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Pottery Trimming Calculator — Shrinkage & Clay Weight

Estimate wet throw size, trimming allowance, and clay weight for any finished pottery dimension and clay body

Throw Size

5.8" tall

Wet Dia.

4.7"

Clay

38.2 oz

Shrinkage

14%

Throwing Guide

5.8" \u00D7 4.7"
wet throw dimensions (height \u00D7 diameter)
Clay Weight
38.2 oz
1,083 g
Total Shrinkage
14%
clay + firing

Trimming Allowances

Throw Wall Thickness0.380"
Throw Bottom Thickness0.440"
Trimming Allowance0.130"
Final Wall Thickness0.25"

Trimming Tips

1. Trim at leather-hard stage—clay should be firm but still carvable
2. Tap the bottom gently; a hollow sound means the wall is too thin
3. Leave extra thickness on the foot ring for stability and weight distribution
4. Porcelain shrinks the most—throw 15–19% larger than your target
5. Use calipers to check wall thickness before and during trimming

Example Calculations

1Mug: 4" H × 3.5" D (Stoneware, Cone 6)

Inputs

Finished Height4"
Finished Diameter3.5"
Wall Thickness1/4"
ClayStoneware
FiringCone 6

Result

Wet Throw Size4.65" × 4.07"
Clay Weight13.2 oz
Total Shrinkage14%

Shrinkage = 12% + 2% (cone 6) = 14%. Wet height = 4 / (1 - 0.14) = 4.65". Wet diameter = 3.5 / 0.86 = 4.07".

2Bowl: 3" H × 8" D (Porcelain, Cone 10)

Inputs

Finished Height3"
Finished Diameter8"
Wall Thickness3/16"
ClayPorcelain
FiringCone 10

Result

Wet Throw Size3.70" × 9.88"
Clay Weight23.8 oz
Total Shrinkage19%

Shrinkage = 15% + 4% (cone 10) = 19%. Wet height = 3 / 0.81 = 3.70". Wet diameter = 8 / 0.81 = 9.88".

Frequently Asked Questions

Q

How much does clay shrink when fired?

Shrinkage depends on clay body and firing temperature. Stoneware shrinks 12–14%, porcelain 15–19%, and earthenware 8–10%. Higher firing temperatures add 2–4% more shrinkage. A 5" tall finished piece in stoneware at cone 6 needs to be thrown at 5.81" wet.

  • Stoneware: 12% base + 2% at cone 6 = 14% total
  • Porcelain: 15% base + 4% at cone 10 = 19% total
  • Earthenware: 8% base + 0% at cone 06 = 8% total
  • B-Mix: 13% base + 2% at cone 6 = 15% total
Clay BodyBase ShrinkageCone 6Cone 10
Stoneware12%14%16%
Porcelain15%17%19%
Earthenware8%10%12%
B-Mix13%15%17%
Q

How thick should I throw pottery walls?

Throw walls 50% thicker than your desired finished thickness to allow for trimming. For a 1/4" finished wall, throw at 3/8". The bottom should be 75% thicker than the wall to provide enough material for foot ring trimming.

  • Walls: throw 50% thicker than desired finished thickness
  • Bottom: throw 75% thicker for foot ring trimming
  • 1/4" finished wall = 3/8" thrown wall
  • Use calipers to check thickness before and during trimming
Q

How much clay do I need for a mug?

A standard 4" tall × 3.5" diameter mug with 1/4" walls needs about 12–15 oz of wedged clay in stoneware. Porcelain mugs need 10–15% more due to higher shrinkage requiring larger wet dimensions.

  • Small mug (3" × 3"): 8–10 oz clay
  • Standard mug (4" × 3.5"): 12–15 oz clay
  • Large mug (5" × 4"): 18–22 oz clay
  • Bowl (3" × 6"): 20–25 oz clay
Q

When should I trim pottery on the wheel?

Trim at leather-hard stage, typically 12–24 hours after throwing depending on humidity and clay thickness. The clay should be firm enough to hold its shape when inverted but soft enough to carve cleanly with a trimming tool.

  • Leather-hard: firm but carvable, 12–24 hours after throwing
  • Too wet: clay deforms under its own weight when inverted
  • Too dry: trimming creates dust and chattering marks
  • Cover loosely with plastic to slow drying for even leather-hard stage

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Last Updated: Mar 19, 2026

This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

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