Ovulation Calculator
Estimate your ovulation day and full fertile window — with adjustable cycle and luteal phase length, a fertile window calendar, and predictions for your next 3 cycles.
This is an estimate based on average cycle timing, not a guarantee. Predictions for future cycles assume your cycle length and luteal phase stay the same each time, so they become less certain the further ahead they go. This is not a substitute for advice from a doctor or fertility specialist, and should not be relied on as a form of contraception.
How to Estimate Your Ovulation Day
- Enter the first day of your last period.
- Enter your average cycle length in days (typically 21-35 days; 28 is the default).
- Enter your average luteal phase length in days if you know it (typically 10-16 days; 14 is the default and most common).
- Your estimated ovulation day, fertile window, and next period start date appear automatically, along with a fertile window calendar and predictions for your next 3 cycles.
- Adjust the cycle length or luteal phase length any time to see how it shifts your estimated dates.
Frequently Asked Questions
How is ovulation day calculated?
Ovulation is estimated to occur a set number of days before your next period — that number is your luteal phase length (14 days by default, but you can adjust it). The calculator adds your cycle length to the first day of your last period to estimate your next period, then subtracts your luteal phase length to estimate ovulation day.
What is the fertile window?
The fertile window is the estimated 6-day span with the highest chance of pregnancy — the 5 days before ovulation through ovulation day itself — since sperm can survive in the body for several days and the egg is viable for about 24 hours after release. The calendar below shows this full window for your current cycle, not just the ovulation date.
What is the luteal phase, and why does its length matter?
The luteal phase is the time between ovulation and the start of your next period. Unlike the first half of the cycle, its length is fairly consistent for a given person — typically 10 to 16 days, most commonly around 14 — so entering your actual luteal phase length, if you know it from tracking, gives a more accurate ovulation estimate than always assuming 14 days.
What if my cycle length varies from month to month?
Enter your average cycle length for the best estimate. If your cycles vary a lot, ovulation timing will vary too, and this calculator's estimate will be less precise — tracking basal body temperature or ovulation test strips over a few cycles can help narrow it down.
How reliable are the predictions for future cycles?
The multi-cycle predictions apply your same average cycle length and luteal phase length to each future cycle, so they become less reliable the further ahead they go — especially if your cycles are irregular. Treat predictions beyond your current cycle as a rough guide, not a firm schedule.
Does this work for cycles outside the typical 21-35 day range?
The calculator will still compute a result, but the underlying assumption — ovulation occurring a fixed number of days (your luteal phase length) before the next period — is best supported by research for cycles in the roughly 21-35 day range and luteal phases in the roughly 10-16 day range. Very irregular cycles are harder to predict with this method.
Is this a reliable form of birth control or a substitute for medical advice?
No — this is an estimate based on average cycle timing, not a guarantee, and individual ovulation varies. It is not a substitute for advice from a doctor or fertility specialist, and should not be relied on as a form of contraception.