Ovulation Calculator
Your six fertile days end on the day you ovulate. This works out which days those are, and your odds on each one.
Your odds on each fertile day
Chance of conceiving from sex on that day alone, from Wilcox, Weinberg & Baird (NEJM, 1995).
Your calendar
Period
Fertile
Ovulation
How it gets your dates
The second half of the cycle, from ovulation to the next period, holds fairly steady at about 14 days. The first half is the part that stretches and shrinks. So the calculator finds your next period first, then counts back 14 days to ovulation.
On a 28-day cycle that lands on day 14, which is where the old rule of thumb comes from. On a 34-day cycle it lands on day 20, not day 14. Adding 14 to the first day of your period is where most people go wrong.
Sperm survive up to five days in fertile cervical mucus. An egg lasts about 24 hours. That asymmetry is why the fertile window opens five days before ovulation and shuts on the day itself.
Why the odds are not flat across the window
Wilcox and colleagues tracked 221 women trying to conceive and measured hormone metabolites in urine to pin down the day each woman ovulated. Every pregnancy in the study came from sex during the six days ending on the day of ovulation. Not one came from sex the day after.
| Day | Chance | What it means |
|---|
| 5 days before ovulation | 10% | Window opens |
| 4 days before | 16% | |
| 3 days before | 14% | |
| 2 days before | 27% | Odds climb |
| 1 day before | 31% | |
| Day of ovulation | 33% | Peak |
| Day after | ~0% | Window shut |
The dip at three days before is in the published data, not a rounding artefact. The sample size on individual days is small, so read the shape of the curve rather than any single number.
When this will be wrong
A calculator does arithmetic on the dates you type. It cannot see your hormones. Here is where the arithmetic breaks down.
Your cycles vary by more than a week
Run it twice, once with your shortest cycle and once with your longest. The two windows you get bracket the range worth covering. If those windows barely overlap, ovulation tests will tell you more than any calendar.
You have PCOS, or you are post-pill or breastfeeding
Ovulation may be late, irregular or absent altogether, and the 14-day luteal phase the maths depends on may not hold. Track with tests rather than dates.
You are treating it as birth control
Do not. Across 696 cycles in 221 women, the fertile window fell entirely inside the textbook days 10 to 17 for only about 30 percent of them. On every day from cycle day 6 to day 21, at least one woman in ten was fertile. That miss rate is fine when you are trying to conceive and dangerous when you are trying not to.
The honest use of this page: it tells you which week to pay attention. An ovulation test strip or your waking temperature tells you which day.
Questions people actually ask
Can I get pregnant right after my period?
On a short cycle, yes. Bleed for six days, ovulate on day 11, and sperm from sex on day 6 are still alive when the egg arrives. The shorter your cycle, the closer ovulation sits to the end of bleeding. Set the cycle length above to your real number and watch the fertile days slide.
How accurate is an ovulation calculator?
It is a prediction from two numbers, not a measurement. Use it to know which week matters, then confirm the day with ovulation tests, basal body temperature, or the change in cervical mucus. Cheap test strips cost a few cents each and remove most of the guesswork.
What if my cycles are irregular?
Enter your shortest cycle, note the window, then enter your longest and note that one too. Together they mark the span worth covering. Once your cycles vary by more than about eight days, the calendar method is too loose to help and tests are the better tool.
When should I take a pregnancy test?
The day after your period was due, which the answer above gives you. Testing before then produces false negatives because hCG has not built up enough to register. If you cannot wait, an early-detection test at 10 days past ovulation catches some pregnancies, and a negative at that point means nothing.
How often should we have sex during the fertile window?
Every day or every other day across the whole six days beats trying to hit one perfect day. Daily sex does not meaningfully lower sperm quality in men with normal counts, and the effort of timing a single day usually costs more than it gains.
Does the day we have sex affect the baby's sex?
No. The Wilcox study looked for exactly this and found no relationship between the timing of sex within the fertile window and whether the baby was a boy or a girl. The Shettles method is popular and does not hold up.