How do I calculate active matter in a surfactant?
Short answer
Divide the active percentage you want by the active percentage of your raw material, then multiply by 100. For 5% active CAPB from a 40% solution: (5 / 40) × 100 = 12.5 g.
Explanation
Surfactants are almost never sold at 100% purity. A bottle of Cocamidopropyl Betaine is typically a 30–40% active solution in water, and a formula that asks for '5% CAPB' usually means 5% active matter, not 5% of the bottle.
The book gives the calculation directly: if a recipe calls for 5% active CAPB and your raw material is a 40% active solution, you weigh (5 / 40) × 100 = 12.5 g of that solution per 100 g of product.
This matters twice over. Weigh the bottle percentage instead of the active percentage and your shampoo will be roughly a third as strong as intended — it will not lather, will not clean, and will feel thin because the extra water thins the whole system. The water that came with the surfactant also has to be subtracted from the water phase, or the batch will overrun its yield.
What to do
- Check the active matter percentage on the supplier's technical data sheet before you weigh anything.
- Subtract the water that arrives with a liquid surfactant from your water phase.
- Write the active percentage next to each surfactant in your own formula sheet so the maths is never repeated from memory.
Source: The Big Book of Homemade Hair Care — The Shampoo Matrix: Beyond Basic Cleansing