Enzyme Activity Units Calculator
Enzyme activity is quantified in international units (U), where one unit converts one micromole of substrate per minute under defined conditions. Enter the micromoles of substrate converted, the reaction time in minutes, and the total protein mass in milligrams. The calculator returns total activity in units, the reaction rate in micromoles per minute, and the specific activity in units per milligram of protein, the key metric for tracking enzyme purity through a purification workflow.
Enzyme activity formula
Rate (micromol/min) = substrate converted / time
Total activity (U) = substrate converted / time
Specific activity (U/mg) = total activity / protein mass
One unit (U) is defined as one micromole of substrate converted per minute. Substrate is in micromoles, time in minutes, protein in milligrams. Time and protein mass must be positive.
Enzymology context
- The international unit (U) is the most widely used measure of catalytic activity in biochemistry.
- The SI unit is the katal (mol/s); 1 U equals approximately 16.67 nanokatal.
- Specific activity (U/mg) increases as an enzyme is purified away from other proteins.
- Always report the assay conditions (temperature, pH, substrate concentration) alongside activity values.
- Use initial-rate (linear) data so the calculated activity reflects true enzyme velocity.
Enzyme activity: frequently asked questions
What is one enzyme unit?
One enzyme unit (U), the international unit, is the amount of enzyme that converts one micromole of substrate per minute under defined assay conditions. Total activity in units equals the micromoles of product formed divided by the reaction time in minutes.
What is specific activity?
Specific activity is enzyme activity per unit mass of total protein, expressed in units per milligram (U/mg). It equals total activity in units divided by the milligrams of protein in the assay. Specific activity rises during purification as contaminating protein is removed.
What is the enzyme activity formula?
Total activity (U) = micromoles of substrate converted divided by reaction time in minutes. Specific activity (U/mg) = total activity divided by protein mass in mg. Enter substrate converted in micromoles, time in minutes, and protein in milligrams.
How does the unit U relate to the katal?
The katal (kat) is the SI unit of catalytic activity: one mole of substrate per second. One unit (U) equals 1/60 micromole per second, which is approximately 16.67 nanokatal. This calculator reports the traditional unit U used in most biochemistry protocols.
Why is specific activity useful?
Specific activity measures enzyme purity and catalytic efficiency relative to total protein. Tracking specific activity across purification steps shows how effectively non-enzyme protein is being removed while preserving the target enzyme's activity.
Official sources
- National Institute of Standards and Technology: SI Units, including the katal.
- National Center for Biotechnology Information: Enzymes (NCBI Bookshelf).
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.