How this calculator works
Multiply target units per millilitre by the final volume, then divide by the preparation's units per milligram. Specific activity must describe the material being weighed: units per mg of solid and units per mg of protein are different when the powder contains salts or stabilizers. Use the supplier's assay conditions and lot-specific activity. Dissolve and make up to the stated final volume.
Formula
Total activity (U) = desired activity (U/mL) x final volume (mL). Mass (mg) = total activity/specific activity (U/mg).
Worked example
Preparing 10 mL at 20 U/mL needs 200 U. At 100 U/mg, weigh 2 mg. A 1 mg preparation rated 50 U/mg in 5 mL gives 10 U/mL.
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Frequently asked questions
What does one unit mean?
A conventional enzyme unit represents 1 micromole converted per minute under specified assay conditions; supplier definitions can differ.
Can I compare two suppliers directly?
Only when the substrate, temperature, pH and unit definitions match.
Does this measure retained activity?
No. Storage losses and inhibitors require an activity assay.