Crude Protein Calculator
Turn Kjeldahl nitrogen and sample weight into a crude protein percentage.
Eleven Jones factors, nine volume units and seven weight units.
Updated October 18, 2026
What the page calculates
Crude protein is the protein figure you get from measuring nitrogen. A Kjeldahl digestion converts the nitrogen in a sample into ammonium, a titration measures how much ammonium came over, and this page carries those titrant readings through to two numbers: the nitrogen content of the sample, and the crude protein percentage that follows from it.
That makes it the page for whoever is standing at the bench with a burette reading in hand. Feed and food laboratories checking incoming soybean meal, cereal or milk powder; a mill verifying the protein figure on a supplier's certificate of analysis; a student working through why the nitrogen figure and the protein figure are two different things. The two are linked by a conversion factor that you choose, and that choice is part of the answer you report.
There are two ways into the calculation. Enter the blank and sample titrant volumes, the titrant concentration, the acid factor and the sample weight, and the nitrogen content is worked out for you. Or, when the nitrogen was measured by something other than a titration — combustion analysis, near-infrared, another laboratory's report — type that nitrogen value in with the sample weight and the page goes straight to the protein figure.
Two equations
Nitrogen from the titration, then protein from the nitrogen
9 volume units, 7 weight units
Volumes normalised to millilitres, weights to grams
11 conversion factors
From 4.59 for red seaweed to 6.38 for milk
Quick start
Work through the five entries below in any order — only the boxes you leave empty are solved for you. The last step tells you what the two answers mean.
Take the blank and the sample titration readings
Run the blank alongside the sample exactly as your method requires. Record the titrant volume each one consumed, in the same unit — millilitres on most burettes, but litres, cubic centimetres and fluid ounces all work.
Enter the two volumes
The blank goes in the first box because it is the volume that gets subtracted. That means the blank is usually the larger of the two numbers. If the nitrogen box comes back negative, the two readings are the other way round for this convention, and swapping them fixes it.
Enter the titrant concentration and the acid factor
Concentration is in moles per litre. The acid factor records how many reacting units each mole of titrant supplies — 1 for hydrochloric acid or for a sodium hydroxide back-titration, 2 for sulfuric acid. If your bottle is labelled by normality, enter that number as the concentration and leave the factor at 1.
Enter the sample weight
The weight you actually digested, in whatever unit the balance reported. Micrograms, decagrams and pounds are all accepted; the arithmetic behind the page always ends up in grams.
Choose the protein conversion factor
The standard factor of 6.25 fits anything whose protein source is unknown or mixed. Otherwise pick the entry for your material — soybean, rice, oats, milk, fish, peanuts, red seaweed — because the factor is part of the result, not a display preference.
Read the two answers
The nitrogen content box is what the titration delivered, as a percentage of the sample's mass. The crude protein percentage multiplies that nitrogen by the conversion factor and by the dilution factor.
A worked example
A 0.25 g portion of a mixed feed is digested and titrated with 0.1 mol/L titrant. The blank took 21 mL and the sample took 1 mL, so the titration itself was responsible for 20 mL.
The 20 mL of 0.1 mol/L titrant that the sample consumed stands for 0.002 mol of nitrogen, about 28 mg, and that mass is 11.2056% of the 0.25 g portion. The page shows 11.2056 in the nitrogen box and 70.035 in the protein box. The protein figure is the one to report for a feed or a food label, and it travels with the factor that produced it: the same nitrogen read with the soybean factor of 5.71 is 63.984%.
The calculation behind the result
Two short equations do all the work here. The first turns the titration into nitrogen; the second turns nitrogen into an estimate of protein.
Where the 1.4007 comes from
The constant is not a fudge factor. It is the molar mass of nitrogen with the two unit adjustments of the equation already folded in:
The 100 turns a fraction into a percentage, and the 1000 turns moles per litre against millilitres of titrant into a mass on the balance. That is why the arithmetic wants millilitres and grams: enter litres or pounds and the page converts them first, so the answer does not depend on the unit you happened to read.
The acid factor and the dilution factor
Both factors are corrections for how the bench work was done, and both change the answer if they are wrong.
The acid factor converts molarity into reacting units, because the titration cares about protons or hydroxide ions rather than molecules: normality is molarity multiplied by this factor. Hydrochloric acid supplies one, sulfuric acid supplies two, and a sodium hydroxide back-titration supplies one. If you prepare the titrant yourself and prefer to work from the weighed mass and the equivalent weight, the Normality Calculator turns those numbers into a normality you can enter as the concentration with the factor left at 1.
