# Soap and Candle Making: Lye, Superfat and How Much Wax

Why every oil needs a different amount of lye, what superfat actually does, and why a jar that holds 8 fluid ounces does not need 8 ounces of wax.

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- **Canonical URL:** https://dothecalculation.com/blog/hobby/soap-and-candle-making-guide
- **Category:** Hobbies & Leisure Utilities
- **Author:** Do The Calculation Team
- **Published:** 2026-09-20
- **Reading time:** 16 min read
- **Publisher:** Do The Calculation (https://dothecalculation.com)
- **Methodology:** https://dothecalculation.com/methodology

---

Both of these crafts are chemistry with a nice finish on it, and both have one calculation you cannot skip. In soap making that calculation is not optional in the safety sense — get it wrong in one direction and you have a caustic bar.

> **Before any soap making** — Sodium hydroxide is caustic. It burns skin and eyes, it heats violently when mixed with water, and the fumes should not be breathed. Always add lye to water and never water to lye. Wear goggles and gloves, work somewhere ventilated, and keep children and animals out of the room. This guide explains the arithmetic; it does not replace learning the process properly before you start.

## Why every oil needs a different amount of lye

Soap is what you get when a fat reacts with a strong alkali. Each fat has its own molecular structure, so each needs its own amount of lye to react completely. That quantity is the saponification value, and it is a property of the oil.

**Sodium hydroxide needed per ounce of oil**
| Oil | NaOH per oz | For 16 oz of it |
| --- | --- | --- |
| Coconut oil | 0.1910 oz | 3.06 oz |
| Palm oil | 0.1420 oz | 2.27 oz |
| Sweet almond oil | 0.1367 oz | 2.19 oz |
| Sunflower oil | 0.1358 oz | 2.17 oz |
| Olive oil | 0.1353 oz | 2.16 oz |
| Avocado oil | 0.1337 oz | 2.14 oz |
| Shea butter | 0.1296 oz | 2.07 oz |
| Castor oil | 0.1286 oz | 2.06 oz |

Coconut oil needs nearly 50% more lye than castor oil for the same weight. This is why you cannot substitute one oil for another in a recipe without recalculating — swapping castor for coconut in a recipe sized for castor would leave a large excess of unreacted lye in the bar.

**Lye for a recipe**

```
NaOH = Σ (weight of each oil × its SAP value) × (1 − superfat)
```
- Every oil is calculated separately and the results added. There is no average.
- Worked, for a 32 oz batch of 50% olive, 30% coconut and 20% palm: olive 16 × 0.1353 = 2.16 oz, coconut 9.6 × 0.1910 = 1.83 oz, palm 6.4 × 0.1420 = 0.91 oz. That is 4.91 oz before superfat, and 4.66 oz at a 5% superfat.

## Superfat: deliberately not enough lye

Superfatting means using slightly less lye than the oils could consume, so a known percentage of the oil stays unreacted in the finished bar. That leftover fat is what makes a bar feel conditioning rather than stripping, and it is also your safety margin against small measuring errors.

**The same 32 oz batch at different superfat levels**
| Superfat | Lye | Water at 33% concentration |
| --- | --- | --- |
| 0% | 4.91 oz | 9.96 oz |
| 3% | 4.76 oz | 9.66 oz |
| 5% | 4.66 oz | 9.46 oz |
| 8% | 4.51 oz | 9.17 oz |
| 10% | 4.42 oz | 8.97 oz |

> **Never soap at 0% superfat** — At zero superfat, any error in the direction of too much lye leaves free alkali in the bar, and a scale that reads a tenth of an ounce heavy is enough. Five percent is the usual default for a general-purpose bar and gives you a real margin. Higher superfat makes a more conditioning but softer bar that lathers less and can go rancid sooner, since the excess oil is just sitting there.

The water figure is sized to a lye concentration rather than picked directly. At 33%, the lye solution is one third lye and two thirds water by weight. Less water gives a firmer bar faster and a shorter cure, but it also accelerates trace and gives you less working time — which matters if you are doing anything decorative.

Tool: [Run a soap recipe](https://dothecalculation.com/calculators/soap-making-lye-calculator) — Lye and water for any combination of oils, at your chosen superfat and lye concentration, with the contribution of each oil shown separately.

