How to Use a Lye Calculator, Step by Step

To use a lye calculator, choose your lye type (NaOH for bars, KOH for liquid soap), enter each oil by weight, set a superfat (5% is standard) and a lye concentration (around 33%), then read off the lye and water amounts. The calculator multiplies each oil by its SAP value, totals it and holds back the superfat. This page walks through every field of our lye calculator using its default recipe, and shows how to check the result by hand.

Why a calculator, and not a rule of thumb

Every oil needs a different amount of lye to turn into soap. A kilo of coconut oil needs about a third more lye than a kilo of olive oil. Change one oil, or even one weight, and the lye changes. There’s no “X grams of lye per pound of oil” shortcut that works across recipes.

A calculator also protects you from arithmetic slips. You still need to understand what it’s doing, though, because a calculator will happily give you the right answer to the wrong question. If you type 500 in the coconut row when you meant 50, it won’t stop you.

Step 1: choose the lye type

The first choice is NaOH · bar soap or KOH · liquid soap.

• NaOH (sodium hydroxide) is for bars: cold process, hot process, most of the recipes on this site.
• KOH (potassium hydroxide) is for liquid and soft soap. When you pick it, the calculator multiplies every SAP value by 1.403 and divides by 0.90, because KOH is a heavier molecule and is usually sold about 90% pure.

Pick this first. Switching later recalculates everything, which is fine, but getting into the habit means you never weigh NaOH from a KOH result. The sodium vs potassium hydroxide page explains the difference.

Step 2: enter your oils by weight

Add each oil from the list and type its weight in grams. Enter the weight of the oils only, not the whole batch. Water, lye and fragrance are calculated or added separately.

As you type, each row shows:

• SAP: grams of lye needed to saponify one gram of that oil. See SAP values.
• %: that oil’s share of the total oils. Recipes usually describe formulas this way.
• Lye: how much lye that oil needs on its own, after superfat.

If a recipe gives only percentages, decide on a total oil weight and multiply. For 1,000 g of oils, a 45% olive oil line is 450 g. For 800 g, it’s 360 g.

Step 3: set the superfat

The superfat slider runs from 0 to 20%. It reduces the lye so that percentage of the oil stays unsaponified. 5% suits most bar soap. Salt bars and 100% coconut bars typically use 15–20%; liquid soap usually 0–3%.

Don’t raise the superfat by adding oil to the pot after calculating. Change the slider and let the lye drop. Our superfat page has a table of typical numbers by soap type.

Step 4: set the lye concentration

This slider (25–40%) sets how much water goes into the lye solution. Lye concentration is the lye’s share of the solution by weight. At 33%, lye is one third of the solution and water two thirds, about 2 parts water to 1 part lye.

• 30–33% is a forgiving range for most beginner cold process recipes.
• Higher (35–40%) means less water: faster unmolding and less soda ash, but faster trace and more heat.
• Lower (25–28%) gives more working time but softer, slower-to-unmold bars.

The calculator also shows the result as a water : lye ratio. Water discount explains all three ways recipes express water and how to convert between them.

Worked example: the calculator’s default recipe

Open the calculator and you’ll see this recipe already filled in, at 5% superfat and 33% concentration, NaOH:

Oil	Weight	SAP (NaOH)	Weight × SAP
Olive oil	450 g	0.135	60.75 g
Coconut oil (76°)	250 g	0.183	45.75 g
Shea butter	150 g	0.128	19.20 g
Avocado oil	100 g	0.133	13.30 g
Castor oil	50 g	0.128	6.40 g
Total	1,000 g		145.40 g

Now check the outputs:

1. Lye at 0% superfat: 145.40 g, the sum above.
2. Apply 5% superfat: 145.40 × 0.95 = 138.1 g NaOH.
3. Water at 33%: 138.13 ÷ 0.33 = 418.58 g of solution; minus the lye, 418.58 − 138.13 = 280.4 g water.
4. Water : lye: 280.4 ÷ 138.1 ≈ 2.03 : 1.
5. Batch weight: 1,000 + 138.1 + 280.4 ≈ 1,419 g, which the calculator rounds to about 10 bars of 140 g.

If your hand-check and the calculator agree, you’ve confirmed you entered the recipe correctly. If they disagree, look for a mistyped weight before anything else.

Reading the bar quality panel

Below the lye and water, the calculator shows five bar quality numbers. For the default recipe they come out at roughly:

Quality	Default recipe	Typical range shown
Hardness	36	29–54
Cleansing	17	12–22
Conditioning	59	44–69
Bubbly	21	14–46
Creamy	24	16–48

These are weighted averages of each oil’s typical fatty acid profile. They’re useful for comparing two versions of a recipe: swap some olive for more coconut and you’ll see hardness and cleansing rise while conditioning falls. A number outside the shaded range is a prompt to think, not an error. A castile soap scores low on hardness and cleansing by these measures and is still a well-loved bar; it just needs a long cure.

Saving and printing your recipe

Use Copy share link to save the whole recipe as a URL you can bookmark or send to someone checking your numbers. Print gives you a sheet to keep beside the scale. Weigh from the printout rather than switching between your phone and the scale with gloves on.

Keep a record of each batch, with the date, the exact weights and anything that surprised you. Our soap batch log is built for that.

Common mistakes

• Entering batch weight as oil weight. If you want a 1,000 g batch, your oils will be around 700 g, not 1,000 g.
• Picking the wrong oil variant. “Coconut oil” in most calculators means refined 76° coconut oil. Fractionated coconut oil and other variants have different SAP values. Choose the closest match and note it.
• Leaving the lye type on KOH after making liquid soap, or the reverse.
• Mixing units. Enter everything in grams. Convert ounces first if you have to (1 oz ≈ 28.35 g).
• Adding extra oil or butter at trace without putting it in the calculator. If it’s an oil, it reacts with lye and belongs in the calculation.
• Trusting a recipe’s lye number without checking. Run every recipe you didn’t write through the calculator.

Where to go next

Try running a recipe you know: castile soap is a one-oil formula you can check against the SAP value chart in a minute. Then try a multi-oil recipe like goat milk soap and compare your numbers with ours.