Units of activity (U) are typically used to describe enzyme catalytic activity, where a unit (U) refers to the amount of enzyme that catalyzes the conversion of 1 micromole (μmole) of substrate per minute. Thus, 1 enzyme unit (U) = 1 μmol/min, where μmol refers to the amount of substrate converted. Because each enzyme has a unique substrate, a unit of activity is different for one enzyme versus another. In addition, the experimental conditions under which enzymatic activity is determined must be specified (e.g., substrate concentration, temperature, pH, ionic strength, etc.). The SI unit of enzyme catalytic activity is the katal, however, it is less commonly used in practice. 1 katal = amount of enzyme that catalyzes the conversion of 1 mole of substrate per second (1 katal = 1 mol/s). Thus, 1 Unit = 1.667 × 10^-8 katal and 1 katal = 6.00 × 10⁷ Units.

It is also important to note that other units of activity exist for various biologically active molecules, and each is defined in a unique way according to a standard established by experts in the field. The calculator below may also be used in these other instances where the units of activity are used in a similar manner to that described above for enzymes. For example, heparin is a naturally found anticoagulant that is present in basophils and mast cells. Upon release from these cells, heparin binds to antithrombin III which causes the inactivation of thrombin and other clotting factors. In this fashion, heparin prevents the formation of blood clots. The physiological significance of heparin as an anticoagulant is being debated. Nevertheless, intravenous administration of heparin has widespread use for its anticoagulation properties. In addition, heparin has tremendous value and use in preventing blood clotting outside of the body, such as for blood samples drawn for clinical analysis of blood components. Because of its ability to prevent blood clotting, a unit of heparin is defined as the amount of heparin required to keep 1 mL of cat's blood fluid for 24 hours at 0 °C. When the solid form of heparin is purchased, the unit of activity is typically noted in Units/mg. Commercial preparations of heparin solutions typically report the activity in Units/mL.

If you wish to perform dilution factor or fold dilution calculations for solutions with units per volume concentration units (e.g., mU/mL, μU/μL, μU/L, etc.), use our Dilution Factor Calculator - Units per Volume.

When the solid form of an enzyme (such as collagenase) or haparin is purchased, the activity unit is usually noted in Units/mg. If you are starting with the solid enzyme/haparin and wish to make a solution with a given enzyme/heparin activity per unit volume, use our Units per Volume Solution Concentration Calculator.

Additional dilution calculators are also available and are suited to more specialized applications (see here).

For convenience, this calculator allows you to select different volume and concentration units, and the necessary conversions are carried out for you to obtain the value of the blank cell in the desired unit. Please note that the symbol U refers to Unit and, by extension, the following can be defined: fU (femtoUnit), pU (picoUnit), nU (nanoUnit), μU (microUnit), and mU (milliUnit).