> **Check your numbers against a second source** — SAP values vary slightly between references because natural oils vary between batches and suppliers. For a recipe you are going to make repeatedly, or one using an oil not in the standard tables, cross-check the value against your supplier's own figure. And cure the finished soap for four to six weeks: cold process soap is not finished when it comes out of the mould.

## Candles: the jar does not hold what it says

Candle wax is simpler and much safer, but the volume-to-weight step catches everyone. An 8 fluid ounce jar does not need 8 ounces of wax, for two independent reasons.

- You do not fill a jar to the brim. Headspace keeps the flame away from the rim and leaves room for a top-off pour. Around 93% is a sensible working figure.
- Wax is lighter than water. Fluid ounces measure volume and ounces measure weight, and they do not even coincide for water: a US fluid ounce of water weighs about 1.043 ounces. Soy wax has a specific gravity of about 0.90, paraffin about 0.88, beeswax about 0.95.

**Wax for a container**

```
Wax weight (oz) = container volume (fl oz) × fill fraction × wax specific gravity
```
- Worked: an 8 fl oz jar filled to 93% is 7.44 fl oz, which as water would weigh 7.44 × 1.0432 = 7.76 oz; at soy's 0.90 specific gravity that is 6.99 oz of wax, not 8.
- Fragrance oil is added as a percentage of the wax weight, so it comes after this step rather than being part of the volume.

**An 8 fl oz jar in soy wax, by how full you pour it**
| Fill level | Wax | Fragrance at 8% |
| --- | --- | --- |
| 85% | 6.38 oz | 0.51 oz |
| 90% | 6.76 oz | 0.54 oz |
| 93% | 6.99 oz | 0.56 oz |
| 100% (brim full) | 7.51 oz | 0.60 oz |

Assuming a brim-full jar over-orders wax by about 7.5% on every candle, which on a batch of a hundred is seven or eight jars' worth of wax you did not need.

## Fragrance load has a ceiling

Fragrance oil is quoted as a percentage of wax weight, and more is not better. Each wax can only bind so much before the excess separates out — it pools on the surface, which makers call sweating, and it can clog the wick and affect how the candle burns.

Soy generally takes up to about 10%, and the calculator flags anything above its per-wax maximum. Start lower than the ceiling: a candle at 6% that throws well is better than one at 10% that sweats, and cold throw in the jar is a poor predictor of hot throw when lit. Test burn before committing to a batch.

**A batch of six 8 fl oz soy candles at 8% fragrance, with a 10% top-off allowance**
| Component | Per candle | Batch of six |
| --- | --- | --- |
| Wax for the main pour | 6.99 oz (198 g) | 41.91 oz (1,188 g) |
| Fragrance oil | 0.56 oz (16 g) | 3.35 oz (95 g) |
| Total pour weight | 7.54 oz (214 g) | 45.26 oz (1,283 g) |
| Extra wax for topping off | 0.70 oz (20 g) | 4.19 oz (119 g) |

The top-off allowance is the other thing beginners leave out. Soy in particular sinks and cracks around the wick as it cools, and the fix is a second small pour once the first has set. Ordering wax without that allowance means coming up short on the last few candles of a batch.

Tool: [Work out wax for a batch](https://dothecalculation.com/calculators/candle-making-calculator) — Wax, fragrance and top-off quantities for any container size, wax type, fragrance load and batch size, in ounces and grams.

## A full recipe in grams

Most modern soap recipes are written as percentages of the oil weight, which makes them easy to scale to any mould. Here is the same 50% olive, 30% coconut, 20% palm recipe from above, scaled to 1,000 grams of oils, with a 5% superfat and a 33% lye concentration.

**1,000 g of oils, 5% superfat, 33% lye concentration**
| Ingredient | Working | Amount |
| --- | --- | --- |
| Olive oil | 50% of 1,000 g | 500 g |
| Coconut oil | 30% of 1,000 g | 300 g |
| Palm oil | 20% of 1,000 g | 200 g |
| Sodium hydroxide | (500 × 0.1353 + 300 × 0.1910 + 200 × 0.1420) × 0.95 | 145.7 g |
| Water | 145.7 ÷ 0.33 − 145.7 | 295.8 g |
| Total batch weight | Oils + lye + water | 1,441.5 g |

Because saponification values are ratios, the same numbers work in ounces, grams or pounds; the unit only has to be consistent. Weigh everything, including the water, on a scale that reads to at least a gram. Measuring lye or water by volume is a common source of error, and the arithmetic cannot correct for an inaccurate measurement.

