Ovulation Calculator
Estimate the day you ovulate, the days when pregnancy is most likely, and when your next period is due. The calendar method works best with regular cycles — it is a prediction, not a measurement.
Enter your details above — results update as you type.
Formula and calculation
Worked example
If your last period started on September 25, 2026 and your cycles run 28 days, the next ovulation is estimated for Friday, November 6, 2026 — that is 21 days after October 16, 2026. The fertile window runs November 1 – 7, 2026, the next period is expected around October 23, 2026, and today is cycle day 22.
Beyond counting days
The calendar is the crudest fertility tracker, because it assumes your next ovulation repeats last month's. Two signs narrow it down: an LH test turns positive about a day before ovulation, and basal body temperature rises a few tenths of a degree after ovulation — useful for confirming the pattern, though it reports after the fact. Cervical mucus turning clear and stretchy is the cheapest signal. Whatever you use, remember that none of these predict an irregular cycle reliably; if your cycles are always shorter than 21 days or longer than 35, or you have been trying to conceive for over a year, a clinician can check for causes.
What the results mean
- Estimated ovulation
- The estimated ovulation day: about 14 days before your next period is due. This is the single most fertile day, but pregnancy is possible over a wider window.
- Fertile window
- The stretch of days when pregnancy is most likely — the five days before ovulation plus the day after it, written as a date range.
- Window opens
- The first day of the fertile window — five days before the estimated ovulation day. Sperm can survive this long, so sex from this day on can lead to pregnancy.
- Window closes
- The last day of the fertile window — the day after ovulation. This is generous by a few hours: the egg lives about 12 to 24 hours after it is released.
- Next period expected
- The date your next period is expected to start: your last period date plus one cycle length.
- Days until ovulation
- How many days from today until the estimated ovulation day. Zero means the estimate falls on today.
- Cycle day today
- Where you are in the current cycle, counting the first day of your period as day 1. Doctors use cycle days when timing tests and scans.
- Cycle pattern
- How well the calendar method fits your cycles. With irregular cycles the date can shift by a week or more, so treat it as a rough guide rather than a schedule.
Frequently asked questions
How accurate is the calendar method for ovulation?
For regular cycles it usually lands within a day or two; for irregular cycles it can be off by a week or more. The 14-days-before-your-period rule is an average of a stable luteal phase, not a fixed setting, and factors like illness, stress, travel and weight change move ovulation. If the exact day matters — for timing a procedure or interpreting a test — use ovulation tests or ultrasound rather than a calendar.
Can I get pregnant outside the fertile window?
Yes, though it is much less likely. The window is where the odds are concentrated: sperm can survive up to about five days, and the egg about a day. If ovulation is a few days earlier or later than predicted, sex you thought was outside the window can still lead to pregnancy.
Can I use this calculator as birth control?
No. This page is a calendar estimate, and calendar-based contraception has high real-world failure rates even when followed carefully — it does not track the body signs (temperature, cervical mucus, hormone tests) that fertility awareness methods rely on. Use a method designed for contraception, and ask a clinician which one suits you.
Why does ovulation move later in a longer cycle?
Because the days after ovulation are the stable part. The luteal phase — ovulation to the next period — runs about 14 days in most people, so the extra days in a 32-day cycle are added before ovulation, not after. That puts ovulation around cycle day 18 instead of day 14.
What if my cycles are irregular?
Then a single predicted date is the wrong tool: ovulation can shift by a week or more from cycle to cycle, and the fertile window moves with it. Track more than one cycle and look for patterns, or use ovulation tests in the cycle you care about. Talk to a clinician if your cycles are consistently shorter than 21 days, longer than 35 days, or have stopped without a clear reason.