Protein Solubility Calculator
Turn blank and sample titers into a protein solubility percentage.
Microgram-to-pound sample weights and four titrant concentration units.
Updated October 19, 2026
Introduction
This page turns a Kjeldahl-type titration into a protein solubility percentage: the share of the sample that the assay recovered as soluble protein, read off the difference between two titrant volumes.
The chemistry behind it is a nitrogen determination. The sample is digested, the nitrogen is distilled into a receiver and titrated, and the volume the titration takes is proportional to the nitrogen it carried. A second titration — the blank — is run on the reagents alone, because they consume titrant too. Subtracting the blank removes that background, which is why the four numbers you enter are a blank volume, a sample volume, the strength of the titrant and the mass of sample the run started from.
That makes this a working page for people who already have a titration in front of them: dairy, food and feed laboratories checking how much protein stays in solution, method development where a target has to be hit, and teaching labs where the arithmetic needs to be shown rather than trusted. It solves in both directions, so whichever of the five boxes you leave empty is the one the page works out for you.
5 boxes, 1 left open
The page solves for whichever one you leave empty
8 weight units
Micrograms to pounds, plus four titrant-strength units
43.78125
The fixed 1.401 × 6.25 × 5 the assay is built on
Everything the page needs is a titration that has already been run: the two volumes, the strength of the titrant and the mass the run started from. It reports the protein that stayed in solution as a percentage of that mass, and it uses the standard 6.25 protein factor — a nitrogen-only result or a product-specific factor is a separate conversion.
Quick start
Blank titer (B)
The volume the blank titration took, in millilitres — the reagents alone, no sample. This is the number that gets subtracted, so it has to come from the same batch of reagents as the sample run. A typical blank sits close to the sample volume; in the example below it is 7.0 mL.
Titer (T)
The volume the sample titration took, also in millilitres: 6.0 mL in the example. Only the difference between the two matters, so both volumes need to come from the same burette and the same endpoint judgement rather than from two different readings of the glassware.
Normality of NaOH (N)
The strength of the sodium hydroxide you titrated with, taken from the certificate or from your own standardisation: 98 g/L in the example. The box accepts grams per litre, grams per millilitre, milligrams per litre or milligrams per millilitre, and 98 mg/mL is exactly the same concentration as 98 g/L.
Effective weight (M)
The mass of sample the titration was run on, in grams by default: 55 g in the example. The menu covers micrograms, milligrams, grams, decagrams, kilograms, grains, ounces and pounds, so the number can be typed exactly as the balance printed it.
Read the protein solubility percentage (P)
With those four entered, the percentage appears. It is the assay formula with its fixed multiplier of 43.78125:
The percentage is reported to two decimals, so the value above is what the box shows. Read it as: on this sample, the assay recovered 78.01% of the protein the titration accounts for.
Leave the box you want answered
All five boxes accept a number, and the page solves for whichever one is left empty. Enter a target percentage and leave the sample weight open and you get the mass to weigh out; enter a measured percentage and leave the blank titer open and you get the volume the run should have taken. Fill in all five and the page keeps every value as you typed it.
The formula and its fixed multiplier
One equation connects the five boxes. It reports the protein percentage directly, so nothing has to be converted by hand before or after it.
Where the three factors come from
The multiplier is not a fitted constant; it is three conventions multiplied together, and each one has a meaning you can check.
- 1.401 is the nitrogen equivalent of the titration step. One millilitre of a 0.1 N titrant accounts for about 1.4 mg of nitrogen, and this factor carries the millilitre-to-litre and percentage scaling with it.
- 6.25 is the standard protein factor: protein averages about 16% nitrogen, so nitrogen is multiplied by to estimate protein.
- 5 is the five-fold dilution this assay formula carries. It is fixed here, not adjustable, which matters if your own protocol dilutes differently.
Any of the five boxes can be the answer
The same equation rearranges into four more forms, one for each remaining box. They are what the page uses when you leave that box empty:
Units are normalised before the arithmetic
The equation reads grams and grams per litre, so the readings you enter are converted into those two units first rather than being mixed into the formula:
Each factor is an exact definition rather than a rounded table value — a grain is exactly 0.06479891 g, an ounce is a sixteenth of the international pound, and 1 mg/mL is exactly the same concentration as 1 g/L. Because the conversion happens first, switching a unit rescales only that one box and never changes the percentage: 55 g, 0.055 kg and 0.121254 lb are the same sample mass and return the same answer.
Two things the page refuses to invent
- A percentage above 100 is flagged rather than accepted. No sample is more than 100% protein, so a value such as 437.81% turns red and keeps its digits instead of being hidden — the number itself is the clue about what went wrong.
- A zero difference stays blank. When the blank and the sample have taken the same volume, the difference is zero and a reverse solve would divide by it. The box stays empty with an explanation instead of showing a meaningless value.
Worked examples
Two situations that come up repeatedly in the laboratory: planning a run around a target, and recording the same run in whatever units the instruments reported.
Weighing out for a target percentage
Situation: A method is being set up for a product that should come out near 80%. The titrant is certified at 98 g/L and the blank runs at 7.0 mL. How much sample should go into the flask if the sample titration is expected to take 6.0 mL?
