Where the percentages come from
The core numbers are the day-specific conception probabilities measured by Wilcox, Weinberg and Baird in 1995, still the standard reference: about 10% five days before ovulation, rising to roughly 30% on the day before and the day of ovulation, then effectively zero once the egg has passed. The calculator works out which of those days your date fell on.
Two adjustments follow. Age scales the baseline down, gently in the early thirties and steeply after 40. Protection scales it further: an intact condom removes almost all of the risk, withdrawal removes some of it. If ovulation is only estimated from your last period, the result is blurred across a few neighbouring days, because a calendar estimate is rarely exact.
What to do with the answer
Whatever the number, the only way to know is a test at the right time. Use the test date shown, or the when to take a pregnancy test calculator for more detail. If it has been under five days and you do not want to be pregnant, the Plan B window calculator shows which emergency options are still available. And if you are trying to conceive, the best time to get pregnant calculator turns the same probabilities into a plan.
Frequently asked questions
What are the chances of getting pregnant from one time?
It depends almost entirely on where that day fell in your cycle. On the day before ovulation the chance is around 30%; five days before it is around 10%; two days after ovulation it is close to zero. Averaged over a whole cycle the chance from a single act of unprotected sex is roughly 3 to 5%. Age and any protection used scale those numbers down.
Can I get pregnant on my period?
It is unlikely but possible, mainly with short cycles. If you ovulate on day 10 or 11, sperm from sex on day 5 or 6 of your period can still be alive. The calculator uses your cycle length to place ovulation, so it will show a small but non-zero chance in that situation.
How soon can I tell if I am pregnant?
Not from symptoms, which overlap with normal luteal-phase changes. A sensitive test can show a positive about 10 days after ovulation, and a test on the day your period is due is reliable for most people. The calculator gives both dates for your timing.
I used withdrawal. Can I still be pregnant?
Yes. Withdrawal fails about 20% of the time in typical use over a year, partly because pre-ejaculate can carry sperm and partly because timing goes wrong. The calculator treats withdrawal as roughly a 70% reduction in per-act chance, not as protection.
Is it too late for emergency contraception?
Levonorgestrel (Plan B, Levonelle) works best within 72 hours; ulipristal (ellaOne) and the copper IUD work up to 120 hours (five days). If you are inside that window and do not want to be pregnant, act now rather than waiting for a test. The Plan B window calculator shows which options are still in time.
These calculators give estimates based on cycle averages and standard formulas. They are for general information only and are not medical advice. For anything concerning your health or pregnancy, talk to a qualified healthcare provider.