## Lye concentration and the water-to-lye ratio

Soap recipes express the water in two different ways, and it helps to be able to convert between them. Lye concentration is the lye as a percentage of the lye-and-water solution. The water-to-lye ratio is the weight of water for each unit of lye. They describe the same thing.

**Converting between the two**

```
Water : lye ratio = (100 − concentration %) ÷ concentration %
```
- A 33% lye concentration is a water-to-lye ratio of about 2 to 1.
- Higher concentration means less water: a firmer bar sooner and a shorter cure, but faster trace and less working time.

**Common lye concentrations and the equivalent ratios**
| Lye concentration | Water : lye ratio |
| --- | --- |
| 25% | 3.00 : 1 |
| 28.6% | 2.50 : 1 |
| 30% | 2.33 : 1 |
| 33% | 2.03 : 1 |
| 35% | 1.86 : 1 |
| 40% | 1.50 : 1 |

Soapmakers also talk about a "water discount", meaning less water than a recipe's default. It changes nothing about the chemistry of saponification, which depends only on the lye and the oils; it changes how the batter behaves and how long the bar takes to harden. The lye amount is set by the oils and the superfat, and the water is set by the concentration you choose.

## Liquid soap: potassium hydroxide instead of sodium hydroxide

Bar soap is made with sodium hydroxide. Liquid and soft soaps are made with potassium hydroxide, which needs a larger weight for the same oil because potassium is heavier than sodium. The conversion comes straight from the two compounds' formula masses: 56.106 for KOH against 39.997 for NaOH, a ratio of 1.4027.

**Saponification values per ounce of oil, sodium versus potassium hydroxide**
| Oil | NaOH | KOH (× 1.4027) |
| --- | --- | --- |
| Coconut oil | 0.1910 | 0.2679 |
| Palm oil | 0.1420 | 0.1992 |
| Olive oil | 0.1353 | 0.1898 |
| Castor oil | 0.1286 | 0.1804 |

The same 1,000 g recipe at a 5% superfat would need about 204.4 g of potassium hydroxide at 100% purity. Commercial potassium hydroxide is often supplied at less than 100% purity, which is stated on the label; divide by the purity to get the weight to measure out. At an illustrative 90% purity that is 204.4 ÷ 0.90 ≈ 227.1 g. Check the purity of your own product rather than assuming a figure.

> **Never swap one lye for the other** — Using sodium hydroxide amounts with potassium hydroxide leaves the oils under-saponified; using potassium hydroxide amounts with sodium hydroxide leaves a large excess of caustic lye in the product. Always recalculate for the lye you actually have, and label your lye containers clearly so the two can never be confused.

## Wax for common container sizes

Here is the calculator's wax figure for a range of container sizes in soy wax, poured to 93% with an 8% fragrance load. Every figure uses the true weight of a US fluid ounce of water, 1.0432 ounces, which is why the numbers are slightly higher than a quick volume-times-gravity estimate.

**Soy wax, 93% fill, 8% fragrance**
| Container | Wax | Fragrance oil | Total pour |
| --- | --- | --- | --- |
| 4 fl oz | 3.49 oz (99 g) | 0.28 oz (8 g) | 3.77 oz (107 g) |
| 6 fl oz | 5.24 oz (149 g) | 0.42 oz (12 g) | 5.66 oz (160 g) |
| 8 fl oz | 6.99 oz (198 g) | 0.56 oz (16 g) | 7.54 oz (214 g) |
| 10 fl oz | 8.73 oz (248 g) | 0.70 oz (20 g) | 9.43 oz (267 g) |
| 12 fl oz | 10.48 oz (297 g) | 0.84 oz (24 g) | 11.32 oz (321 g) |
| 16 fl oz | 13.97 oz (396 g) | 1.12 oz (32 g) | 15.09 oz (428 g) |

**The same 8 fl oz jar in four waxes, 93% fill, no fragrance**
| Wax | Typical specific gravity | Wax needed |
| --- | --- | --- |
| Paraffin | about 0.88 | 6.83 oz (194 g) |
| Soy | about 0.90 | 6.99 oz (198 g) |
| Coconut | about 0.90 | 6.99 oz (198 g) |
| Beeswax | about 0.95 | 7.37 oz (209 g) |

The spread between waxes is about 8% from paraffin to beeswax for the same jar, which is enough to leave you a candle or two short on a batch if you switch waxes without recalculating. The specific gravities are typical figures; your wax supplier's data sheet gives the value for your specific product.

