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To calculate crude protein in fish feed, measure the sample’s nitrogen percentage and multiply it by 6.25: crude protein (%) = nitrogen (%) × 6.25. For example, a measured nitrogen value of 5.0% gives an estimated crude-protein value of 31.25%. This is a calculation example, not a laboratory result. The factor is a conventional estimate, not a direct measurement of the amino-acid protein fish can use.
Use measured nitrogen, not the ingredient list
The calculation starts with a laboratory measurement of nitrogen in the feed sample. It does not let you calculate nitrogen from a recipe or ingredient list. A common method for animal feed is Kjeldahl: the sample is digested, then nitrogen is measured through distillation and titration. ISO 5983-2:2009 specifies a Kjeldahl method and crude-protein calculation for animal feeding stuffs; ISO reports this edition was reviewed and confirmed as current in 2026.
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Dumas combustion is another method used to measure nitrogen in some feed and food analyses. Both methods produce a nitrogen-based estimate when nitrogen is converted to crude protein; neither directly measures amino-acid protein.
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Apply the formula and keep the basis consistent
- Get the nitrogen result. Ask the laboratory whether its report gives nitrogen as a percentage and whether the result is on a dry-matter or as-fed basis.
- Multiply by 6.25. For a nitrogen result of 5.0%, the calculation is 5.0 × 6.25 = 31.25% crude protein on the same basis as the nitrogen result.
- Compare like with like. A dry-matter nitrogen result yields a dry-matter crude-protein estimate. To compare it with an as-fed package value, account for the feed’s moisture. UF/IFAS explains the nitrogen basis of crude protein on fish-food guaranteed-analysis labels; the applicable labeling rules may depend on jurisdiction.
The 6.25 factor comes from the assumption that protein contains an average of 16% nitrogen by weight (100 ÷ 16 = 6.25), as explained by FAO and UF/IFAS. It is a convention, not a universal property of every protein source.
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What the crude-protein number does—and does not—tell you
Crude protein is an estimate based on total nitrogen. Kjeldahl does not separate protein nitrogen from non-protein nitrogen (NPN), which can include compounds such as urea, uric acid, and nucleic acids. Dumas also does not distinguish protein nitrogen from NPN. If a feed ingredient contains appreciable NPN, the calculated crude-protein value can overstate the amino-acid protein available to fish.
Proteins vary in nitrogen content. FAO reports variation from 12% to 19% among proteins; its aquatic-feed reference notes that using 6.25 when a material’s nitrogen composition differs can produce errors of 15–20% in some cases. Those figures describe potential variation and error, not a correction that applies to every fish feed. Use another conversion factor only when the relevant standard or laboratory supports it, and identify the factor used in the result.
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What to check when ordering a feed analysis
When comparing a Kjeldahl service with a Dumas service, ask for the details that make results interpretable:
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- Which analytical method was used.
- Whether the report gives measured nitrogen, converted crude protein, or both.
- Which conversion factor was applied.
- Whether results are reported as-fed or on a dry-matter basis.
- Whether the lab accepts your feed sample type and whether it can address NPN concerns. A nitrogen method alone does not establish how much amino-acid protein fish can use.
For context, FAO’s laboratory manual describes nitrogen analysis in feed, while AOAC Official Method 954.01 is another cited reference for crude-protein analysis.
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