Relative Risk Calculator
Risk ratio, absolute risk reduction and number needed to treat.
Enter your values
Enter the exposed with the outcome you are working with.
Enter the exposed without you are working with.
Enter the unexposed with the outcome you are working with.
Enter the unexposed without you are working with.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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.
RR = [a ÷ (a+b)] ÷ [c ÷ (c+d)]
At a glance
| What it does | Risk ratio, absolute risk reduction and number needed to treat. |
|---|---|
| Category | Statistics |
| Inputs needed | Exposed with the outcome, Exposed without, Unexposed with the outcome, Unexposed without |
| Main output | Relative risk |
| Formula | RR = [a ÷ (a+b)] ÷ [c ÷ (c+d)] |
| Cost | Free — no sign-up, no download |
How to use the Relative Risk Calculator
- 1Enter the outcome counts for the exposed group.
- 2Enter the same for the unexposed or control group.
- 3Read the relative risk alongside the absolute difference — both matter.
Inputs explained
- Exposed with the outcome
- Enter the exposed with the outcome you are working with.
- Exposed without
- Enter the exposed without you are working with.
- Unexposed with the outcome
- Enter the unexposed with the outcome you are working with.
- Unexposed without
- Enter the unexposed without you are working with.
Worked example
Using the values the calculator loads with:
Inputs
- Exposed with the outcome30
- Exposed without70
- Unexposed with the outcome15
- Unexposed without85
Results
- 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
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.
What do I need to enter into the Relative Risk Calculator?
Just 4 values: exposed with the outcome, exposed without, unexposed with the outcome, unexposed without. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Relative Risk Calculator work out the 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. It applies the formula RR = [a ÷ (a+b)] ÷ [c ÷ (c+d)] and shows the working so you can check each step by hand.
Is the Relative Risk Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.