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Number Needed to Harm Calculator

Calculate the number needed to harm from adverse event rates.

About Number Needed to Harm Calculator

Number needed to harm mirrors number needed to treat but for adverse effects. A low NNH means harm is common, and it must be weighed directly against the NNT for benefit. Rare but severe harms can outweigh a favourable NNH, so severity must be considered alongside frequency.

Formula

Number Needed to Harm

NNH = 1 ÷ absolute risk increase

NNH
Patients treated before one additional harm occurs

Worked example

With the default values below, Number Needed to Harm Calculator returns Number Needed to Harm of 20.00. Change any input above and every figure here is recalculated in your browser.

Inputs used

Adverse event rate with treatmentaccepts 0–100 %
8 %
Adverse event rate without treatmentaccepts 0–100 %
3 %
Population treatedminimum 1
5,000

Results

Number Needed to Harm
20.00
Absolute Risk Increase
5.000%
Relative Risk Increase
166.667%
Excess Harms in the Population
250
Interpretation
Moderate harm frequency

Step by step

  1. 1Values used
  2. 2Number Needed to Harm
  3. 3Number Needed to Harm= 20.00
  4. 4Absolute Risk Increase= 5.000
  5. 5Relative Risk Increase= 166.667
  6. 6Excess Harms in the Population= 250
  7. 7Interpretation= Moderate harm frequency

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is Number Needed to Harm calculated?

NNH = 1 ÷ absolute risk increase Number needed to harm mirrors number needed to treat but for adverse effects. A low NNH means harm is common, and it must be weighed directly against the NNT for benefit.

Why does Number Needed to Harm matter?

Rare but severe harms can outweigh a favourable NNH, so severity must be considered alongside frequency.

What values do I need to enter?

This calculator takes 3 inputs: Adverse event rate with treatment, Adverse event rate without treatment, Population treated. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

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