Selectivity Fold Calculator
Compare target and off-target potency as unit-normalized fold selectivity.
Off-targets / isoforms
Fold selectivity results
Enter target and off-target values.
For potency measures where lower values mean stronger activity, fold = off-target / target. Compare values from the same metric, assay format, biological state, and conditions. Do not mix IC50, EC50, Ki, and KD or treat single-concentration % inhibition as potency. This tool does not set an automatic selectivity acceptance threshold.
Sources: NIH/NCATS Assay Guidance Manual; NCBI “Finding a better path to drug selectivity” (PMCID: PMC3210374).
Tool Guide
Definition
Selectivity Fold Calculator — For potency measures where lower values mean stronger activity (IC50, EC50, Ki, KD), divide off-target potency by target potency to calculate fold selectivity and the log10 difference. Concentration units are normalized through molar units.
Purpose
(1) Compare target potency against isoforms and off-targets (2) Cross-check fold selectivity in SAR tables (3) Prevent unit errors in mixed nM and µM data (4) Preserve measured bounds (>, <) in reported ratios
How to Use
① Choose the potency metric shared by every value. ② Enter the reference target value and unit. ③ Enter each off-target name, =/>/< qualifier, value, and unit. ④ Copy fold and log10 differences. Only compare values measured with the same endpoint, assay format, biological state, and conditions. Do not mix IC50, EC50, Ki, and KD.
Examples
Example) Target A IC50=10 nM • Isoform B=250 nM → 25×, log10=1.398 • Kinase X>10 µM → >1,000×, log10>3 • Kinase Y=5 nM → 0.5×, indicating stronger off-target potency The tool reports numeric direction without an automatic pass/fail threshold.