Effect size and NNT calculator

Three calculators researchers keep reaching for: NNT and absolute risk from any effect measure, conversion between OR, RR, RD, and SMD, and standard error from a confidence interval.

Open the calculator Free core tools. Runs in your browser, nothing to install.

NNT and absolute risk from any effect measure

Enter an odds ratio, risk ratio, risk difference, or hazard ratio plus the baseline (control) risk, and get the number needed to treat, the absolute risk reduction, and the event rates behind them. This is the step that turns "OR 0.75" into "treat 18 people to prevent one event", the sentence your discussion section and your patients actually need.

Convert between effect measures

OR to RR, RD, and NNT, given a baseline risk. RR to OR and RD. SMD to OR and Cohen's d. RD to NNT. A meta-analysis needs every study on one measure, and published studies rarely cooperate; this converter is for that step.

Standard error from a confidence interval

Papers report an effect and its 95% CI; a meta-analysis needs the standard error. Enter the estimate and both limits, mark whether the measure is on a log scale (OR, RR, HR) or not (MD, SMD), and get the SE and variance back, ready to paste into a generic inverse-variance analysis.

Part of a full analysis platform

The calculator feeds the rest of Covexe: a meta-analysis tool validated against R's metafor that takes events and totals, means and SDs, or precomputed effects, with forest plots, funnel plots, subgroup analysis, and meta-regression, free in the browser. Results update as you type, and every tab has a worked example.

Frequently asked questions

How do I get NNT from an odds ratio?

Enter the OR and the baseline (control) risk. The calculator converts to absolute risks, reports the absolute risk reduction, and returns NNT as 1 divided by the ARR.

Can I convert an odds ratio to a risk ratio?

Yes. The conversion needs the control event rate, so the converter asks for it and returns the RR together with the RD and NNT.

How do I get a standard error from a 95% CI?

Enter the estimate and both CI limits, and mark whether the measure is on a log scale (OR, RR, HR) or a linear one (MD, SMD). You get the SE and variance, ready for meta-analysis.

Is it free?

Yes. The calculator is free, needs no account, and results update as you type.

Can I calculate Cohen's d from means and SDs?

Through the meta-analysis tool: enter each group's mean, SD, and n and it computes the standardized mean difference per study. The converter here maps between SMD and Cohen's d and on to an odds ratio.