Number Needed to Treat (NNT) Calculator

Compute NNT, NNH, Absolute Risk Reduction (ARR), Control Event Rate (CER) and Experimental Event Rate (EER) from trial data or percentages.

1. Enter trial data (2×2 table)

Use raw counts from a randomized trial or cohort study.

Control group

Treatment group

2. Results

What is Number Needed to Treat (NNT)?

The Number Needed to Treat (NNT) is the number of patients who must receive a treatment for a specified period in order to prevent one additional bad outcome compared with a control group. It is widely used in evidence-based medicine to express the clinical impact of a therapy.

NNT is derived from the absolute risk reduction (ARR) between treatment and control. A lower NNT means a stronger treatment effect.

Formulas used by this NNT calculator

From patient counts (2×2 table):

Let:

  • a = events in treatment group
  • b = non-events in treatment group
  • c = events in control group
  • d = non-events in control group

Then:

\[ \text{EER} = \frac{a}{a + b} \quad\quad \text{CER} = \frac{c}{c + d} \]

For a treatment that reduces bad events:

\[ \text{ARR} = \text{CER} - \text{EER} \]

\[ \text{NNT} = \frac{1}{\text{ARR}} \]

From event rates (%):

\[ \text{CER} = \frac{\text{Control rate (\%)}}{100} \quad\quad \text{EER} = \frac{\text{Treatment rate (\%)}}{100} \]

For a beneficial treatment (reduces risk):

\[ \text{ARR} = \text{CER} - \text{EER} \]

For a harmful effect (increases risk):

\[ \text{ARI} = \text{EER} - \text{CER} \]

Then:

\[ \text{NNT} = \frac{1}{|\text{ARR}|} \quad\text{or}\quad \text{NNH} = \frac{1}{\text{ARI}} \]

This calculator automatically determines whether the effect is beneficial (NNT) or harmful (NNH) based on the direction of the risk difference and the type of outcome you select.

Interpreting NNT and NNH

  • NNT = 1–5: very strong effect for most clinical contexts.
  • NNT = 6–20: moderate effect; may still be worthwhile for serious outcomes.
  • NNT > 20: small effect; consider costs, side effects and patient preferences.
  • NNH: the smaller the NNH, the more frequent the harm.

Always interpret NNT together with:

  • the severity of the outcome (e.g. death vs. mild symptom),
  • the time horizon (e.g. 1 month, 5 years),
  • the baseline risk of your population,
  • the Number Needed to Harm (NNH) for adverse events.

Worked example

Suppose a randomized trial compares a new drug vs. placebo for preventing myocardial infarction (MI) over 5 years:

  • Control group: 40 MI out of 200 patients
  • Treatment group: 20 MI out of 200 patients

Then:

  • CER = 40 / 200 = 0.20 (20%)
  • EER = 20 / 200 = 0.10 (10%)
  • ARR = 0.20 − 0.10 = 0.10 (10 percentage points)
  • NNT = 1 / 0.10 = 10

Interpretation: 10 patients must be treated for 5 years to prevent one additional MI compared with placebo.

Limitations

  • NNT depends on the baseline risk; it can differ across populations.
  • NNT is usually reported with a confidence interval, which this simple tool does not compute. For borderline effects, confidence intervals are essential.
  • NNT assumes a constant effect over the time period; in reality, treatment effects may vary over time.

Clinical use

NNT is particularly useful for:

  • Summarizing randomized controlled trials in a way that is easy to explain to patients.
  • Comparing alternative treatments for the same condition.
  • Balancing benefits (NNT) against harms (NNH) and costs.

This calculator is for educational and research support only and does not replace clinical judgment, local guidelines, or individual patient assessment.

NNT Calculator – FAQ