About the Soap Lye Calculator
This soap lye calculator works out how much sodium hydroxide (NaOH, for bar soap) or potassium hydroxide (KOH, for liquid soap) your oils need to turn into soap. Choose up to four oils, enter their weights, set a superfat and a water amount, and it returns the lye and water weights along with the total batch weight and the lye concentration.
It is meant for cold process and hot process soapmakers designing or resizing a recipe. Each oil has its own saponification (SAP) value — the milligrams of KOH needed to saponify one gram of that oil. The values here are the midpoints of published ranges: Codex Alimentarius figures from the FAO for food oils, and a soap-supplier chart for castor, shea, butters, lard and tallow. Because real oils vary from batch to batch, a superfat of 5% or more is the usual safety margin.
Use any one weight unit (grams or ounces) for every oil — lye and water come out in the same unit. Always run a recipe through a second lye calculator before making it, weigh with a digital scale, and wear goggles and gloves: sodium hydroxide causes severe skin and eye burns.
With the default inputs, the lye to weigh is 140.9. Change any value above to recalculate instantly.
How to use the soap lye calculator
- 1Choose bar soap (NaOH) or liquid soap (KOH) and enter the lye purity from the label.
- 2Pick up to four oils and enter each weight in the same unit (grams or ounces).
- 3Set a superfat — 5% is a common starting point for bar soap.
- 4Choose how to set the water: lye concentration, water-to-lye ratio or % of oils.
- 5Weigh the lye and water shown, and double-check the recipe in a second calculator before making soap.
Formula and method
A saponification (SAP) value is the milligrams of potassium hydroxide needed to completely saponify one gram of an oil. Dividing by 1,000 gives grams of KOH per gram of oil; dividing by 1,402.5 converts to sodium hydroxide, because NaOH (40.00 g/mol) is lighter than KOH (56.11 g/mol): 1,000 × 56.11 ÷ 40.00 ≈ 1,403, and soapmakers conventionally round the divisor to 1,402.5. Each oil’s weight is multiplied by its factor and the results are summed.
The superfat (lye discount) reduces the lye so a small share of oil stays unsaponified, which protects against measuring errors and SAP-value variation and makes a milder bar. Dividing by purity adds lye when the chemical is not 100% pure. Water is set by lye concentration (lye ÷ lye + water), by a water-to-lye ratio, or as a percentage of the oil weight.
- SAP
- Saponification value, mg KOH per g of oil
- Superfat%
- Share of lye withheld so some oil stays free
- Purity
- Lye purity as a fraction (1.00 = 100%)
- C
- Lye concentration as a fraction
Worked examples
1,000 g bar soap: olive, coconut, shea, castor at 5% superfat
Olive needs 500 × 190 ÷ 1402.5 = 67.74 g NaOH, coconut 300 × 256.5 ÷ 1402.5 = 54.87 g, shea 150 × 180 ÷ 1402.5 = 19.25 g and castor 50 × 180.5 ÷ 1402.5 = 6.43 g — 148.29 g in total. A 5% superfat cuts this to 140.87 g, and a 33% lye concentration needs 140.87 × 0.67 ÷ 0.33 = 286.0 g water.
Pure olive (Castile) soap with water at 38% of oils
In ounces: 32 × 190 ÷ 1402.5 = 4.335 oz NaOH at 0% superfat; a 6% discount gives 4.075 oz. Water at 38% of 32 oz is 12.16 oz, a fairly weak 25.1% lye solution, which suits slow-tracing Castile.
Liquid soap with 90% pure KOH and a 2:1 water ratio
KOH needed is 400 × 0.2565 + 400 × 0.190 + 200 × 0.1805 = 214.7 g at 0% superfat. A 3% superfat gives 208.26 g, and dividing by 0.90 purity means weighing 231.40 g of flakes; twice that, 462.8 g, is the water.
Frequently asked questions
How do you calculate how much lye to use in soap?+
Multiply each oil’s weight by its NaOH saponification value (SAP ÷ 1402.5), add them up, then reduce the total by your superfat. For 500 g olive oil that is 500 × 0.1355 = 67.7 g of NaOH before superfat, or 64.4 g at 5%.
What superfat should I use for soap?+
Most cold process bar soap recipes use a 5–8% superfat, which leaves extra oil for mildness and a margin against measuring error. Facial or very gentle bars may go higher; liquid soap is usually 0–3% so it stays clear.
What is the difference between NaOH and KOH?+
Sodium hydroxide (NaOH) makes hard bar soap; potassium hydroxide (KOH) makes soft or liquid soap. KOH is heavier per mole, so the same oils need about 1.4 times as much KOH by weight as NaOH.
How much water do I need for cold process soap?+
A lye concentration of 28–35% is typical — for 33%, use about twice as much water as lye by weight. Less water (higher concentration) speeds trace and unmolding; more water slows trace but can cause soda ash or longer curing.
Are saponification values the same for every bottle of oil?+
No. SAP values are published as ranges because fatty-acid makeup varies with crop, region and processing; Codex lists olive oil at 184–196 mg KOH/g. That variation is why soapmakers always include a superfat.
Is lye dangerous to handle?+
Yes. Sodium hydroxide causes severe burns to skin and eyes. Wear goggles and gloves, work in a ventilated space, always add lye to water (not water to lye), and flush any splash with plenty of water immediately.