The dilution factor matters when only part of the digest reaches the burette. Titrate a 25 mL aliquot out of a 250 mL digest and the factor is 10. It multiplies the protein figure only: the nitrogen box reports what the portion you titrated contained, while the protein box is scaled back up to the whole sample.
Which direction the page solves
The titration relation is a single statement about six quantities, so it runs both ways. Fill any five of the six boxes — the two volumes, the concentration, the acid factor, the sample weight, the nitrogen content — and the sixth is worked out. In practice the acid factor starts at 1 and the two volumes, the concentration and the sample weight are the entries you type, so the nitrogen content is what appears. Measure the nitrogen some other way, enter it, and the same relation will instead tell you the titrant concentration or the sample weight that its value implies.
What is always an output
The crude protein percentage is calculated, never entered. It is the product of the nitrogen content, the conversion factor and the dilution factor, so there is nothing to type into it — change one of those three and it follows.
Choosing a protein factor
The conversion factor is the number of grams of protein that one gram of nitrogen is taken to stand for. It is the reciprocal of the material's nitrogen content: 6.25 assumes 16% nitrogen, 5.71 assumes about 17.5%, and 4.59 assumes about 21.8%. Choosing a factor is therefore a statement about how much nitrogen your material's protein carries.
Proteins from animal sources generally carry a little less nitrogen than the 16% average and sit at the top of the factor range; plant proteins carry more, which is why their factors are lower. That is the whole logic behind the menu:
| Menu entry | Factor | Read it when the sample is |
|---|---|---|
| Milk | 6.38 | Milk and dairy protein, taken to contain about 15.7% nitrogen |
| Standard factor, Meat, Eggs | 6.25 | Anything unknown or mixed; the 16% average |
| Rice | 5.95 | Rice, taken to contain about 16.8% nitrogen |
| Oats | 5.83 | Oats, taken to contain about 17.2% nitrogen |
| Soy Bean | 5.71 | Soybean and soybean meal, about 17.5% nitrogen |
| Fish | 5.6 | Fish and shrimp, about 17.9% nitrogen |
| Peanuts | 5.46 | Peanuts and peanut meal, about 18.3% nitrogen |
| Cereal products | 5.4 | Cereal work that assumes about 18.5% nitrogen |
| Red Seaweed | 4.59 | Red seaweeds, the highest nitrogen content in the menu at about 21.8% |
The same sample, read four ways
The nitrogen content never moves when you change the factor — only the protein figure does. Taking the 11.2056% nitrogen from the worked example:
| Factor used | Crude protein reported |
|---|---|
| 6.38 (Milk) | 71.4917% |
| 6.25 (Standard) | 70.035% |
| 5.71 (Soy Bean) | 63.984% |
| 4.59 (Red Seaweed) | 51.4337% |
A swing of twenty percentage points
Nothing about the sample changed, only the assumption about its protein. If a factor is not written next to a protein figure, you cannot tell whether 63% and 70% describe the same meal or two different ones. Report the factor with the result.
When the material is unknown or the sample is a mixture, the general factor is the honest choice, and it is also the one most methods and certificates use for comparison. Where a specification or a customer names a factor, use theirs.
When the result looks wrong
The nitrogen and protein boxes can show a warning instead of an answer, and each warning points at a specific kind of mistake. The number stays visible in every case, so you can see how far off it is.
Nitrogen above 20%, protein above 100%
The two ceilings sit in different places, and the arithmetic is what puts them there. Protein is roughly a sixteenth nitrogen, so at the standard factor the protein figure passes 100% as soon as nitrogen passes 16% — a nitrogen content in the high teens describes a sample that is nearly all protein. The nitrogen box itself turns red above 20%, where the result can no longer be protein at all.
The usual causes are a nitrogen-rich compound that is not protein — urea is the classic one — or numbers that do not come from the same run: a sample weight that is smaller than the portion actually digested, or a volume difference that is larger than the titration delivered.
A negative nitrogen content
The blank volume is subtracted from the sample volume, so a negative nitrogen content means the sample reading was the larger one. Either the two readings are in the wrong boxes for this convention, or the method titrates the other way round: swap them and the sign corrects itself.
A nitrogen content of exactly zero
That is a real answer, not a failure. Both titrations consumed the same volume, so the difference contains no nitrogen: the protein figure follows as zero. A genuine blank subtraction looks like this whenever the sample carries no measurable nitrogen.
An empty box with an explanation under it
Some directions of the calculation are genuinely undefined. Ask for the titrant concentration from two identical volumes and there is nothing to divide by; ask for the sample weight from a nitrogen content of zero and the same thing happens. The box stays empty and says so. Enter the missing value or clear the one you no longer mean.
