Enzyme Activity Calculator
Work out the enzyme mass that gives a buffer its target activity.
Four volume units, five mass units, and a reverse solve for each of the four values.
Updated October 19, 2026
Introduction
This page turns a target activity into a mass you can put on a balance. Give it the activity you want the finished buffer to have, the activity your enzyme preparation carries per unit of mass, and the volume you are preparing, and it returns the enzyme mass to weigh out. Fill in any three of the four boxes — including a mass you have already weighed — and it works out the fourth.
It is written for the moment a protocol says something like “prepare the assay buffer at 4 units/ml” and your enzyme arrives as a powder or a preparation whose certificate quotes activity against mass. It also covers the reverse question, the one that comes up after the fact: you weighed a certain amount into a certain volume, so what activity did you actually prepare?
Two different denominators, and only one of them can be weighed
An activity per volume () describes a solution: how much activity a millilitre of that liquid contains. An activity per mass () describes the preparation itself: how much activity each milligram carries. Multiplying the first by a volume gives a total amount of activity, and dividing that total by the second gives a mass.
Entering a per-volume number in the per-mass box, or the other way round, is the one mistake that moves the answer by a factor of a thousand or more, so it is worth checking which denominator is printed on the certificate before you type.
Any three of four values
Target activity, stock activity, volume, mass
4 volume units, 5 mass units
µl, ml, cl, L and µg, mg, g, dag, kg on the activity boxes too
No fixed mode
The same balance runs in every direction
Quick start
Enter the activity your protocol asks for
This is a concentration, so it belongs in the box labelled with the volume unit — for example 4 units/ml. Keep the per-volume unit the protocol uses. If a protocol is written per litre and you work in millilitres, switching the unit rescales the number rather than silently changing what it means.
Enter the activity of your stock, as its certificate states it
Take the specific activity from the certificate or from your own assay and pick the unit it is printed in. It is normal for one preparation to be quoted per milligram and another per microgram, and the unit you choose is part of the number: 0.25 units/µg is the same preparation as 250 units/mg.
Enter the volume you are preparing
The final volume of the buffer, in microlitres, millilitres, centilitres or litres. A larger volume needs proportionally more enzyme, so this is the box to change when you scale a protocol up or down.
Read the enzyme mass
Preparing 250 ml at 4 units/ml from a stock of 20 units/mg asks for:
The box that was left empty is the one the page solved, and it stays editable: type a different mass into it and the page moves the derived answer to another box instead.
Sanity-check the mass against the volume
50 mg in 250 ml is 0.2 mg/ml, which is an ordinary amount of protein for an assay buffer. If the mass comes out comparable to the volume itself — grams of powder for millilitres of buffer — one of the two activity numbers is probably on the wrong scale, and the page turns that box red to say so.
The activity balance
Everything on the page comes from one statement: the activity the finished buffer must contain equals the activity the weighed enzyme brings with it.
The same balance, rearranged
Because it is a single balance, each of the four values can be the unknown, which is why any three boxes are enough:
Unit scales run in opposite directions
Plain volume and mass scale the way you expect — 1 ml is 1000 µl, 1 g is 1000 mg. The two activity boxes scale the other way, because the unit sits in the denominator: the same solution that reads 8 units/ml reads 0.008 units/µl, and 1 unit/µg is 1000 units/mg. Two neighbouring entries in those menus are always a factor of a thousand apart, which is the usual explanation when an answer looks wrong by exactly that much.
A zero denominator is not an answer
A zero target activity is meaningful — a buffer with no enzyme in it needs no enzyme mass — and so is a zero mass, which asks what activity no added enzyme delivers. Both are accepted and both work backwards:
What has no finite answer is a zero divisor. Asking for a positive mass while the target is zero, or a positive target from a zero mass, would need the stock to carry an infinite amount of activity per milligram. The page leaves that box blank with a short explanation instead of printing an impossible number.
When the number cannot be dissolved
More solid than solution
A millilitre of dilute aqueous buffer weighs about a gram, so the whole final volume weighs roughly milligrams. When the required mass crosses that line the preparation is physically impossible — it asks you to dissolve more enzyme than the liquid it goes into weighs. The digits stay on screen so you can see the size of the error, and the box is flagged:
For example, a target of 1000 units/ml over 10 ml from a stock of 0.5 units/mg evaluates to 20,000 mg — twenty grams of powder for ten millilitres of buffer:
A much more common route to that warning is a unit scale: a certificate read as units/mg when it is printed per microgram is a thousand times too small, and the mass comes out a thousand times too large.