## Measuring a container's real volume

Container capacities printed by suppliers are often rounded, and some are measured to the brim and some to a fill line. The most accurate way to know what your container holds is to weigh it full of water.

**Container volume from water weight**

```
Volume (fl oz) = weight of water in grams ÷ 29.57
```
- Tare the empty container on the scale, fill it with water to the level you will pour wax to, and read the weight in grams.
- Illustrative example: 237 g of water is 237 ÷ 29.57 ≈ 8.01 fl oz. A container sold as 250 mL is about 8.45 fl oz.

If you fill to your actual pour line rather than the brim, enter 100% as the fill level, because you have already measured the poured volume directly. Either approach works; the point is not to apply the fill correction twice.

## Percent of what? The fragrance load trap

Fragrance loads are almost always quoted as a percentage of the wax weight, and that is how the calculator applies them. Some makers and some suppliers use a percentage of the total blend instead. The two are not the same number.

**An 8% fragrance load on 6.99 oz of soy wax, calculated two ways**
| Basis | Fragrance oil | Fragrance as a share of the finished blend |
| --- | --- | --- |
| 8% of the wax weight | 0.559 oz | 7.41% |
| 8% of the total blend | 0.608 oz | 8.00% |

The difference is about 9% more fragrance oil on the total-blend basis. That matters when you are working close to a wax's maximum load, because the same "10%" can mean two different amounts. Check which basis your wax manufacturer's maximum load uses, and stay consistent with it.

## Wicks, test burns and why the arithmetic is not the end

The calculations in this guide give you quantities. They cannot choose a wick, and the wick decides whether a candle burns well. Wick size depends on the container diameter, the wax, the fragrance and any dye, and wick manufacturers publish sizing charts for their own wicks.

- Start from the wick manufacturer's chart for your wax and container diameter.
- Make a test candle and let it cure for the time your wax supplier recommends before test burning.
- Burn it for a few hours at a time and watch the melt pool: it should reach the edge of the container without the flame becoming too large or sooting.
- Change one thing at a time, wick size or fragrance load, and record the result.
- Only scale up to a full batch once a test candle burns cleanly from top to bottom.

## Common soap and candle quantity mistakes

- **Swapping oils or lye type without recalculating.** Every oil has its own saponification value, and potassium hydroxide needs about 1.4 times the weight of sodium hydroxide.
- **Measuring by volume.** Weigh lye, water, oils, wax and fragrance.
- **Soaping at zero superfat.** Keep a margin against measuring error.
- **Treating fluid ounces as ounces.** Wax weight comes from volume, fill level, the weight of water and the wax's specific gravity.
- **Mixing fragrance-load bases.** Know whether a percentage is of the wax or of the total blend.
- **Skipping the test burn.** The quantities can be perfect and the candle can still burn badly with the wrong wick.

## Tracking a soap cure by weight

The advice to cure cold process soap for four to six weeks is a guide, not a measurement. Weighing bars gives you the measurement. Most of what happens during a cure that you can observe on a kitchen scale is water leaving the bar, and the recipe tells you how much water went in.

In the 1,000 g recipe above, 295.8 g of the 1,441.5 g batch is water, about 20.5% of the fresh weight. A freshly cut 100 g bar therefore started with roughly 20.5 g of water in it. It cannot lose more weight than that from evaporation, and in practice some water stays in the finished bar, so the weight loss levels off before reaching it.

**How the lye concentration changes the water in the same 1,000 g recipe**
| Lye concentration | Water | Water as a share of fresh batch weight |
| --- | --- | --- |
| 25% | 437.1 g | 27.6% |
| 30% | 340.0 g | 22.9% |
| 33% | 295.8 g | 20.5% |
| 35% | 270.6 g | 19.1% |
| 40% | 218.5 g | 16.0% |

- Weigh two or three bars from each batch when you cut them, and write the weights on a label or in a notebook.
- Weigh the same bars once a week, on the same scale.
- When the weight stops falling between weekly readings, the bars have stopped losing water, whatever the calendar says.
- A batch made with less water starts drier and levels off sooner, which is one practical reason soapmakers use higher lye concentrations.