Entered
Result
In practice: Weigh out 53.632 g. Because the percentage is inversely proportional to the mass, a 5% weighing error moves the result by about 4 percentage points at this scale, so use the balance the method calls for.
The same run recorded in other units
Situation: The balance report is in kilograms and the titrant certificate states its strength in milligrams per millilitre. Nothing about the chemistry has changed — only the scales the numbers were written on.
Entered
Result
In practice: Enter the readings as they were printed and let the page convert. If you convert by hand first, you are adding a second arithmetic step — and a misplaced decimal is exactly the error that quietly crosses the 100% ceiling.
Getting a number you can trust
Run the blank with the sample's own reagents
The blank is subtracted, so it only cancels the background if it shares the sample's digestion, distillation and titrant. A blank carried over from another day, or from another batch of sodium hydroxide, moves straight into the difference.
A negative percentage means the two volumes are swapped
If the sample titer is larger than the blank, the difference goes negative and the page shows it in red — 6.0 mL blank against 7.0 mL sample reads −78.01%. That is not a calculation error but a bookkeeping one, and the sign finds it immediately.
Use the 100% ceiling as a unit check
Leave the box on grams but enter the kilogram reading, 0.055, and the page answers 78,010.23% in red. Any 1000-fold unit slip on the sample weight lands far above 100%, so that ceiling is the fastest way to catch it.
Enter the strength your certificate states
The percentage is directly proportional to the titrant strength, so half the value halves the answer and twice the value doubles it. If the sodium hydroxide is not certified, the titration calculator works out its concentration from a standardisation run.
Write the unit next to every number
A reading of 0.055 means 0.055 g, 0.055 kg or 0.055 lb depending on the scale selected, so record the scale with the value. Small unit mismatches — an ounce where a gram was meant — may not cross the 100% ceiling and can pass unnoticed, while the four concentration units hide one another neatly: 98 g/L, 98 mg/mL and 0.098 g/mL are all the same strength.
Questions
Why is my result above 100%?
Because one of the entered numbers is on a different scale from what the formula expects. The percentage is a share of the sample mass, so it cannot exceed 100, and the page leaves the impossible value on screen in red so you can see how far out it is. The usual causes are a sample weight 1000 times too small — 0.055 left in a box set to grams answers 78,010.23% — a titrant strength 1000 times too large, such as 98 typed into a box still set to g/L when the certificate reads 98 mg/L, or a decimal slip in a titer, where a blank of 70 mL instead of 7.0 mL answers 4,992.65%.
The strength box is suddenly empty and mentions a zero difference. What happened?
The blank and sample volumes are equal, so there is no titration difference for the percentage to come from. Working backwards from a percentage then means dividing by zero and the box stays blank with that explanation instead of inventing a value. Check which volume went into which box: the formula subtracts the sample volume from the blank, so a positive percentage always has the larger number in the blank box.
Which protein factor is used, and what if my product needs another one?
6.25 — the general factor, on the grounds that protein averages about 16% nitrogen. It is fixed in this page's multiplier, and for a product whose published factor is different the result shifts in proportion:
| Factor | Published use | The run above reads |
|---|---|---|
| 6.25 | General factor, used by this page | 78.01% |
| 6.38 | Milk and dairy products | 79.63% |
| 5.95 | Rice | 74.27% |
| 5.71 | Soy bean | 71.27% |
Those are the Jones factors offered as a menu in our crude protein calculator, and they can be applied here by multiplying the percentage by your factor and dividing by 6.25 — a soy result of 78.01% becomes . If your method specifies a factor of its own, that conversion is the step to take.
My protocol dilutes differently from the formula. Does that matter?
Yes. The multiplier carries a fixed five-fold dilution, so a protocol whose dilution factor is 10 or 20 needs the result multiplied by that factor and divided by five — a 20-fold dilution gives a result four times larger than the page reports. Work out the ratio with the dilution factor calculator and apply it to the percentage, or follow your own method's formula if it embeds the dilution some other way.
How far to trust it
- One fixed convention. The 6.25 protein factor and the five-fold dilution are built into the multiplier and cannot be changed here, so results are comparable within one method rather than across methods. Official methods may state a different factor, a different dilution or a nitrogen-only result; check the method you are working to before the number leaves the bench.
- It converts four numbers; it does not verify the chemistry. Complete digestion, distillation losses, an over-run endpoint and a drifting titrant all change the real result without changing anything the page can see. The percentage is only as good as the titration that produced the two volumes.
- Both titers are treated as one burette scale. A reading past 100 mL is kept and flagged in red rather than refused, on the assumption that a single burette cannot deliver that much — so a number that large usually points at a unit or a scale mistake rather than at a long titration.
- An empty box means undefined, not zero. When a reverse solve needs a division by a difference of zero or by a zero sample weight, the box stays empty with an explanation. A genuine zero — a blank and sample that took the same volume — is reported as , which is a result rather than an error.
- Treat it as a bench check, not a certificate. For product release, labelling, regulatory submissions or any decision with a cost attached, run the determination under your accredited method and have the result reviewed by a qualified analyst.
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