total pottery shrinkage is typically 10-16% from wet to fired. Calculate final size: Starting Size × (1 - Shrinkage Rate). For 12% shrinkage: 6" wet becomes 5.28" fired (6 × 0.88 = 5.28).
Understanding the Three Shrinkage Stages
Clay shrinks at different rates during each stage of the pottery process
Wet to Dry5-8%
Clay loses moisture as it air dries before first firing
Formula:
Dry Size = Wet Size × 0.93 (for 7% average shrinkage)
Affecting Factors:
Clay composition
Humidity
Drying speed
Wall thickness
Bisque Firing8-12%
Chemical water evaporates and clay particles bond permanently
Formula:
Bisque Size = Dry Size × 0.90 (for 10% average shrinkage)
Affecting Factors:
Clay type
Firing temperature
Firing speed
Clay maturity
Glaze Firing2-5%
Final shrinkage as glaze melts and clay fully vitrifies
Formula:
Final Size = Bisque Size × 0.97 (for 3% average shrinkage)
Affecting Factors:
Final temperature
Clay maturity
Glaze type
Firing atmosphere
Shrinkage Rates by Clay Type
| Clay Type | Total Shrinkage | Firing Temperature | Notes |
|---|---|---|---|
| Earthenware | 12-15% | 1800-2100°F | Higher shrinkage, lower firing temperature |
| Stoneware | 10-13% | 2100-2300°F | Moderate shrinkage, versatile clay body |
| Porcelain | 13-16% | 2200-2400°F | Higher shrinkage, very high firing temperature |
| Raku | 8-12% | 1800-2000°F | Lower shrinkage, specialized firing process |
Step-by-Step Calculation Method
1Determine Your Clay Type
Identify your clay body and its documented shrinkage rate from supplier specifications.
Example:
Stoneware clay with 12% total shrinkage rate
2Measure Starting Dimensions
Measure your piece while wet/plastic, using consistent units throughout.
Example:
Bowl measures 6 inches diameter when wet
3Apply Total Shrinkage Rate
Calculate final size using: Final Size = Starting Size × (1 - Shrinkage Rate)
Example:
6 inches × (1 - 0.12) = 6 × 0.88 = 5.28 inches final
4Plan for Target Size
Work backwards: Starting Size = Target Size ÷ (1 - Shrinkage Rate)
Example:
For 5" final bowl: 5 ÷ 0.88 = 5.68 inches starting size needed
How to Test Your Specific Clay's Shrinkage Rate
- Make a test tile exactly 10cm × 10cm when wet
- Mark the tile clearly and let it dry completely
- Measure again after drying (calculate wet-to-dry shrinkage)
- Bisque fire the tile and measure again
- Glaze fire and take final measurements
- Calculate total shrinkage: ((10 - Final) ÷ 10) × 100 = % shrinkage
Pro Tip: Test each new clay body and firing schedule. Even the same clay from different batches can vary by 1-2%.