Not a method of birth controlCycles vary from month to month, so these dates are estimates. Do not use this calculator to avoid pregnancy. It cannot tell you which days are "safe". For contraception advice, speak with a doctor, nurse or sexual health clinic.
The short answer
An ovulation calculator estimates the day you ovulate, usually about 14 days before your next period starts. Enter the first day of your last period and your average cycle length. It shows your ovulation date and the six-day fertile window that ends on that day, so you can time intercourse when trying to conceive.
Formulas and sources are named on this page. How we calculate.
How this is calculated
The calculator counts ovulation back from your next period rather than forward from your last one. The time between ovulation and the next period, the luteal phase, is far more consistent than the time before ovulation, and is commonly taken to be about 14 days.
Ovulation date = first day of last period + (cycle length − luteal phase)
Fertile window = ovulation date − 5 days, up to and including ovulation day
Next period = first day of last period + cycle length
Worked examples
28-day cycle, last period started 1 October 2026. Ovulation = 1 October + (28 − 14) = 15 October. The fertile window runs from 10 to 15 October, and the next period is due on 29 October.
35-day cycle, same start date. Ovulation = 1 October + (35 − 14) = 22 October. A longer cycle means later ovulation, not a longer luteal phase, so the fertile window is 17 to 22 October.
28-day cycle with a 12-day luteal phase. Ovulation = 1 October + (28 − 12) = 17 October.
The fertile window
An egg survives for about a day after ovulation, but sperm can survive in the body for several days. A study of couples in the New England Journal of Medicine found that pregnancy was possible only in a six-day window: the five days before ovulation and the day of ovulation. The chance was highest in the two days before ovulation and on ovulation day itself, which the calculator marks as your most fertile days.
If your cycles are irregular
This method works best when your cycles are fairly regular. If your cycle length changes by more than a few days, or you have no regular periods, your real ovulation date can be several days away from the estimate. Ovulation (LH) test kits, which detect the hormone surge before ovulation, or advice from a doctor will serve you better than a calendar estimate.
Ovulation date by cycle length
Estimated day of ovulation, counting the first day of your period as day 1 (assuming a 14-day luteal phase)| Cycle length | Ovulation on cycle day | Fertile window (cycle days) |
|---|
| 21 days | 8 | 3 – 8 |
| 24 days | 11 | 6 – 11 |
| 26 days | 13 | 8 – 13 |
| 28 days | 15 | 10 – 15 |
| 30 days | 17 | 12 – 17 |
| 32 days | 19 | 14 – 19 |
| 35 days | 22 | 17 – 22 |
Cycle day 1 is the first day of your period. With a 14-day luteal phase, ovulation falls on cycle day 15 of a 28-day cycle, counting day 1 as the first day.
Frequently asked questions
When do I ovulate?
Most commonly about 14 days before your next period starts. In a 28-day cycle that is around cycle day 15, counting the first day of your period as day 1. If your cycle is longer or shorter, ovulation moves with it, and the calculator works out the date for your cycle length.
How long is the fertile window?
About six days: the five days before ovulation and the day of ovulation. Sperm can survive in the body for several days while the egg lasts only about a day, so intercourse in the days before ovulation can lead to pregnancy.
Which days are the most fertile?
The two days before ovulation and the day of ovulation itself are when the chance of conception is highest. The calculator highlights these as your most fertile days.
Can I use this to avoid pregnancy?
No. Ovulation can come early or late, and cycles change from month to month, so calendar estimates are unreliable for contraception. Please ask a doctor, nurse or sexual health clinic about contraception that is suitable for you.
How can I tell that I have ovulated?
Signs include a change in cervical mucus to a clear, stretchy texture around ovulation. Ovulation test kits detect a surge in luteinising hormone that comes shortly before ovulation. A calculator only predicts the timing and cannot confirm that ovulation happened.
What if my periods are irregular?
The estimate becomes less reliable the more your cycle length varies. Using your average cycle length gives a rough guide only. Ovulation test kits can help, and if your periods are very irregular or absent, a doctor can look for the cause and advise you.
Disclaimer and editorial note
For information only, not medical advice. This calculator gives general estimates from the numbers you enter. It cannot diagnose, treat or prevent any condition and does not replace advice from a doctor, midwife, registered dietitian or other qualified professional who knows your history.
If you are worried about your periods, fertility or a possible pregnancy, speak with a doctor or midwife. This calculator cannot diagnose a fertility problem and is not a contraceptive.
Written and maintained by the HealthFitnessHQ editorial team. We name the source of every formula and category used, and we review each calculator when guidance changes. Read how we work in our editorial policy and methodology.