The crude protein calculator above performs the conversion that sits at the end of every Kjeldahl or Dumas nitrogen determination: measured nitrogen multiplied by a nitrogen-to-protein conversion factor. It runs in both directions, so you can take a laboratory nitrogen figure to a protein percentage, or take a published protein percentage back to the nitrogen content it implies. It also handles the moisture correction, because a feed analysis quoted as received and the same analysis quoted on a dry-matter basis are different numbers describing the same sample.
Arb Digital publishes free calculators for the step where people actually lose accuracy, and with crude protein that step is almost never the multiplication. It is choosing the right factor, and then reporting which basis the answer is on. This page keeps the factor and the basis on screen next to the result rather than hiding them, so the number you copy out carries its own context.
What This Crude Protein Calculator Does
In its default mode it takes the nitrogen content of a sample as a percentage by mass, multiplies by the factor you select, and returns crude protein as a percentage. The supporting grid converts that same result onto a dry-matter basis using the moisture you supply, prints the grams of protein and grams of nitrogen contained in the sample or serving mass you entered, and shows nitrogen expressed on a dry basis as well.
Reverse mode does the opposite. Give it a crude protein percentage from a label, a feed tag or a trade specification, and it returns the nitrogen content that figure was derived from. That is more useful than it first appears: nitrogen is the quantity actually measured, so working back to it is how you compare two products quoted with different factors, and how you check whether a stated protein figure is even arithmetically consistent with an analysis you have in hand.
One boundary worth stating plainly. The live protein concentration calculator measures protein directly by absorbance, using A280 or a Bradford or BCA standard curve on a purified or partly purified sample. This page never touches absorbance. It converts an elemental nitrogen measurement into a protein estimate for a whole food or feed, which is a completely different analytical route with a completely different set of assumptions.
How to Use It
- Enter the nitrogen percentage exactly as your laboratory reported it, and note whether that report was on an as-received or a dry sample.
- Choose the conversion factor that matches the commodity. The general 6.25 is the default because most feed and food specifications use it, but grains and dairy have their own published values.
- Enter the moisture percentage so the dry-matter figure is computed. Leave it at zero if the nitrogen figure you entered was already on a dry basis.
- Set the sample or serving mass to see the result in grams rather than percent, which is the form a ration or a nutrition panel usually needs.
- Record the factor with the result. A crude protein percentage without its factor is an incomplete report, and two laboratories using different factors on the same sample will disagree by several percent.
The Formula and How It Is Calculated
The core relationship is a single multiplication: crude protein % = nitrogen % × F, where F is the nitrogen-to-protein conversion factor. Reversed, nitrogen % = crude protein % ÷ F. The moisture correction converts between bases: value on dry matter = value as received ÷ (1 − moisture/100), and the mass in a sample is simply the percentage divided by one hundred and multiplied by the sample mass.
Working the default example: 2.72 percent nitrogen multiplied by 6.25 gives 17.00 percent crude protein as received. With 10 percent moisture the dry matter fraction is 0.90, so the dry-matter crude protein is 17.00 ÷ 0.90 = 18.89 percent. In a 100 g sample that is 17.00 g of protein and 2.72 g of nitrogen. Change the factor to 6.38 for a dairy sample and the same nitrogen gives 17.35 percent, which is the size of disagreement a mismatched factor produces.
The factor is not arbitrary. It is the reciprocal of the average nitrogen fraction of the protein in that commodity. A factor of 6.25 assumes protein is 16 percent nitrogen by mass, since 100 ÷ 16 = 6.25. Commodity-specific factors, commonly attributed to Jones, come from measured amino acid compositions, and the FAO report on food energy conversion factors discusses both the Jones factors and the general 6.25 and the difference between them. For composition data on specific foods, the USDA FoodData Central database is the reference to check against.
Why It Is Called Crude
The method measures nitrogen and then infers protein. Everything nitrogenous in the sample is therefore counted, whether or not it is protein. Free amino acids, nucleic acids, urea, ammonium salts, nitrate, creatine, betaine, chitin in insect meal and alkaloids all contribute nitrogen and all inflate the answer. In most whole foods the non-protein nitrogen fraction is small enough to live with, which is why the method survived a century. In some it is not: silage, fermented feeds and pasture at certain growth stages can carry a substantial non-protein nitrogen load.
This is also why the method is vulnerable to deliberate adulteration. Any cheap nitrogen-rich compound added to a product raises the apparent protein without adding any protein at all, and the assay cannot tell the difference. That vulnerability is a property of the method, not of the arithmetic on this page. It is the reason modern specifications increasingly ask for a direct amino acid profile alongside a crude protein figure rather than accepting the nitrogen result on its own.
The practical consequence for anyone reading a result is that crude protein is an upper bound on true protein, not an estimate that scatters evenly either side. If two samples of the same commodity give the same crude protein but one has been fermented, they do not contain the same protein.
Choosing the Factor, and Why 6.25 Persists
The general factor of 6.25 assumes a uniform 16 percent nitrogen content across all proteins. Real proteins vary from roughly 15 to 18 percent nitrogen depending on their amino acid composition, because arginine, histidine, lysine and tryptophan carry extra nitrogen while the rest do not. Cereal proteins are relatively rich in the low-nitrogen amino acids, which is why wheat flour takes 5.70 and rice takes 5.95. Milk proteins run the other way, hence 6.38.