Weight tells you about water, not about everything a cure does, so treat it as one signal alongside the time your recipe or supplier recommends. It is also a good check on consistency: two batches of the same recipe that lose very different amounts of weight were probably not made quite the same way.

## Sources

- Saponification values for sodium hydroxide, from the published SAP chart: https://www.certified-lye.com/lye-soap.html — these vary slightly between references because natural oils vary between batches, so cross-check against your supplier for anything unusual.
- Wax specific gravities of about 0.90 for soy, 0.88 for paraffin and 0.95 for beeswax are the standard working figures used across candle supply. The fill percentage, the top-off allowance and the fragrance ceilings are craft conventions rather than published standards, and the calculator exposes each as an input rather than burying it.

## Where to go next

Both of these are really the same craft problem as [epoxy resin mixing and curing](/blog/hobby/epoxy-resin-guide): measure two things in the correct ratio, get the temperature right, and wait. The resin guide covers why a ratio by volume is not a ratio by weight, which is the same distinction that makes candle wax weigh less than the jar it fills.

## Common questions

**How much lye do I need for my soap recipe?**

Multiply each oil's weight by its own saponification value and add the results, then subtract your superfat. A 32 oz batch of 50% olive, 30% coconut and 20% palm needs 4.91 oz of lye at 0% superfat and 4.66 oz at 5%. There is no single figure for "oil" — coconut needs nearly 50% more lye per ounce than castor.

**What is superfat and what percentage should I use?**

Using slightly less lye than the oils could consume, so some oil stays unreacted in the bar. It makes the soap more conditioning and gives you a margin against measuring error. Five percent is the usual default. Never use zero — a scale reading slightly heavy would then leave free lye in the bar.

**Can I swap one oil for another in a soap recipe?**

Only if you recalculate the lye. Each oil has its own saponification value, and the spread is large: 0.1910 oz of lye per ounce of coconut oil against 0.1286 for castor. Substituting without recalculating can leave unreacted lye in the finished bar.

**How much wax do I need for an 8 oz jar?**

About 7.0 oz of soy wax (6.99 oz), not 8. A jar is filled to roughly 93% rather than the brim, a fluid ounce of water weighs about 1.043 oz, and soy's specific gravity is about 0.90. Assuming a brim-full jar over-orders by about 7.5% on every candle.

**How much fragrance oil should I add?**

A percentage of the wax weight, below whatever your wax can bind — around 10% for soy. Above that the excess separates and pools on the surface, and can clog the wick. Starting at 6 to 8% and test burning is better than going straight to the ceiling.

**Why does my soy candle crack around the wick?**

Soy shrinks as it cools and often sinks around the wick. The standard fix is a second small pour once the first has set, which is why wax orders should include a top-off allowance of around 10% on top of the main pour.

**How do I convert a soap recipe to potassium hydroxide for liquid soap?**

Multiply each oil's sodium hydroxide saponification value by 1.4027, the ratio of the two compounds' formula masses. Olive oil goes from 0.1353 to 0.1898, coconut from 0.1910 to 0.2679. Then divide by your potassium hydroxide's stated purity, since it is often less than 100%.

**What does a 33% lye concentration mean?**

The lye is 33% of the weight of the lye-and-water solution, which is a water-to-lye ratio of about 2 to 1. For 145.7 g of sodium hydroxide that means about 295.8 g of water.

**How do I measure how much my candle container holds?**

Weigh it full of water. Tare the empty container, fill it to the level you will pour to, and divide the grams of water by 29.57 to get fluid ounces. 237 g of water is about 8.0 fl oz.

**Is fragrance load a percentage of the wax or of the total?**

Usually of the wax weight, which is how this calculator applies it, but some suppliers use the total blend. On 6.99 oz of soy wax, 8% of the wax is 0.559 oz of fragrance while 8% of the total is 0.608 oz, about 9% more. Check which basis your wax's maximum load uses.

**Why does beeswax need more wax by weight for the same jar?**

It is denser. With a typical specific gravity around 0.95 against soy's 0.90, an 8 fl oz jar at 93% fill takes about 7.37 oz of beeswax against 6.99 oz of soy.

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_Source: [Do The Calculation](https://dothecalculation.com/blog/hobby/soap-and-candle-making-guide). Quote freely with attribution and a link to this page._