What this free soap calculator does
Every soap maker eventually needs the same math: how much lye do I add to this specific blend of oils, and how much water does the lye need to dissolve? The answer depends on each oil's saponification value (SAP) — the precise amount of lye it takes to turn that oil into soap. Coconut oil needs more lye per gram than olive oil; shea butter needs less than coconut. A soap calculator does that lookup, multiplies each oil's SAP by the weight you are using, and adds it all together.
On top of the lye and water amounts, this calculator also predicts the qualities of the finished bar — hardness, cleansing power, conditioning, bubbly lather, creamy lather, iodine value, and the INS hardness gauge. Those numbers come from the fatty acid balance of your oils. Get them in a sensible range, and the bar will perform well in the shower.
The calculator is completely free, runs in your browser, requires no signup, and has no usage limits. Use it for personal soap making, gifts, or a small-batch shop.
How to use the soap calculator
- Pick your lye. Choose NaOH (sodium hydroxide) for solid bar soap, or KOH (potassium hydroxide) for liquid soap. Cold process and hot process both use NaOH — the difference between the two methods is the cooking step, not the chemistry.
- Set the total oil weight. 500–1000 g is typical for a home batch. The unit toggle lets you switch between grams, ounces and pounds at any time.
- Choose a water method. "Water as % of oils" is the classic approach (38% is the cold-process default). "Lye concentration" gives you direct control (33% is standard). "Water:lye ratio" is the old-school version (2:1 is standard). All three end at the same numbers, just expressed differently.
- Set superfat. 5% is the standard starting point. Lower means a stronger, more cleansing bar; higher means a milder bar that spoils faster. For liquid soap, 0–3% is more typical.
- Set fragrance percentage. 3–4% of oil weight is typical for cold and hot process soap. Skip it if you are making unscented soap.
- Add your oils and percentages. The total must sum to 100%. Pick from the dropdown — 60+ oils and butters are available, organized by hard fats, oleic-dominant liquids, linoleic-dominant liquids, and specialty oils.
- Review the results. The amounts table shows the exact lye, water and fragrance weights you need to measure out. The bar-quality and fatty-acid panels show what your finished bar will feel like. Adjust until you like the balance.
- Print the recipe. The Print recipe button creates a clean one-page summary you can take into the kitchen.
What the soap qualities mean
Each bar quality is a number derived from the fatty acid composition of your oil blend. The grey band on each row is the typical "balanced" target range — a value inside the band means a well-rounded bar, a value outside means the bar leans one way.
- Hardness — how solid the cured bar will be. Target: 29–54.
- Cleansing — how stripping the lather will feel. Target: 12–22. Very high values (laundry soap) feel drying; very low (Castile) feel almost too gentle.
- Conditioning — how moisturizing the bar will feel. Target: 44–69.
- Bubbly — how foamy the lather will be on first contact. Target: 14–46.
- Creamy — how dense and stable the lather will feel as you keep washing. Target: 16–48.
- Iodine value — a measure of unsaturated oils. High iodine bars are softer and more prone to rancidity. Target: 41–70.
- INS — an empirical hardness and quality gauge. Target: 136–170.
There is no perfect score. A bar tuned for max cleansing will feel drying; a bar tuned for max conditioning will be soft and use up fast. The numbers are a guide to your trade-offs.
Lye safety
Sodium hydroxide and potassium hydroxide are strongly alkaline and will burn skin on contact. Working with lye is safe when you follow the rules; it goes wrong when the rules are skipped. Always:
- Wear goggles, gloves, long sleeves and closed shoes — every single batch.
- Work in a ventilated space; the brief vapor when lye hits water is sharp.
- Always add lye to water, never water to lye.
- Keep children and pets out of the room.
- Label your lye solution clearly while it cools.
New to soap making? Read the complete soap-making starter guide before you make your first batch.
Frequently asked questions
Is this soap calculator free?
Yes. The DIY Cosmetica soap calculator is completely free to use, with no signup, no paywall, and no usage limits. Use it as many times as you want, for personal or commercial soap-making.
What does the soap calculator do?
It tells you exactly how much lye (sodium hydroxide for bar soap, or potassium hydroxide for liquid soap) and water you need for any oil blend, based on the saponification value of each oil. It also predicts the finished bar’s qualities — hardness, cleansing, conditioning, lather, iodine value and INS — and shows the fatty acid profile of your blend.
Does the calculator work for cold process, hot process and liquid soap?
Yes. Choose NaOH for cold or hot process bar soap, or KOH for liquid soap. The math is the same either way, only the lye type differs. Cold process and hot process use the same recipe; only the cooking step changes.
How accurate is the lye amount?
Within roughly 1% of any other calculator using the same superfat. Saponification (SAP) values are measured physical properties of each oil but published numbers vary by ±0.5–1.5% between sources. Our data uses consensus values across multiple references, and the slight conservative bias is on the safe side (a touch less lye = a slightly milder bar, never caustic excess).
What superfat should I use?
5% is the standard starting point for cold process and hot process bar soap. Lower (0–3%) gives a stronger cleansing bar; higher (6–10%) gives a milder, more conditioning bar but spoils faster. For liquid soap with KOH, 0–3% is more typical.
Which water method should I pick?
"Water as % of oils" is the classic approach — 38% is the default for cold process. "Lye concentration" gives you direct control, with 33% as the standard. "Water:lye ratio" is the old-school option, with 2:1 as the standard. All three produce the same result mathematically.
Why does the bar quality bar sometimes show outside the green range?
The grey-band column shows the typical "balanced" range for each quality. A value below or above the range isn’t wrong — it just means your bar will lean one way (e.g., very high cleansing means a stripping bar suited to laundry or oily skin; very low cleansing means a luxurious bar that doesn’t foam much). The numbers guide your trade-offs.
Can I print the recipe?
Yes. Tap the green Print recipe button to save a clean one-page summary with your inputs, the calculated amounts in pounds/ounces/grams, the oils breakdown, soap qualities, fatty acid profile, and the safety reminder.