An ovulation calculator estimates when an egg is likely to be released in a cycle, and the handful of days around it when conception is possible. It uses the first day of the last period and the usual cycle length - nothing else.
The second half of a cycle (from ovulation to the next period) is fairly constant at around 14 days. So ovulation is estimated as 14 days before the next expected period. Sperm can survive up to about 5 days, so the fertile window is the 5 days before ovulation plus ovulation day itself. The last 3 days of that window are the most fertile.
Trying to conceive is a shared project, but the timing usually lives in one person's head or app. Knowing the fertile window yourself means you can plan around it - keep those evenings free, skip the late work trip - without having to be reminded.
FAQ
How ovulation and the fertile window are estimated, and how far to trust the dates.
Ovulation usually happens about 14 days before the next period starts. For a 28-day cycle that is day 14; for a 32-day cycle it is day 18. The calculator adds your cycle length to the last period date, then counts back 14 days.
The fertile window is the 6 days when sex can lead to pregnancy: the 5 days before ovulation and ovulation day. Sperm can live for up to 5 days, while an egg survives only 12-24 hours after release. The 2 days before ovulation and the day itself are the most fertile.
It is an estimate based on averages. Cycles vary month to month, and stress, illness, and travel can shift ovulation. Ovulation test kits, basal body temperature, and cervical mucus changes can confirm timing more precisely. Never use a calculator as contraception.
If cycle length varies by more than a few days, a calendar estimate is less reliable. Use the average cycle length as a starting point, and consider ovulation tests. If cycles are very irregular or there's no pregnancy after 12 months of trying (6 months if 35 or older), talk to a GP.
Most home tests are reliable from the first day of a missed period - roughly 14 days after ovulation. Testing earlier can give a false negative because hCG levels are still rising.
Work out the likely conception date and window from the last period or the due date — plus the due date that goes with it.
Use tool →Find your due date from the first day of the last period, or from how many weeks pregnant you are today.
Use tool →Learn why couples often wait 12 weeks, who to tell earlier, and how to decide when to announce your pregnancy.
Read article →A practical guide to the first 12 weeks of pregnancy for partners. What is happening, what your partner needs, and how to show up well.
Read article →Built for dads & partners
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