Ovulation Window Calculator
This calculator estimates your likely ovulation day and fertile window from the first day of your last period, your average cycle length, and your luteal phase length. It uses the standard rule that ovulation occurs about one luteal-phase length before your next period. The fertile window is taken as the five days before ovulation plus ovulation day. All inputs are editable so you can match your own cycle. This is an educational estimate; for trying to conceive or to avoid pregnancy, combine it with direct ovulation signs and professional advice.
Ovulation window formula
Next period = last period + cycle length
Ovulation = next period - luteal phase length
Cycle day of ovulation = cycle length - luteal phase length + 1
Fertile window = (ovulation - 5 days) to ovulation
The luteal phase (ovulation to next period) is the more constant part of the cycle, so ovulation is pinned relative to the next period rather than to the last one. Cycle day 1 is the first day of bleeding.
Worked example
Last period 1 June 2026, 28-day cycle, 14-day luteal phase:
- Next period = 1 June + 28 = 29 June 2026.
- Ovulation = 29 June - 14 = 15 June 2026 (cycle day 15).
- Fertile window = 10 June through 15 June 2026.
Ovulation window: frequently asked questions
How is the ovulation day estimated?
Ovulation typically happens about 14 days before the start of your next period, because the luteal phase (from ovulation to the next period) is relatively constant at around 14 days. So ovulation day is estimated as the first day of your last period plus your cycle length minus the luteal phase length. With a 28-day cycle and a 14-day luteal phase, that places ovulation around day 14.
What is the fertile window?
The fertile window is the span of days when intercourse can lead to pregnancy. Because sperm can survive several days and the egg lives about a day, the window is usually taken as roughly the 5 days before ovulation plus the day of ovulation itself: about 6 days total. This calculator marks the window from 5 days before estimated ovulation through ovulation day.
How accurate is this estimate?
It is an estimate, not a measurement. Cycle length and ovulation timing vary month to month and person to person, and irregular cycles make calendar prediction unreliable. Ovulation predictor kits, basal body temperature tracking, and cervical mucus observation give more direct signals. Use this as a planning guide, not a guarantee.
Can I use this for contraception?
No. Calendar-based estimates are not a reliable contraceptive method on their own. If you are trying to avoid pregnancy, discuss effective contraception with a healthcare provider. Fertility-awareness methods require training and careful daily tracking to be used for that purpose.
Official sources
- Office on Women's Health, U.S. Department of Health and Human Services: Your menstrual cycle.
- U.S. National Library of Medicine, MedlinePlus: Ovulation.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.