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Calculation Machine

Relative Risk Calculator

Risk ratio, absolute risk reduction and number needed to treat.

Reviewed by James Whitfield · How we check our maths

Your values

ⓘ Live results

Results
Relative risk
2
95% confidence interval
1.149 to 3.482
Risk in the exposed group
30%
Risk in the unexposed group
15%
Absolute risk difference
15 percentage points
Number needed to treat/harm
6.7

Quick answer

Relative risk divides the risk in the exposed group by the risk in the unexposed group; RR = 2 means the outcome is twice as likely.

The formula

RR = [a ÷ (a+b)] ÷ [c ÷ (c+d)]

Worked example

Worked example

Using the values the calculator loads with.

Inputs

  • Exposed with the outcome30
  • Exposed without70
  • Unexposed with the outcome15
  • Unexposed without85

Result

  • Relative risk2
  • 95% confidence interval1.149 to 3.482
  • Risk in the exposed group30%
  • Risk in the unexposed group15%
  • Absolute risk difference15 percentage points
  • Number needed to treat/harm6.7

How to use the Relative Risk Calculator

  1. 1Enter the outcome counts for the exposed group.
  2. 2Enter the same for the unexposed or control group.
  3. 3Read the relative risk alongside the absolute difference — both matter.

Frequently asked questions

Why does absolute risk matter more?

A 50% relative reduction sounds huge, but if the base risk is 2 in 10,000 it saves one person in 10,000. Always read both numbers.

What is number needed to treat?

How many people must receive the treatment for one extra good outcome — the inverse of the absolute risk reduction.

About this calculator