Do Different Soap Calculators Give Different Lye Amounts?
Every now and then, soapmakers notice that different soap calculators produce slightly different lye amounts for the exact same recipe.
Understandably, this can cause concern — after all, nobody wants to end up with a lye-heavy batch!
But should these differences really worry us? And if so, is there a way to stay on the safe side?
Why the differences exist
The answer lies in the SAP (saponification) values — the numbers used by every soap calculator to determine how much lye is needed to convert each oil or fat into soap. While these values are based on scientific testing, there is no single universal list of SAP values that all calculators share. They can vary slightly depending on:
- Plant variety and growing conditions (climate, soil, harvest time, etc.)
- Processing method (cold-pressed, refined, hydrogenated, etc.)
- Data source used by the calculator’s creator (lab analyses, published averages, supplier data)
As a result, even reputable soap calculators may use slightly different SAP values for the same oil — and this naturally leads to small variations in the calculated lye amount.
How big are these differences?
To get a clearer picture, I tested several recipes across five well-known and reputable soap calculators:
- SoapCalc (SC)
- SoapmakingFriend (SMF)
- The Cosmetics Lab (TCL)
- LyeCalc (LC)
- BrambleBerry (BB)
The results are presented in the table below.
If numbers aren’t your thing and you’d rather jump straight to the main takeaways, you can skip ahead to the section “What this means for soapmakers.”
For those who’d like to dive into the results, a few quick notes:
- Columns SC, SMF, TCL, LC, and BB indicate the amount of NaOH calculated by each soap calculator, expressed in grams.
- SoapMakingFriend, The Cosmetics Lab, and LyeCalc allow you to specify the lye purity. In contrast, SoapCalc assumes 100% NaOH purity and BrambleBerry assumes 97%. Because of this difference, these two calculators are not included in the same test recipes — their results are not directly comparable.
- The 5-oil recipe mentioned in the table is: 45% Olive oil, 23% Palm oil, 22% Coconut oil, 7% Shea butter and 2% Castor oil.
- The purpose of the single-oil recipe tests was to make it easy to compare results across calculators and verify the assumed SAP values for each selected oil or fat.
- In all tests, the total amount of oils was 1000g.
| # | Recipe | SC | SMF | TCL | LC | BB | Median | Std. Dev. | Range (%) |
|---|---|---|---|---|---|---|---|---|---|
| 1 | Olive oil 100%, SF 0%, NaOH purity 100% | 135.5 | 134.1 | 134.0 | 133.0 | - | 134.05 | 1.03 | 1.86 |
| 2 | Olive oil 100%, SF 5%, NaOH purity 100% | 128.7 | 127.4 | 127.3 | 126.4 | - | 127.35 | 0.95 | 1.81 |
| 3 | Coconut oil 100%, SF 0%, NaOH purity 100% | 183.3 | 181.4 | 183.2 | 184.0 | - | 183.25 | 1.11 | 1.42 |
| 4 | Coconut oil 100%, SF 5%, NaOH purity 100% | 174.1 | 172.3 | 174.1 | 174.8 | - | 174.10 | 1.07 | 1.44 |
| 5 | Beef tallow 100%, SF 0%, NaOH purity 100% | 142.6 | 141.1 | 142.6 | 140.0 | - | 141.85 | 1.27 | 1.83 |
| 6 | Beef tallow 100%, SF 5%, NaOH purity 100% | 135.5 | 134.1 | 135.5 | 133.0 | - | 134.80 | 1.21 | 1.85 |
| 7 | Palm oil 100%, SF 0%, NaOH purity 100% | 141.9 | 140.42 | 141.9 | 139.0 | - | 141.16 | 1.39 | 2.05 |
| 8 | Palm oil 100%, SF 5%, NaOH purity 100% | 134.8 | 133.4 | 134.8 | 132.1 | - | 134.10 | 1.30 | 2.01 |
| 9 | Shea butter 100%, SF 0%, NaOH purity 100% | 127.6 | 126.3 | 127.6 | 126.0 | - | 126.95 | 0.85 | 1.26 |
