Ovulation Calculator
Ovulation usually happens about 14 days before the next period starts. With a 28-day cycle that began on 1 January 2024, the estimated ovulation day is Mon, Jan 15, 2024, with the fertile window from Wed, Jan 10, 2024 to Mon, Jan 15, 2024. This is an estimate and does not work as birth control.
How to use
- Enter the first day of your last period.
- Enter your usual cycle length in days, counted from the first day of one period to the day before the next. Leave 28 if you are not sure.
- Read the estimated ovulation day and fertile window, and the same dates for the following two cycles.
How it works
Ovulation is estimated at the cycle length minus 14 days after your period starts, because the time from ovulation to the next period (the luteal phase) is usually about 14 days. For a 28-day cycle that is day 14. For a 32-day cycle it is day 18.
The fertile window is the 5 days before ovulation plus the day of ovulation, 6 days in total. Sperm can survive for up to 5 days, and the egg lives about a day after release.
Real cycles vary. Ovulation can move from month to month, and the estimate is less reliable when cycles are irregular or you have conditions such as PCOS. Ovulation tests, basal body temperature and cervical mucus give more direct signs.
This is NOT a form of contraception. You can become pregnant on days outside the estimated window. It is general information, not medical advice. If you have been trying to conceive for a year (6 months if you are over 35) without success, see a doctor.
Values used
| Item | Value |
|---|---|
| Luteal phase | 14 days (fixed) |
| Ovulation day | Cycle length minus 14, counted from day 1 of the period |
| Fertile window | Ovulation day minus 5 days, through the ovulation day |
| Cycle length accepted | 21 to 45 days |
Sources and disclaimer
Fertile window: Wilcox, Weinberg and Baird, "Timing of sexual intercourse in relation to ovulation", New England Journal of Medicine 1995 (six-day window ending on the day of ovulation). Luteal phase length: NHS and Office on Women's Health (US HHS) cycle guidance.
This tool is not medical advice and is not a contraceptive method.
Sources: NHS · Office on Women's HealthLast reviewed 2026-10-13
FAQ
How accurate is an ovulation calculator?
It is a calendar estimate. It works best for regular cycles and can be off by several days. Ovulation tests, which detect the LH surge, are more direct.
Can I use it to avoid pregnancy?
No. Calendar methods alone have a high failure rate with typical use. If you want to prevent pregnancy, talk to a doctor or clinic about reliable contraception.
What if my cycles are irregular?
Use your average cycle length, but treat the dates as very rough. Tracking with ovulation tests or temperature, and talking to a doctor, will help more.
Is my data sent anywhere?
No. The calculation runs in your browser and nothing you enter is sent to a server.