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Protein Quality (PDCAAS) Calculator

Rate any protein's quality with the standard PDCAAS score.

Result

0.78
0.86
Good

How it is calculated

You supply three numbers for the limiting essential amino acid, the one furthest below the reference pattern. The first is its content in the test protein in mg per g of protein; the second is the reference pattern value for that same amino acid, in the same unit; the third is digestibility in percent (above 0 up to 100). The amino acid ratio is AAS = test mg ÷ reference mg. The score is PDCAAS = AAS × digestibility ÷ 100, truncated at 1.00 and rounded to two decimals. The rating is decided on that displayed, rounded score: 0.90 or more Excellent, 0.70 to 0.89 Good, 0.40 to 0.69 Fair, below 0.40 Low. Amino acid amounts above 1000 mg per g, a reference of zero or digestibility above 100 show no result. The tool does not find the limiting amino acid for you or provide reference values, so both must come from tables.

Worked example

A protein has 50 mg of its limiting amino acid per g, the reference pattern has 58 mg per g, and digestibility is 90%. Amino acid ratio = 50 ÷ 58 = 0.86. PDCAAS = 0.862 × 0.90 = 0.776, shown as 0.78. That is between 0.70 and 0.89, so the rating is Good. If the content were 70 mg, the ratio would be 1.21 and the raw score above 1, but the result is capped at 1.00.

Try this example →

Common mistakes

  • Entering digestibility as a fraction (0.9) instead of a percent (90). The tool reads 0.9 as 0.9%, giving a score of 0.01.
  • Using the content of the most abundant amino acid rather than the limiting one. PDCAAS uses only the amino acid with the lowest ratio, so use the worst case.
  • Mixing units between test and reference, such as mg per g of protein against mg per 100 g of food. Both must be the same basis or the ratio is meaningless, and values above 1000 are refused.
  • Expecting a displayed 0.70 to rate as Fair. The verdict uses the rounded score, so 0.70 is already rated Good.
  • Believing a score of 1.00 means the same quality for every protein. The cap hides how far above the reference some proteins are, so several different proteins can all show 1.00.

Sources

About this calculator

The PDCAAS (Protein Digestibility-Corrected Amino Acid Score) is the method the FAO and WHO proposed in 1991 and the FDA later adopted for food labeling to rate protein quality. It is calculated as the amino acid score of the protein’s limiting essential amino acid — the milligrams of that amino acid per gram of test protein divided by the milligrams found in a gram of a reference protein pattern — multiplied by the protein’s true or fecal digestibility percentage, with the final score capped at 1.0 even if the raw calculation exceeds it.

Food scientists, dietitians, and supplement manufacturers use PDCAAS to compare protein sources on a standardized scale: whey and egg protein typically score at or near the 1.0 cap, soy protein isolate scores around 0.91, and grain proteins like wheat gluten score much lower because lysine is their limiting amino acid and is only partially digestible.

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