IC50 Calculator

IC50, the half maximal inhibitory concentration, is the concentration of an inhibitor, drug, or test compound required to reduce a biological or biochemical response by 50%. It is one of the most widely reported potency metrics in pharmacology, drug screening, enzymology, and toxicology. When you run a dose-response assay and plot percent inhibition against compound concentration, the IC50 is the point on that curve where exactly half of the maximal effect is achieved.

This IC50 calculator takes the raw concentration and percent-inhibition pairs you enter, finds the two data points that bracket the 50% inhibition level, and interpolates between them in logarithmic concentration space to estimate the IC50. Because dose-response relationships are sigmoidal when plotted on a log scale, log-space interpolation gives a far more accurate estimate than a naive linear average of the raw concentrations.

A lower IC50 means a more potent compound: less of it is needed to halve the response. Reporting IC50 in consistent units (this tool assumes micromolar, uM, on input) lets researchers compare candidate molecules, rank hits from a high-throughput screen, and characterize structure-activity relationships across a chemical series. IC50 is closely related to EC50 (half maximal effective concentration) for stimulatory assays, and to the inhibition constant Ki once you account for substrate competition.

You supply at least three data points, each a pair of compound concentration (in uM) and the corresponding percent inhibition. The calculator first parses every row, discards any entry that is not a valid positive number, and sorts the surviving points by ascending concentration. It then scans the sorted list for the first consecutive pair where the lower point sits at or below 50% inhibition and the next point sits at or above 50%. These two points bracket the IC50.

Once the bracketing pair is found, the tool converts both concentrations to their base-10 logarithms and performs a linear interpolation in that log space to find the log of the IC50, then raises 10 to that power to recover the concentration. This is the standard approach used when you do not want to fit a full four-parameter logistic curve but still need a reliable point estimate. The result panel reports the IC50 in both micromolar and log units, along with an approximate Hill slope, the concentration range tested, and the inhibition range observed.

If the maximum inhibition in your dataset never reaches 50%, the calculator cannot interpolate and tells you the IC50 lies above the tested range. If every point already exceeds 50%, it reports that the IC50 is below the tested range. Both messages are signals to extend your concentration series so that the assay properly brackets the half-maximal response.

The headline number is the IC50 itself. A compound with an IC50 of 13 uM is roughly ten times less potent than one with an IC50 of 1.3 uM, since potency scales inversely with concentration. The log IC50 value is convenient for tabulating and comparing potencies across many orders of magnitude, and it is what you would plot on the x-axis of a classic sigmoidal dose-response graph.

The calculator also returns an approximate Hill slope, which describes how steeply the response rises across the transition region. A Hill slope near 1 suggests simple single-site binding behavior, a slope steeper than 1 can indicate cooperativity or a multi-step mechanism, and a shallow slope below 1 may signal heterogeneous binding or assay artifacts. Because this estimate is derived from only the two bracketing points, treat it as a rough guide rather than a rigorously fitted parameter.

The tool formats concentrations automatically: values at or above 1000 uM display as mM, values from 1 to 1000 as uM, smaller values as nM, and the smallest as pM. Watch the inhibition range as well. If your lowest point already shows substantial inhibition or your highest point falls short of full inhibition, your curve may not capture the true plateau, which biases both the IC50 and the Hill slope.

It is easy to confuse the family of half-maximal metrics, so it helps to keep them straight. IC50 measures inhibition: the concentration that halves an activity, signal, or growth. EC50 measures the opposite, the concentration that produces half of a maximal stimulatory or agonist effect. Many platforms, including this site, offer a dedicated EC50 calculator for stimulatory dose-response data.

Ki, the inhibition constant, is a true thermodynamic binding affinity that, unlike IC50, does not depend on the substrate concentration used in the assay. The Cheng-Prusoff equation converts an IC50 to a Ki once you know the substrate concentration and the Michaelis constant. Kd is the equilibrium dissociation constant for a binding interaction and is also assay-independent. Because IC50 shifts with assay conditions such as substrate level, incubation time, and enzyme concentration, always report those conditions alongside the IC50 so other labs can reproduce and compare your potency values.

The quality of any IC50 estimate is only as good as the underlying data. Use log-spaced concentrations, typically half-log or full-log dilution series such as 0.1, 1, 10, 100, and 1000 uM, so that points are evenly distributed across the sigmoidal curve. Aim for at least eight to twelve concentrations spanning roughly two orders of magnitude on either side of the expected IC50, although this interpolation tool works with as few as three points.

Always include points that bracket 50% inhibition. If you only test concentrations that produce 10% to 40% inhibition, the calculator must extrapolate or simply cannot solve, and the reported IC50 becomes unreliable. Run replicates at each concentration to control for pipetting and biological variability, and normalize your raw signal to no-inhibitor and full-inhibition controls before converting it to percent inhibition.

Be mindful of compound solubility limits, since precipitation at high concentrations can flatten the top of the curve and inflate the apparent IC50. Keep the solvent concentration, usually DMSO, constant across all wells to avoid vehicle effects. Finally, fit a full four-parameter logistic model in dedicated software when you need publication-grade confidence intervals; use this calculator for fast, in-the-moment estimates during screening and assay development.