Contraceptive Effectiveness Calculator

Work out contraceptive effectiveness instantly with clear inputs, formula shown and shareable results.

About Contraceptive Effectiveness Calculator

Compounds published first-year failure rates over the chosen number of years so typical and perfect use can be compared over a realistic horizon.

Formula

Cumulative failure

Cumulative pregnancy risk = 1 - (1 - annual failure rate)^years

Worked example

With the default values below, Contraceptive Effectiveness Calculator returns Cumulative pregnancy chance with typical use of 37.60%. Change any input above and every figure here is recalculated in your browser.

Inputs used

Typical-use failure rateaccepts 0.01–30 % per year: 9 % per year
Perfect-use failure rateaccepts 0.01–30 % per year: 0.3 % per year
Years of useaccepts 0.5–30: 5

Results

Cumulative pregnancy chance with typical use: 37.60%
Cumulative chance with perfect use: 1.49%
Annual gap between typical and perfect use: 8.70%

Step by step

1Values used
2Cumulative failure
3Cumulative pregnancy chance with typical use= 37.60
4Cumulative chance with perfect use= 1.49
5Annual gap between typical and perfect use= 8.70

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 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Educational estimate only. Not a substitute for professional clinical judgement, and must not be used to make treatment or dosing decisions.
This is an estimate for general information and not professional medical advice. See the full disclaimer for the limits that apply to this kind of calculator.

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

Educational estimate only. Not a substitute for professional clinical judgement, and must not be used to make treatment or dosing decisions.

Frequently Asked Questions

Why do typical and perfect use differ so much?

Methods needing daily or per-act action are vulnerable to missed doses; long-acting methods have almost no gap.

Do failure rates fall with time?

For most methods yes, as users become more consistent, so a constant-rate model is conservative.