Absolute Risk Reduction Calculator
Enter the event rate in a control group and a treatment group to calculate absolute risk reduction, relative risk reduction, and number needed to treat.
Last updated: September 5, 2026
Frequently asked questions
What's the difference between absolute and relative risk reduction?
Absolute risk reduction (ARR) is the simple difference between two event rates (e.g., 20% − 12% = 8 percentage points). Relative risk reduction (RRR) expresses that same difference as a percentage of the original rate (8 ÷ 20 = 40%) - RRR often sounds more dramatic than ARR for the same underlying data, which is why both numbers matter for context.
What does 'number needed to treat' (NNT) mean?
NNT is the number of people who would need to receive a treatment for one additional person to benefit (avoid the event), calculated as 1 ÷ ARR (expressed as a decimal). A smaller NNT means a more impactful treatment effect - an NNT of 12.5 means roughly 1 in every 12-13 people treated sees the benefit.
Is this appropriate for making medical treatment decisions?
No - this is a statistics reference calculator for understanding how these numbers are computed, not a substitute for a full clinical study's confidence intervals, population characteristics, and a qualified professional's interpretation. Always discuss treatment decisions with a healthcare provider.
How do I calculate ARR if the control group event rate is 25% and the treatment group is 15%?
ARR = 25% − 15% = 10 percentage points (0.10). RRR = 0.10 ÷ 0.25 = 40%. NNT = 1 ÷ 0.10 = 10, meaning about 10 people need treatment to prevent one additional event.
Why can RRR make a treatment sound more effective than ARR suggests?
Because RRR is a ratio relative to the baseline rate, a treatment reducing a rare event from 0.2% to 0.1% has a dramatic-sounding 50% RRR but only a 0.1 percentage point ARR (and an NNT of 1,000) - both numbers are mathematically correct, but ARR better reflects the real-world impact on a population.
What does a negative ARR mean?
A negative ARR means the treatment group actually had a higher event rate than the control group - the treatment appears harmful rather than beneficial for the outcome measured, and this calculator will show the reduction as negative to reflect that.