Enzyme activity calculator explained
Suppliers specify enzymes by activity units per gram or per milliliter, but most protocols call for a target activity per milliliter of final solution. This calculator multiplies the desired activity {desiredactivity} by the final volume {desiredvolume} and divides by the stock potency {stockactivity} to tell you how much solid or concentrate {enzymemass} to add.
Use it when preparing lysis buffers, digestion cocktails, or QC blends so every batch hits the same U/mL specification regardless of lot potency.
How the conversion works
Activity units behave like concentration:
where is the required activity per milliliter (U/mL), is the total volume (mL), and is the supplier's potency (U/mg or U/mL). The calculator assumes consistent activity units and outputs either mass or volume depending on how you enter .
Units and conversions
| Quantity | Units | Notes |
|---|---|---|
| U/mL | Desired activity in the working solution. | |
| mL | Total batch size. | |
| U/mg or U/mL | Supplier potency; choose the unit that matches your input (mass vs. volume). | |
| mg or mL | Mass or volume of concentrate to add. |
Worked examples
- Solid lysozyme
Need 50 mL of lysozyme buffer at 100 U/mL. Stock lyophilized powder is rated 20,000 U/mg.
Weigh 0.25 mg (or 250 µg) and dissolve before bringing the volume to 50 mL.
- Liquid restriction enzyme
Prepare 500 mL of reaction mix at 5 U/mL using a 1,000 U/mL stock supplied in glycerol.
Pipette 2.5 mL of stock enzyme into the buffer, then add solvent to reach 500 mL total.
Tips and pitfalls
- Always keep potencies in the same temperature units the vendor used; some enzymes lose units if warmed.
- Correct for storage buffer volume when adding viscous glycerol stocks, as they can skew final volume.
- Record both the calculated amount and the lot-specific potency in your lab book so you can trace future discrepancies.
- If activity decays over time, adjust based on QC assays before using this calculator.