How to Use This Tool
Follow these simple steps to calculate your pottery firing shrinkage:
- Enter the pre-firing dimension of your clay piece (length, width, or height) in the input field.
- Select the unit of measurement you used from the dropdown (Inches, Centimeters, or Millimeters).
- Choose your clay type from the dropdown. If using a custom clay blend, select "Custom" and enter the known shrinkage percentage for your clay.
- Select the firing temperature you plan to use (Low, Mid, or High Fire).
- Click the "Calculate Shrinkage" button to see your results.
- Use the "Reset" button to clear all inputs and start over, or click "Copy Results" to save the breakdown to your clipboard.
Formula and Logic
This calculator uses standard linear shrinkage calculations used by pottery studios and hobbyists:
Post-Firing Dimension = Pre-Firing Dimension × (1 - (Total Shrinkage Percentage / 100))
Total Shrinkage Percentage = Clay Type Base Shrinkage + Firing Temperature Adjustment
The base shrinkage rates for each clay type are industry-standard averages:
- Earthenware: 7% (low-fire clay, porous after firing)
- Stoneware: 11% (mid- to high-fire, durable dinnerware clay)
- Porcelain: 13.5% (high-fire, translucent white clay)
- Raku: 9% (low- to mid-fire, used for decorative raku firing)
Firing temperature adjustments account for additional shrinkage at higher kiln temperatures:
- Low Fire (Cone 06-04): 0% adjustment
- Mid Fire (Cone 5-6): +0.5% shrinkage
- High Fire (Cone 10): +1% shrinkage
Practical Notes
Pottery shrinkage can vary slightly based on factors not accounted for in this calculator. Keep these real-world tips in mind:
- Always test fire a small sample of your clay before working on large projects to confirm actual shrinkage rates.
- Thick-walled pieces may shrink slightly less than thin-walled pieces due to slower drying and firing.
- Clay that is not fully dry before firing may crack or shrink more than expected—ensure pieces are bone dry first.
- If mixing clay types, use the higher shrinkage rate of the blend to avoid undersized pieces.
- Shrinkage rates are linear—volume shrinkage will be roughly three times the linear rate, but this calculator focuses on linear dimensions for planning sizing.
Why This Tool Is Useful
Home potters and hobbyists often struggle with sizing mistakes after firing, leading to pieces that don't fit lids, stack properly, or match planned dimensions. This tool eliminates guesswork by giving accurate pre-firing size targets. It saves time and clay by reducing failed pieces, and helps plan projects like matching dinnerware sets, custom planters, or decorative tiles. Even small shrinkage miscalculations can ruin hours of work, so using this tool upfront protects your time and materials.
Frequently Asked Questions
Does this calculator account for glaze shrinkage?
No, this tool only calculates clay body shrinkage during bisque and glaze firing. Glazes add a separate layer that may shrink slightly, but most glaze shrinkage is minimal compared to clay body shrinkage. Test your glaze on a sample tile to check for fit.
Can I use this for volume measurements?
This calculator is designed for linear dimensions (length, width, height). For volume, calculate the linear shrinkage first, then apply it to each dimension of your piece. For example, a cubic piece with 10% linear shrinkage will have ~27% volume shrinkage.
What if my clay doesn't match the preset types?
Select "Custom" from the clay type dropdown and enter the shrinkage rate provided by your clay manufacturer. Most clay suppliers list shrinkage rates on their packaging or website. If unavailable, test a 10cm sample strip to measure actual shrinkage.
Additional Guidance
For best results, always measure your pre-firing pieces after they are fully dry but before loading the kiln. Keep a log of your clay types, firing temperatures, and actual shrinkage rates to refine your calculations over time. If you're new to pottery, start with preset clay types and test samples before committing to large projects. Remember that kiln conditions (like uneven heating) can cause slight variations, so leave a small margin of error for critical sizing.