Soap Batch Scaler Calculator

How soap recipe scaling works:

Soap recipes are proportional. Every ingredient must scale by the same factor. If you double the oils, you must double the lye and water too. Getting this wrong means your soap will be lye-heavy (caustic and unsafe) or oil-heavy (soft and rancid).

Scaling formula:

Scale factor = New batch size / Original batch size

New ingredient amount = Original amount × Scale factor

Important: Scale factor applies to ALL ingredients: oils, lye, water, fragrance, colorant, and additives.

Batch size means oil weight. That is the convention every lye calculator uses, and it is the only figure that stays stable, because water amounts vary with how concentrated you like your lye solution. When a recipe is called a 2-pound batch, that is 2 pounds of oils, not 2 pounds of finished soap.

Worked example:

Original recipe, 1 lb (16 oz) of oils at 5% superfat:

• Olive oil: 10 oz
• Coconut oil: 6 oz
• Lye (sodium hydroxide, NaOH): 2.3 oz
• Water: 6 oz

You want 2.5 lbs of oils. Scale factor = 2.5 / 1 = 2.5×

• Olive oil: 10 × 2.5 = 25 oz
• Coconut oil: 6 × 2.5 = 15 oz
• Lye: 2.3 × 2.5 = 5.8 oz
• Water: 6 × 2.5 = 15 oz

Those 2.3 oz are not a round number anyone guesses. Olive oil takes about 0.134 g of sodium hydroxide per gram of oil and coconut takes 0.183, which is 2.44 oz between them, less 5% held back as superfat. If a recipe hands you a lye figure that looks generous against that arithmetic, do not scale it. Re-run it through a lye calculator first, because scaling multiplies the error along with everything else.

Mold capacity check:

Before scaling up, check your mold can hold it. Fresh batter is roughly the density of water, so a standard 3.5 x 9 inch loaf mold takes about 18 oz per inch of depth (31.5 cubic inches, near enough 1 g per mL). Batter also weighs about 1.45 times the oil figure once the lye solution is in, so 2.5 lbs of oils makes around 58 oz of batter and fills a little over 3 inches of that mold.

Tips for scaling:

• Always weigh in the same unit (oz or grams): never mix
• Small batches (under 1 lb total oils) are harder to work with because trace happens fast
• Large batches (over 5 lbs) generate more heat: use lower temperatures to avoid gel cracking
• Fragrance load stays proportional: if the original uses 3% fragrance by oil weight, the scaled version uses the same 3%
• When scaling down, round lye DOWN (safer to have slightly more oil than too much lye)
• When scaling up, consider splitting into two batches if you are new: large batches leave no room for error

Superfat does not change: A 5% superfat recipe stays 5% regardless of batch size, because the lye amount was already calculated to leave 5% of the oil unsaponified.