How do I calculate how much lye I need for soap?

Multiply each oil weight by its SAP value (saponification value) to get the lye needed for that oil. Add all results together to get total NaOH required. Reduce by your superfat percentage. For liquid soap, use KOH instead — divide the NaOH amount by 0.798 to convert, then account for KOH purity.

What is superfat in soap making?

Superfat (or lye discount) is the percentage of oils left unsaponified in your soap — these free oils make the soap more moisturising and provide a safety margin. A typical superfat is 5–8% for bar soap. Higher superfat = more moisturising but shorter shelf life. 0% superfat is used in cleaning soap.

What is the difference between NaOH and KOH for soap?

NaOH (sodium hydroxide) makes hard bar soap. KOH (potassium hydroxide) makes soft/liquid soap. Both work by saponifying oils, but KOH produces a more fluid soap. For liquid soap, KOH at 90% purity is most common. For bar soap, use NaOH.

How much water do I use in soap making?

A common starting point is 38% water as a percentage of oil weight (water:oil ratio of 38:100). A 33% lye concentration is another popular approach. Less water = faster unmoulding, harder bar, but more risk of soda ash or cracking. More water = softer bar, longer cure time.

What oils are best for cold process soap?

Common soap making oils: Coconut oil (cleansing, lather), Palm oil (hardness), Olive oil (conditioning, mildness), Castor oil (boost lather, up to 5–10%), Sweet almond oil (skin feel), Shea butter (conditioning), Cocoa butter (hardness). Most recipes blend 2–4 oils for balanced properties.

Soap Making Calculator — Lye & Water Amounts

⚠️ Safety first: Lye (NaOH/KOH) is caustic and dangerous. Always add lye to water (not water to lye), wear gloves and eye protection, and work in a ventilated area. Never use aluminium containers.

Units

Soap Type

Quick Recipe Templates

Oil Recipe

Oil / Butter	Weight (g)	SAP

Lye Settings

Superfat % (Lye Discount)

5% = standard. Higher = more moisturising. 0% = cleaning soap.

KOH Purity %

Most KOH is 90% pure. NaOH is typically 99%+.

Water Method

Water % of Oil (38% typical)

NaOH (Lye) Needed

0 g

Total oil: 0g · Superfat: 5%

NaOH

lye needed

Water

distilled water

Total Oils

combined weight

📋 Batch Recipe Summary

Total Oil Weight0g

NaOH (Lye)0g

Distilled Water0g

Superfat5%

Lye Concentration—

Estimated Batch Size0g

📌 Cure time: Cold process soap needs 4–6 weeks to cure. Hot process soap can be used sooner (1–2 weeks) but benefits from curing. Always test pH before use — safe soap is between pH 7–10.

▶ 💰 Cost Estimator — how much does each bar of soap cost to make?

Avg Oil Price per kg

Average across your oil blend

Lye (NaOH/KOH) Price per kg

Fragrance / Essential Oil (per batch)

Colorant + Additives (per batch)

Packaging + Label (per bar)

Number of Bars from Batch

How many bars you cut from this batch

🛒 Selling soap online? Use the Cost Estimator above to calculate your material cost, then check your real profit on Etsy, eBay, or Shopee with our free fee calculators.

Frequently Asked Questions

What is SAP value? +

SAP (saponification value) is the amount of NaOH (in grams) needed to saponify 1 gram of a specific oil. Each oil has a unique SAP value. This calculator uses standard SAP values for each oil to compute the exact lye needed.

What is superfat in soap? +

Superfat is the percentage of oils left unsaponified — they make the soap more moisturising. 5% is standard for bar soap. Calculate by reducing the lye amount by 5%, leaving extra oil unreacted.

How much water do I add? +

A common ratio is 38% water relative to oil weight. Example: 500g oils × 38% = 190g water. Less water speeds hardening; more water gives a softer bar needing longer cure time.

Can I substitute one oil for another? +

Yes — but each oil has a different SAP value so the lye amount will change. Always recalculate when changing oils. This calculator handles multiple oils at once.

Soap Making Lye Calculator

Frequently Asked Questions

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