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Last updated: September 2025
How to Use Our Calculator
- Choose a contraceptive method (optional) — leave as “No contraceptive” if none was used.
- Enter the number of intercourse events you want to estimate for (any time frame).
Only one event? Try the One-Time Calculator. - Enter your age (optional) and select Calculate.
We don’t store your inputs. Results are estimates and show a capped range (never exactly 0% or 100%).
How Our Calculators Work: The Science Behind Your Estimate
We start with a per-act baseline probability informed by age and method-specific failure rates. For multiple acts, we combine those probabilities over the window you chose. The model is calibrated to peer-reviewed data and avoids showing impossible values (we display “Under 0.001%” or “Over 99.999%” at the extremes).
Try the Advanced Calculator to factor in BMI and lifestyle and run quick “what-if” scenarios.
The Key Factors in Our Algorithm
Female Age
A primary driver of fertility. Our model accounts for the natural decline in egg quantity and quality over time, based on guidelines from the American College of Obstetricians and Gynecologists (ACOG).
Male Age
Male fertility also declines with age, affecting sperm health. Our algorithm considers this factor, especially for partners over 40, as supported by research on paternal age.
Intercourse Timing
We analyze the timing of intercourse relative to the fertile window, incorporating data showing that intercourse frequency can be more effective than targeted ovulation timing alone.
Body Mass Index (BMI)
A healthy BMI is crucial for hormonal balance. Our calculations are adjusted based on studies linking BMI to fertility outcomes.
Lifestyle Factors
Our advanced models incorporate data on how choices like smoking or alcohol consumption can impact hormonal regulation and overall fertility.
Contraceptive Use
We use large-scale study data, including the landmark Trussell (2011) study on contraceptive failure, to factor in typical and perfect-use rates.
Data Sources & Primary Research
- Contraceptive failure rates: Trussell 2011; Kost et al. 2008
- Age & fertility: Hendershot et al. 1982
- Behavior/timing: Frequency vs timing
For a complete list of the dozens of studies and data sets used in our models, please visit our full Research Library.
Medical Disclaimer
The Pregnancy Chance Calculator is based on peer-reviewed scientific research to provide accurate estimates of pregnancy chances. However, it is not a substitute for professional medical advice, diagnosis, or treatment. Always consult with a healthcare provider for any medical concerns or questions about your fertility.