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

Shape of your soap mould

Length

Internal length of the mould

Width

Internal width of the mould

Height

Internal height of the mould

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.