| 10 | Shea butter 100%, SF 5%, NaOH purity 100% | 121.3 | 120.0 | 121.2 | 119.7 | - | 120.60 | 0.82 | 1.33 |
| 11 | 5-oil recipe: SF 0%, NaOH purity 100% | 146.7 | 145.2 | 146.0 | 144.9 | - | 145.60 | 0.81 | 1.24 |
| 12 | 5-oil recipe: SF 5%, NaOH purity 100% | 139.4 | 137.9 | 138.7 | 137.7 | - | 138.30 | 0.78 | 1.23 |
| 13 | 5-oil recipe: SF 0%, NaOH purity 97% | - | 149.7 | 150.5 | 149.4 | 147.30 | 149.55 | 1.36 | 2.14 |
| 14 | 5-oil recipe: SF 5%, NaOH purity 97% | - | 142.2 | 143.0 | 141.9 | 139.90 | 142.05 | 1.32 | 2.18 |
| 15 | Olive oil 100%, SF 0%, NaOH purity 97% | - | 138.2 | 138.1 | 137.1 | 137.00 | 137.60 | 0.64 | 0.87 |
| 16 | Olive oil 100%, SF 5%, NaOH purity 97% | - | 131.3 | 131.2 | 130.26 | 130.15 | 130.73 | 0.61 | 0.88 |
| 17 | Coconut oil 100%, SF 0%, NaOH purity 97% (1) | - | 186.9 | 188.7 | 189.7 | 178.0 | 187.80 | 5.34 | 6.23 |
| 18 | Coconut oil 100%, SF 5%, NaOH purity 97% (1) | - | 177.6 | 179.2 | 180.2 | 169.1 | 178.40 | 5.06 | 6.22 |
| 19 | Beef tallow 100%, SF 0%, NaOH purity 97% | - | 145.5 | 147.4 | 144.3 | 140.0 | 144.90 | 3.14 | 5.11 |
| 20 | Beef tallow 100%, SF 5%, NaOH purity 97% | - | 138.2 | 140.1 | 137.1 | 133.0 | 137.65 | 3.00 | 5.16 |
| 21 | Palm oil 100%, SF 0%, NaOH purity 97% | - | 144.8 | 146.4 | 143.3 | 144 | 144.38 | 1.33 | 2.15 |
| 22 | Palm oil 100%, SF 5%, NaOH purity 97% | - | 137.5 | 139.1 | 136.1 | 136.8 | 137.15 | 1.28 | 2.19 |
For most oils, the variation in calculated NaOH amount turned out to be relatively small — typically within about 1–2%. However, a few specific cases, such as coconut oil and beef tallow, showed noticeably higher deviation when calculated with the BrambleBerry soap calculator. This suggests that BrambleBerry may rely on slightly different underlying SAP values than the other tools, which leads to a wider spread in lye results for these particular fats.
In all other cases, the discrepancies remained modest and consistent across calculators. Even when including the outliers, the overall variation stayed below about 2%, which is well within the range that can be attributed to normal differences in SAP data, rounding methods, or oil composition.
In practical terms, this means that while some divergence between calculators is perfectly normal, these differences are not large enough to cause concern — especially when a sensible level of superfatting is applied.
What this means for soapmakers
These results confirm that small discrepancies between calculators are normal and expected. They don’t mean that any of the calculators are “wrong” — they simply reflect the natural variability of oils and the lack of absolute, fixed SAP values.
To stay on the safe side, it’s good practice to:
- Always apply a superfat (typically 3–8%) to provide a margin of safety.
- When you’re formulating or comparing recipes, it’s a good idea to stick with one calculator for consistency. You can, of course, cross-check your results with others — just make sure to use the same settings and recipe parameters (superfat, lye purity, etc).
- When using new or unusual oils, consider running a small test batch first.
Conclusion
While different soap calculators may yield slightly different lye amounts, these variations are small and predictable — not a cause for alarm. What matters most is understanding why they occur and formulating with a sensible safety margin. A well-chosen superfat ensures your soap remains mild, safe, and beautifully balanced — no matter which reputable calculator you use.