# Specific Activity Calculator (U/mg protein) Specific activity = U ÷ mg protein (or U/mL ÷ mg/mL): the purity metric of every purification table, with nkat/mg conversion and the kcat/MW purity ceiling. Educational use only. > Canonical page: https://elysiatools.com/en/tools/specific-activity-calculator - **Category:** Education - **Keywords:** specific activity calculator, u mg protein, enzyme purity metric, purification table, protein purification specific activity, nkat per mg conversion, enzyme activity per protein mass, theoretical max specific activity kcat, bradford bca protein assay, biochemistry calculator, lab calculator ## Overview The Specific Activity Calculator finds enzyme activity per milligram of protein using total activity and protein mass, or activity and protein concentrations. It reports the result in U/mg and converts it to nkat/mg for purification records and educational laboratory planning. ## Inputs - **Calculation Mode** (select): Enter totals (U and mg) or concentrations (U/mL and mg/mL) — the ratio is the same. - **Enzyme Activity (U)** (number): Total activity in the sample (mass mode). - **Protein Mass (mg)** (number): Total protein mass in the same sample (mass mode). - **Activity Concentration (U/mL)** (number): Activity per mL of sample (concentration mode). - **Protein Concentration (mg/mL)** (number): Protein per mL of the same sample (concentration mode). - **Decimal Places** (number) ## When to use - Compare enzyme enrichment across purification fractions. - Calculate specific activity directly from U and mg protein measurements. - Convert a U/mg result to nkat/mg for SI-based reporting. ## How it works - Choose total activity plus protein mass, or activity concentration plus protein concentration. - Enter the matching activity and protein values from the same sample. - The calculator divides activity by protein amount; in concentration mode it divides U/mL by mg/mL. - The result is rounded to the selected number of decimal places and shown in U/mg and nkat/mg. ## Use cases - Fill purification tables with specific activity for each fraction. - Compare enzyme enrichment after chromatography or other purification steps. - Summarize routine activity-assay and protein-assay results in U/mg and nkat/mg. ## Frequently asked questions ### What is specific activity? Specific activity is enzyme activity per milligram of total protein, expressed as U/mg. ### Which calculation modes are available? You can use total activity and protein mass, or activity concentration and protein concentration. ### Do I need to enter a sample volume? No. In concentration mode, divide U/mL by mg/mL directly; the volume units cancel. ### How is U/mg converted to nkat/mg? The calculator uses 1 U = 16.67 nkat, so it multiplies U/mg by 16.67. ### Can this calculator determine enzyme purity? It calculates specific activity, a commonly used purity metric. It does not independently measure purity or replace experimental assays. ## Related tools - [PCR Annealing Temperature Calculator (Ta = Tm − 5°C)](https://elysiatools.com/en/tools/annealing-temperature-calculator): PCR Ta by the simple rule (lower primer Tm − 5°C) or the Rychlik 1990 formula (0.3·Tm_primer + 0.7·Tm_product − 14.9), with a ±3°C gradient window and primer-Tm mismatch warning. 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