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Cycle estimate
Ovulation calculator
Estimate an ovulation date, a fertile window and up to six upcoming cycles. The result is a calendar estimate, not confirmation that ovulation occurred.
Estimate only
Do not use this result as birth control
Cycle information
Estimated dates
- Estimated fertile window
- Estimated next period
- Estimated current period
Cycle forecast
| Cycle | Period | Ovulation | Fertile window | Next period |
|---|
Actual ovulation can occur earlier or later. These dates do not confirm ovulation.
How the estimate works
The calculator estimates the next period by adding the average cycle length. It then places ovulation 14 days before that estimated next period and shows a window from five days before through one day after.
Next period
cycle start + average cycle lengthThe first day of the following cycle is projected from the average length.
Estimated ovulation
next period - 14 daysA conventional midpoint estimate; real luteal-phase length varies.
Fertile window
ovulation - 5 days to ovulation + 1 dayReflects the possible survival time of sperm and the egg around ovulation.
Important limitations
- The calculation assumes future cycles resemble the average cycle entered.
- It cannot confirm whether or when an egg was released.
- It is less reliable for irregular cycles or when cycles are changing.
- Symptoms, ovulation tests and clinical advice may provide different information.
Examples
28-day cycle
Last period: 1 July 2026. Cycle: 28 days.
Estimated ovulation: 15 July 2026. Fertile window: 10-16 July.
32-day cycle
Last period: 1 July 2026. Cycle: 32 days.
Estimated ovulation: 19 July 2026. Fertile window: 14-20 July.
Six-cycle forecast
Forecast: 6 cycles.
The same average interval is applied successively to create six calendar estimates.
Frequently asked questions
Does this confirm ovulation?
No. It only estimates dates from the cycle information entered.
Why is ovulation placed 14 days before the next period?
This is a common calendar convention. In real cycles, the interval can be shorter or longer.
Can I use it to avoid pregnancy?
No. Do not rely on this calculator alone for contraception.
What if my cycles are irregular?
The result may be substantially inaccurate. Discuss irregular cycles with a qualified health professional when appropriate.
Sources and review
- Office on Women’s Health - Ovulation calculator
Official estimate tool and warning that fertile-window predictions are not birth control or medical advice.
- ACOG - Fertility awareness-based methods
Explains fertility awareness methods and their practical limits.
- NHS - Periods and fertility in the menstrual cycle
Explains that ovulation timing varies and often occurs around 10 to 16 days before the next period.