In a solid shampoo bar, the active detergent percentage - also called active surfactant matter (ASM) or total active surfactant content - is the amount of actual cleansing surfactant in the formula after excluding water, fillers, salts, preservatives, and non-surfactant additives. It is one of the most useful numbers for predicting lather, cleansing strength, and mildness.

The core formula

Active detergent % = (Total weight of active surfactant in the batch ÷ Total batch weight) × 100

Most surfactant raw materials are not 100% active. For example, sodium cocoyl isethionate (SCI) may be sold as 85% active noodles or powder, and cocamidopropyl betaine is commonly a 30% active solution. Always check the supplier’s Certificate of Analysis or technical data sheet for the exact active content.

Step-by-step calculation

  1. List every surfactant in your formula.
  2. For each surfactant, note its active concentration as supplied - for example, 85% SCI, 30% cocamidopropyl betaine, 95% sodium methyl cocoyl taurate.
  3. Calculate the active contribution of each: ingredient weight × active percentage.
  4. Add all active surfactant contributions together.
  5. Add up the total batch weight, including water, butters, oils, additives, preservatives, and fragrance.
  6. Divide total active surfactant weight by total batch weight, then multiply by 100.

Worked example

Imagine a 500 g batch with the following formula:

  • 300 g sodium cocoyl isethionate at 85% active
  • 50 g cocamidopropyl betaine at 30% active
  • 20 g cetyl alcohol
  • 5 g panthenol
  • 10 g fragrance
  • 115 g distilled water

Calculate the active surfactant:

  • SCI: 300 g × 0.85 = 255 g active
  • Cocamidopropyl betaine: 50 g × 0.30 = 15 g active
  • Total active surfactant = 270 g

Total batch weight = 500 g, so:

Active detergent % = (270 ÷ 500) × 100 = 54%

This would be a reasonably cleansing syndet shampoo bar. Many solid shampoo bars fall in the 40-70% active surfactant range, depending on how strong or mild the formula is intended to be.

Important details to get it right

  • Use the active content, not the raw ingredient weight. If you use 100 g of an 85% active surfactant, you have 85 g of active detergent, not 100 g.
  • Do not count non-surfactant emollients and thickeners. Cetyl alcohol, cetearyl alcohol, butters, oils, and botanical extracts are not primary detergents.
  • Count primary and secondary surfactants. Ingredients such as SCI, SLSa, cocamidopropyl betaine, sodium cocoyl glutamate, and disodium laureth sulfosuccinate are typically counted. Conditioning emulsifiers that are not intended as cleansers are usually excluded.
  • Adjust for water loss if measuring finished bars. If a batch is calculated before drying, the active percentage is based on the as-made batch weight. After water evaporates, the finished bar will have a slightly higher active percentage because the total weight is lower.
  • Soap-based shampoo bars are different. In a lye-based shampoo bar, the detergent is the soap formed by saponification. You would calculate the theoretical soap yield from the oils used, subtract the superfat, and express that as a percentage of the final bar. For syndet shampoo bars, the surfactant active calculation above is the standard method.

In short, the active detergent percentage is a simple but powerful quality-control calculation: find the true active contribution of each surfactant, add them together, and divide by the total formula weight.