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Enzyme Activity Calculator

Find the mass of an enzyme preparation needed for a target activity concentration and final volume.

By ChemistryCalc•Updated October 28, 2026•References
Your calculationLive results · change any value

Results update as you type. Choose units beside a measurement; changing units converts the current value.

U/mL
U/mg

On this pageCalculatorHow to useFormula & methodWorked exampleActivity is an assay-dependent quantityUse the same assay basis on both sidesFrequently asked questionsRelated calculatorsSources

01 Set up your calculation

  1. Enter the target enzyme activity per milliliter.
  2. Enter the preparation’s specific activity in units per milligram.
  3. Specify the final total volume and read the required preparation mass.

02 The relationship

Required mass (mg) = desired activity (U/mL) × final volume (mL) / specific activity (U/mg).

For the assumptions behind these results, read the explanation below. You can also explore Michaelis–Menten Equation and Solution Dilution.

WORKED EXAMPLE

Follow the numbers

To prepare 10 mL at 8 U/mL from a preparation rated 200 U/mg, the target is 80 U and the mass is 80/200 = 0.4 mg.

03 Activity is an assay-dependent quantity

An enzyme unit commonly represents conversion of one micromole of substrate per minute under specified assay conditions. A preparation’s rated activity therefore belongs to a particular substrate, temperature, pH, and measurement method.

Specific activity in U/mg is activity divided by mass of the stated preparation or protein basis. Check the product or assay definition carefully; U/mL describes a liquid stock concentration and cannot be entered directly as U/mg.

04 Use the same assay basis on both sides

The mass calculation assumes the preparation retains its stated activity in the final conditions. Storage, dilution, inhibitors, and differences between the rating assay and your experiment can change the effective activity.

Final volume means the total prepared volume. The result gives an amount to use within an established experimental protocol; it does not identify a suitable buffer, incubation time, or biological dose. Very small masses may require a calibrated stock solution for accurate transfer.

Questions from the notebook

Is an activity unit the same as enzyme mass?

No. Different preparations can have very different activity per milligram.

Can I enter a liquid stock in U/mL here?

No. This mass calculation needs specific activity in U/mg. Use an activity dilution calculation for a liquid stock.

Can the target be zero?

Yes. Zero desired activity corresponds to zero enzyme mass in this model.

References & reading

Our explanations are written for ChemistryCalc. Read how we build and check our tools. These calculations are educational estimates where the underlying model requires assumptions.