Despite this, 6.25 remains the legal and commercial default in most feed trading and in many food labelling regimes, and that is a feature rather than an oversight. A trading standard needs everyone to compute the same number from the same measurement. Switching factor by commodity would make a load of mixed feed impossible to specify consistently. The right approach is to use the factor your specification, contract or regulation names, and to state it. Use the commodity factor when the question is nutritional accuracy for a single ingredient, and the general factor when the question is compliance with a stated specification.
If you are comparing two products quoted with different factors, convert both back to nitrogen first using reverse mode on this page, then re-apply a single common factor. Comparing the protein figures directly is comparing two different calculations.
As Received Against Dry Matter
Moisture is the largest single source of confusion in feed analysis, and it is entirely avoidable. A hay at 12 percent moisture and the same hay at 25 percent moisture have identical dry-matter protein but very different as-received protein, because water dilutes everything else in the sample. Comparing an as-received figure for one feed against a dry-matter figure for another produces a difference that is pure bookkeeping.
The rule is simple: compare like with like, and convert to dry matter whenever you are comparing ingredients or building a ration. Convert back to as-received when you are weighing material out, because that is the form the material physically arrives in. The grid on this page prints both at once so the gap between them is always visible. If you need the moisture figure itself from wet and dry weights, the percent error calculator is not the tool for that, but a straightforward mass loss ratio is, and the mass percent calculator will handle the general form of the same arithmetic.
Where This Sits in a Wider Analysis
Crude protein is one line of a proximate analysis, and it means little on its own. The same sample is normally reported with moisture, crude fat, crude fibre, ash and a nitrogen-free extract calculated by difference. Protein quality — the amino acid balance and its digestibility — is a separate measurement entirely, and a high crude protein figure tells you nothing about it.
If your nitrogen figure came from a titration, the titration calculator handles the volumetric step that precedes it, and Kjeldahl finishes with exactly that kind of acid-base titration. For expressing a component as a share of a formula rather than a sample, the percent composition calculator works from the chemical formula instead. Where a protein has a known sequence and you need its actual molecular mass rather than a bulk composition, the protein molecular weight calculator is the relevant page, and for laboratory enzyme work the enzyme activity calculator normalises activity to protein content.
Arb Digital publishes hundreds of free calculators across chemistry, maths, finance and marketing — no sign-up, no limits. If something you need is missing, tell us and we will look at building it.
Browse All Free Tools Suggest a ToolCommon Mistakes to Avoid
- Reporting a protein percentage without the factor — 2.72 percent nitrogen is 17.00 percent protein at 6.25 and 17.35 at 6.38, and the reader cannot tell which you used.
- Mixing bases in a comparison — an as-received figure against a dry-matter figure exaggerates or hides a real difference by whatever the moisture happens to be.
- Treating crude protein as true protein — non-protein nitrogen is counted in full, so the result is an upper bound.
- Applying a commodity factor to a mixture — a blended feed has no single characteristic nitrogen fraction, which is one reason the general factor exists.
- Assuming a high figure means high quality — crude protein says nothing about amino acid balance or digestibility, which are measured separately.
Related Free Tools From Arb Digital
For direct protein measurement from a spectrophotometer rather than from nitrogen, use the protein concentration calculator. The mass percent calculator and percent composition calculator cover the general composition arithmetic, the titration calculator handles the volumetric finish of a Kjeldahl determination, and the protein molecular weight calculator works from a sequence. Growers comparing harvest quality across fields may also want the crop yield calculator. The full free online tools hub lists everything else.
Frequently Asked Questions
Crude protein is an estimate of protein content obtained by measuring the total nitrogen in a sample and multiplying it by a conversion factor. It is called crude because every nitrogen-containing compound in the sample contributes, whether or not it is protein.
Because 6.25 is the reciprocal of 0.16, and the general method assumes protein is 16 percent nitrogen by mass. Real proteins range from about 15 to 18 percent nitrogen, which is why commodity-specific factors exist.
Use a commodity factor such as 5.70 for wheat flour or 6.38 for dairy when the question is nutritional accuracy for a single ingredient. Use the general 6.25 when a specification, contract or regulation names it, and always state which you used.
Divide the as-received percentage by the dry matter fraction. At 10 percent moisture the dry matter fraction is 0.90, so 17.00 percent as received becomes 18.89 percent on a dry basis.
Yes. Urea, nitrate, ammonium salts, free amino acids and nucleic acids all contain nitrogen and are all counted as protein by the method. This makes crude protein an upper bound rather than a symmetric estimate.
Yes, that is the second mode on this page. Divide the crude protein percentage by the factor. It is the correct first step when comparing two products that were quoted using different factors.
Both measure total nitrogen but by different routes. Kjeldahl digests the sample in acid and finishes with a titration, while Dumas combusts it and measures nitrogen gas. Results are usually close, and both feed the same conversion factor.
Not on its own. Crude protein reports quantity inferred from nitrogen and says nothing about amino acid balance or digestibility, which are measured by separate methods and reported separately.
This calculator is provided for education and general reference. It describes how a published conversion is computed and is not nutritional, veterinary, feed formulation or analytical guidance; follow the method, specification and quality procedures issued by your own laboratory or regulator.