Practices that keep numbers reproducible
The arithmetic here is exact. Almost all of the variation between two laboratories comes from the bench work and the records that go with it, and these are the habits that keep those under control.
- Run a blank with every batch. The blank captures the nitrogen that reagents, catalyst and water contribute, and it is subtracted from every sample. A blank that drifts between batches quietly shifts every result with it.
- Read both volumes in one unit. Millilitres for both, or litres for both; the page converts, but a mismatched pair is the easiest way to produce an answer that looks plausible and is wrong by a factor of a thousand.
- Weigh the portion you actually digested. The sample weight divides the whole result, so a 1% error in weighing is a 1% error in the protein figure — the same size as most of the differences you are trying to measure.
- Track the aliquot. If only part of the digest reaches the burette, the dilution factor is not optional. Divide the total digest volume by the volume you titrated; the Dilution Factor Calculator does that from the volumes you measured.
- Standardise the titrant and write the value down. A concentration that has aged, or a factor entered from memory rather than from the label, matters as much as a mis-titration and is much harder to spot afterwards.
- Keep the factor with the sample record. Protein figures are compared across suppliers, seasons and laboratories. A number without its factor cannot be compared with anything.
Questions
What if my blank titration uses no titrant at all?
The blank box has to hold a positive volume, because it is the number being subtracted. If your blank genuinely consumed nothing measurable, the blank reading is whatever your burette resolves to, and entering that small value is closer to the truth than skipping it.
If you would rather not enter a blank at all, work the subtraction out yourself and type the nitrogen content that comes from it — the sample weight and the nitrogen content are enough to reach the protein figure, and the titration boxes can stay empty.
Can I use this with a nitrogen value from another method?
Yes, and that is the shortest route through the page. Enter the nitrogen content and the sample weight, leave the titration boxes empty, choose your conversion factor, and the crude protein percentage appears. The dilution factor still applies when that nitrogen figure came from a portion of a diluted digest rather than from the whole digest.
What the page cannot tell you is whether that nitrogen value is any good. It came from a different instrument and a different set of assumptions, so compare it with the titration result before treating the two as interchangeable.
Does the conversion factor change the nitrogen content?
No. The nitrogen box is what the titration measured, and it stays exactly where it is whatever factor you choose. The factor and the dilution factor act on the step that turns nitrogen into protein, which is where the assumption about the material belongs.
How far to trust the result
- •Crude protein counts every nitrogen the digestion released, not only the nitrogen in protein. Free amino acids, nucleotides, urea and ammonium salts are all included and all read as protein. That is the meaning of the word crude: FAO's review of protein analysis methods sets out the same caveat and notes that protein nitrogen actually varies from about 13% to 19% between materials, which is where factors from 5.26 to 7.69 come from.
- •The factor you choose is part of the result, not a cosmetic setting. Using the general 6.25 where a specific factor applies moves an individual food's protein figure by roughly -2% to +9% relative to the specific factor — small next to the difference between feeding a soybean meal and a fish meal, large enough to matter in a specification.
- •This page performs the arithmetic of the Kjeldahl relation. It has no way to know whether the digestion was complete, whether the distillation lost ammonia, whether the indicator was read at the right end point or whether the burette was calibrated. A single titration says nothing about the repeatability of the method, so run the sample in duplicate before you trust a difference of a few percent.
- •For labelling, trade, regulatory or contractual work, report what your required method and an accredited laboratory give you, and keep this page for checking, planning and teaching. Do not report more digits than the method supports: the arithmetic runs to many decimals, the measurement does not.
Related Calculators
Chemical Equation Balancer
Balance a chemical reaction and get the smallest whole-number coefficients. Supports bracketed groups such as Fe(OH)3, user-supplied coefficients, and unicode subscripts.
Net Ionic Equation Calculator
Reduce a molecular equation to its net ionic form. Balances the reaction, splits soluble compounds into ions, and names the spectator ions that cancel out.
Atom Calculator
Calculate atomic properties including protons, neutrons, electrons, atomic number and mass number
Electron Configuration Calculator
Get the electron configuration for any element from the periodic table. View atomic number, atomic mass, and valence electrons.
Atomic Mass Calculator
Calculate atomic mass from protons and neutrons. Get atomic mass in various units including atomic mass units (u), kilograms, and more.
Electronegativity Calculator
Calculate electronegativity difference between two elements and determine bond type (ionic, polar covalent, or nonpolar covalent).