How do I calculate oil weight from mould volume?

A common rule of thumb is: total oil weight (g) = mould volume (ml) x 0.40. This accounts for the water and lye that make up the rest of the soap batter. More precise: oils fill about 38-42% of the final batter volume.

What is a standard soap bar size?

A typical UK handmade soap bar is about 8 x 5.5 x 2.5 cm, weighing 100-120g. A loaf mould that holds 1 litre will produce roughly 8-10 bars at standard thickness.

Should I fill the mould completely?

Leave 1-2 cm from the top to prevent overflow. Many recipes expand slightly during the gel phase. For a 1000 ml mould, calculate for about 900-950 ml of batter.

How do I measure an irregular mould?

Fill the mould with water and pour the water into a measuring jug. This gives the exact volume in ml. Dry the mould thoroughly before using it for soap.

CraftCalcs

Mould Volume Calculator — Soap Mould Sizing

Calculate soap mould volume and the total oil weight needed to fill it.

Mould Shape

Length

Width

Height

How We Calculate This

How the Calculation Works

Rectangle: L x W x H. Cylinder: pi x r² x H. Oval: pi x (L/2) x (W/2) x H.

Oil weight is estimated at 40% of the mould volume (the remaining 60% is water, lye, and air during trace).

Number of bars is calculated by dividing the total volume by 100 ml (approximate volume of a standard bar).

Frequently Asked Questions

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Last updated: 2025-02-20

All calculations are estimates. Always verify quantities before purchasing materials.