Worked examples
What did I just prepare?
Situation: A buffer is already made: 12 mg of a preparation carrying 150 units/mg was dissolved to a final volume of 400 ml. The assay needs an activity of about 4 units/ml, and you want to know whether it is close before running it.
Entered
The page returns
In practice: 4.5 units/ml is about 12% above the 4 units/ml the assay calls for. Either dilute the buffer or use it where that small excess does not matter — the page gives you the number to decide with, not the dilution itself.
A certificate quoted per microgram
Situation: A small vial is labelled 0.25 units/µg. The protocol asks for 500 µl at 10 units/ml, so the mass needed will be tiny — small enough that it is worth knowing the exact figure before deciding how to weigh it.
Entered
The page returns
First put the certificate on the per-milligram scale the balance works in:
Then balance the activity, with the volume read as 0.5 ml:
Switching the mass box to micrograms shows the same result as 20 µg; the value behind it is one and the same, only the scale on screen changes.
In practice: 20 µg is far below what an ordinary balance can weigh directly. Weigh a larger amount of the same preparation, dissolve it to a known volume, and dilute that stock to the target activity — the dilution step is where the volume units have to be kept straight.
Getting a mass you can trust
Use the certificate's own number
A certificate states the assay its activity value refers to, and activity is defined by that assay — substrate, pH and temperature included. Enter the value as printed rather than converting it to a figure from a different method, and if you re-assay the stock yourself, use your own result.
Check the scale before believing the answer
Neighbouring activity units differ by a factor of a thousand. Two quick checks catch most mistakes: the mass should be a small fraction of the volume’s own mass (1 ml ≈ 1 g), and switching the activity box between per µg and per mg should move the number by exactly 1000.
When the mass is too small to weigh
Below the resolution of your balance, weighing is the wrong route. Weigh a convenient amount, dissolve it to a known volume, and then dilute that stock to the target activity — the solution dilution calculator solves the concentration and volume that step needs.
Re-check old or thawed stocks
Activity can fall during storage, freeze–thaw cycles and handling, and no label stays accurate forever. If a preparation has been around for a while, an activity measurement of your own beats the number printed on the vial; the page takes either one.
One more habit worth keeping: after preparing, note both the mass and the activity you were aiming for. If the assay later disagrees with the calculation, that record tells you whether the problem was in the weighing or in the preparation itself.
Questions
My certificate quotes units per millilitre. Can I type that into the stock activity box?
No — that box needs activity per mass, and a concentration in units/ml says nothing about how much activity a milligram of the preparation carries. Convert it first with the protein concentration of the stock solution:
If the protein concentration is unknown, the per-volume value cannot be turned into a mass. Work by volume instead: dilute the stock solution to the target activity rather than weighing a portion of it.
Why does the enzyme mass stay blank instead of showing a very large number?
That box is the one the page is trying to derive, and the derivation divides by one of the other values. When that divisor is zero there is no finite answer — a positive target from a zero mass, or a positive mass with a zero target, would require an infinite specific activity. Rather than print “infinite”, the box stays empty with a note naming the zero it ran into, and the values you typed are left untouched.
Is a zero target activity, or a zero mass, a real answer?
Both are. A target of zero units/ml means a buffer prepared with no enzyme activity, and the mass to weigh is zero; a mass of zero means no enzyme was added, so the activity is zero. Either can be entered and either can be solved for. The combination that makes no sense is a zero target together with a non-zero mass, because it would demand a stock with no activity at all — the page flags the derived stock activity rather than pretending it is valid.
Where it stops
- The mass is of the preparation, not of pure enzyme. A certificate’s units per milligram already includes whatever else is in the vial, so the number on screen is the amount of that preparation to weigh, not the amount of active protein it contains.
- Activity is a measured quantity, not a constant. It depends on the assay it was measured with and falls with storage, freeze–thaw cycles and handling. The page multiplies the number you give it; it cannot tell you whether that number still applies today.
- The dissolvability check assumes a dilute aqueous buffer. It treats 1 ml of solution as about 1 g. Solvents and solutions with a very different density, or large amounts of other additives, shift that bound.
- It balances activity, volume and mass only. Molar concentrations, substrate and inhibitor levels, pH and temperature are outside this balance. When the prepared buffer has to meet a specification — a regulated assay, a clinical method or a batch release — confirm the activity with your own measurement rather than with this calculation alone.
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