Soap Superfat Calculator

While the Cold Process Soap Calculator works forward from a target superfat to a lye amount, this tool works in reverse: given the oil weight, the lye (NaOH) you actually used, and the oil blend's SAP value, it tells you what superfat percentage that lye amount really produces. It first computes how much oil the lye you used is capable of saponifying — saponifiedOil = lyeUsed ÷ SAP value — then subtracts that from your total oil weight to get the excess oil left unreacted, floored at zero so it never reports negative "excess" if you added more lye than the recipe calls for. Dividing that excess oil by total oil weight gives the actual superfat percentage, which is useful for auditing an existing recipe, checking a formulation you got from someone else, or verifying your own math before mixing.

It also runs the calculation the other direction: given a target superfat percentage you'd prefer, it computes the ideal lye amount (totalOil × SAP × (1 − target/100)) and reports the lye adjustment needed — positive means add more lye to bring superfat down to target, negative means you've already used too much lye and are running a lower superfat than intended. As with the forward calculator, everything here hinges on an accurate SAP value for your specific oil blend; a wrong SAP value will produce a confidently wrong superfat reading even though the arithmetic itself is correct, so cross-check it against a reputable saponification chart, especially for blended oils where you need a weighted average across each oil's proportion.