Estimate your chance of pregnancy from a single encounter using the withdrawal method (sometimes called the ‘pull-out’ method). This simple calculator uses cycle timing and whether use was typical vs perfect.
* An approximate per-act estimate, not a measured personal chance. Calendar dates cannot confirm ovulation or identify a safe day. The test date is 21 days after this encounter; you can also test from the first day of a missed period. If you have sex again, count from the latest potentially unprotected encounter.
Methodology & limits
This calculator estimates pregnancy chance from one encounter using withdrawal, with optional additional contraception and personal inputs. The percentage and "about 1 in N" are approximate model outputs, not a measurement of your individual risk.
Calendar timing. Cycle day 1 is the first day of your period. The day-to-day pattern comes from Li, Wilcox and Dunson (2015), Table 3, all-women estimates for cycle days 1-40. Those data include variation in ovulation timing. They describe unprotected intercourse, not withdrawal. The calculator applies their relative timing pattern to its existing average per-act estimate. It divides the selected day's reference by the mean reference over your entered cycle length, limited to the 40 published days. This scaling is an approximation; it does not predict your ovulation day.
Your entered cycle length controls that average, but does not move the peak to a supposedly certain ovulation date. Dates before the entered period or after day 40 use an average, non-day-specific estimate instead. The calculator never assumes a new period started just because the usual cycle length has passed. For hormonal contraception, IUDs, sterilization or lactational amenorrhea combinations, it also uses the average estimate rather than applying a natural-cycle curve.
Withdrawal and additional methods. The existing server model uses your typical/perfect-use selection, age and optional inputs, and combines up to three distinct contraceptive methods. Annual contraceptive-failure rates are not per-encounter rates; their conversion and the personal-factor adjustments are modeling assumptions. The calendar study does not validate those adjustments or provide a precise pregnancy probability for an individual withdrawal encounter. Default ages are 30, with 165 cm and 68 kg when height/weight are not entered.
Testing and emergency contraception. The displayed date adds 21 calendar days to the encounter, following NHS guidance when the next period date is unknown. You can test from the first missed period. Emergency-contraception advice uses elapsed calendar days, not the unknown time of intercourse, so ask promptly rather than relying on a cutoff or a low estimate.
Small results retain the existing display threshold; estimates above 40% retain the existing cap. The estimate does not establish where ejaculation occurred, whether ovulation happened, or the effect of emergency contraception. If withdrawal was late or semen entered the vagina, the displayed withdrawal estimate may not apply. This tool is educational, not a diagnosis.
- Li, Wilcox and Dunson (2015): calendar-day pregnancy benchmarks, Table 3.
- Wilcox et al. (2000): variation in fertile-window timing.
- NHS: when to take a pregnancy test.
- CDC: emergency-contraception timing.
Last updated: October 10, 2026.
This simple estimator uses published typical vs perfect failure rates for withdrawal and adjusts for cycle timing (higher risk near ovulation, lower risk far from it). It does not account for:
- multiple rounds in one day
- where ejaculation occurred (inside/outside/on)
- emergency contraception (EC) taken after the encounter
If any of the above apply, see the “What’s next?” options below for tools that better fit your situation.
Educational tool only; not medical advice.
Good to know
- Typical vs perfect use matters. If withdrawal was late or there were slip-ups, choose typical use for a more realistic estimate.
- Repeat rounds increase risk. Residual sperm can carry into pre-ejaculate during later encounters the same day.
- Only condoms help with STIs. Withdrawal does not reduce STI risk.
Withdrawal FAQ
Does this calculator handle multiple rounds?
No. It assumes a single encounter. For repeat rounds or more nuance, use the Advanced Pregnancy Chance Calculator.
Where ejaculation happened — does that change the result?
This page’s estimator doesn’t take that input. If ejaculation occurred inside the vagina, risk is higher than withdrawal; consider the Emergency Contraceptive Effectiveness tool and the Daily Chance Calendar.
Does it include emergency contraception?
Not here. Use the dedicated Emergency Contraceptive Pill Effectiveness Calculator to see expected impact by timing and pill type.