Ovulation & Fertile Window Calculator
Estimate your ovulation day, fertile window, next period and due date from your last period and cycle length.
An estimate from averages, not a guarantee — real cycles vary from month to month. Do not rely on it as contraception. For trying to conceive, ovulation tests or temperature tracking narrow it down further.
What this calculates
Ovulation is when an ovary releases an egg, usually once per cycle. The useful fact behind every ovulation calculator is that the second half of the cycle — from ovulation to the next period — is fairly constant at about 14 days, while the first half is what changes from person to person. So the calculator counts back 14 days from when your next period is due. Pregnancy is possible on roughly six days of each cycle: the five days before ovulation, because sperm can survive that long, and the day of ovulation itself; the day after is included here as a margin. The due date uses Naegele's rule, adjusted for your cycle length.
Formula
next period = last period + cycle length ovulation = next period − 14 days fertile days = ovulation − 5 days … ovulation + 1 day due date = last period + 280 days + (cycle length − 28)
Example
With a last period starting on 1 January and a 28-day cycle, the next period is due on 29 January, ovulation falls around 15 January, and the fertile window runs from 10 to 16 January. If conception happened that cycle, the estimated due date would be 8 October — exactly 40 weeks later.
Frequently asked questions
How accurate is an ovulation calculator?
It gives a good starting estimate if your cycles are regular, but ovulation can shift by several days from one month to the next — stress, illness and travel all move it. Ovulation predictor kits, which detect the hormone surge a day or so before ovulation, or tracking your temperature and cervical mucus, are more precise.
What if my cycles are irregular?
Use your average cycle length over the last few months and treat the result as a wide window rather than a date. If cycles regularly vary by more than a week, or are shorter than 21 or longer than 35 days, it is worth mentioning to a doctor.
Can I use this to avoid pregnancy?
No. Calendar estimates are not reliable enough to be used as contraception on their own, because ovulation can come earlier or later than predicted.
Why is the due date 280 days after my last period?
Pregnancy is counted from the first day of the last period, about two weeks before conception, so a full-term pregnancy is about 40 weeks or 280 days on that count. For cycles longer or shorter than 28 days, ovulation — and so the due date — moves by the same number of days.
Is what I type here stored anywhere?
No. The calculation runs in your browser and nothing is sent to us or anyone else. Cycle data is among the most sensitive there is, which is why CycleKept keeps it only on your own phone, with no account.