About the Ovulation Calculator
This Ovulation Calculator works forward from the first day of your last period. Enter that date and your usual cycle length, and it counts back from your next period by the length of the luteal phase (14 days unless you change it) to estimate the day you ovulate. It then marks the fertile window, from five days before ovulation to one day after, shows when your next period is expected, and lists the next six cycles. It also shows the due date you would have if you conceived on the estimated ovulation day. The result is an estimate for regular cycles only. Ovulation can move in any cycle, so it cannot be used as contraception or to diagnose fertility problems. The dates you enter stay on your device.
Updated September 2026
How to use the Ovulation Calculator
- 1Pick the first day of your last period.
- 2Check the average cycle length (28 days if you are not sure).
- 3Open More options only if you know your luteal phase is not 14 days.
- 4Read your ovulation day, fertile window and next period, then scroll for the next six cycles.
How ovulation is estimated
Ovulation is when an ovary releases an egg. It happens before your next period, not after your last one, so a calendar method works backwards. The calculator takes the first day of your last period, adds your cycle length to predict the next period, and subtracts the luteal phase. For a 28-day cycle and a 14-day luteal phase, ovulation is expected on day 14.
That is why a longer cycle moves ovulation later. A 35-day cycle gives ovulation on day 21, not day 14. Many websites assume day 14 for everyone, which can be a week out.
The fertile window
You can only become pregnant during a few days each cycle. Sperm can live for up to five days in the body, and the egg survives for roughly a day after it is released. The calculator therefore marks the five days before ovulation, the day itself and the day after.
- The two days before ovulation and ovulation day are usually the most fertile.
- The window is an estimate, so treat its edges as soft.
- If you are not trying to conceive, do not rely on it to avoid pregnancy.
Cycle length and luteal phase
Count your cycle from the first day of one period to the day before the next. Average a few cycles if you can. Cycles of 21 to 45 days are accepted here. The luteal phase is 14 days by default; if you have tracked it with tests and know it is 12, change it under More options and the ovulation date moves two days later.
The tool asks that the luteal phase is at least seven days shorter than the cycle. A short cycle with a long luteal phase would put ovulation almost on top of your period, which is not a useful result.
Other ways to track ovulation
A calendar estimate is a starting point. Other signs can confirm it: ovulation predictor kits detect a hormone surge in urine, cervical mucus turns clear and stretchy near ovulation, and a slight rise in morning body temperature shows it has already happened.
See a doctor if you have irregular cycles, if you have been trying to conceive for a year (six months if you are 35 or older), or if you have pain, very heavy bleeding or any other concern. This tool does not diagnose anything. Once a due date matters, the Pregnancy Due Date Calculator works it out, and Days Between Dates counts the days to a date you pick. Track dates with the Days Between Dates tool, and the Sleep Calculator helps with rest times.
Features
- Estimated ovulation day and fertile window
- Next period date and six cycles ahead
- Cycle length from 21 to 45 days
- Adjustable luteal phase from 10 to 16 days
- Due date if you conceive on ovulation day
- Copy the summary in one tap
- Private: worked out on your device
Frequently asked questions
How do I calculate my ovulation day?
Add your cycle length to the first day of your last period to find when the next one starts, then count back the luteal phase, usually 14 days. With a 28-day cycle starting on 1 January, that lands on 15 January. Enter your date here and the sum is done for you.
How long is the fertile window?
Here it runs for seven days: the five days before ovulation, the day itself and the day after. Sperm can survive for up to five days, while an egg lasts about a day, so the days just before ovulation matter most.
Can I use this calculator if my cycle is irregular?
Only as a rough guide. The estimate assumes each cycle has the same length, which is not true when periods vary by more than a few days. If your cycles are irregular, try an ovulation test kit or track body signs, and ask a doctor for advice.
What is the luteal phase?
It is the time between ovulation and the start of your next period. It is fairly steady for each person, often 12 to 14 days, while the first half of the cycle varies more. You can change it under More options if you know yours.
Is the ovulation calculator a form of contraception?
No. Ovulation can shift, and sperm can survive for days, so a calendar estimate cannot prevent pregnancy. Use a proper contraception method and talk to a doctor or nurse about what suits you.
How is my due date worked out?
The tool assumes you conceive on the estimated ovulation day and adds 266 days, the usual gap between conception and birth. A doctor usually counts from the last period instead, and our Pregnancy Due Date Calculator does that. Both are estimates.