Ovulation Calculator

How ovulation is estimated

Ovulation is the release of an egg from an ovary. The NHS says that in most women it happens around 10 to 16 days before the next period starts. The time from ovulation to the next period is called the luteal phase, and the calculator follows the usual calendar convention of counting backward from your expected next period:

1. Your next period is expected one cycle length after the first day of your last period.
2. Ovulation is estimated 14 days before that, or the luteal phase length you enter.
3. The fertile window is the five days before ovulation plus ovulation day itself.

The six-day window comes from a study by Wilcox and colleagues, published in the New England Journal of Medicine in 1995. They followed 221 women trying to conceive and pinpointed ovulation from daily urine hormone measurements. Conception happened only from intercourse in the six days ending on the day of ovulation. The chance of conception on a single day ranged from about 10% five days before ovulation to about 33% on the day of ovulation. Earlier days count because sperm can survive in the body for several days; the NHS says up to 7 days.

The table below the result repeats the calculation for the next six cycles, assuming each one is the same length as your average.

Ovulation and fertile window formulas

LMP

first day of the last menstrual period (cycle day 1)

C

average cycle length in days

L

luteal phase length in days, 14 if unknown

Counting the first day of bleeding as day 1, ovulation falls on cycle day C − L + 1: day 15 for a 28-day cycle with a 14-day luteal phase, and the fertile window runs from day 10 to day 15. The NHS describes women with a regular 28-day cycle as likely to be fertile around day 14; a difference of a day is well within the natural variation of ovulation from cycle to cycle.

A longer cycle moves ovulation later by the same number of days, because the luteal phase is assumed not to change: a 35-day cycle puts ovulation on day 22.

Worked example

Your last period started on Tuesday, September 1, 2026, and your cycles are usually 28 days:

1. Next period: September 1 + 28 days = September 29, 2026.
2. Ovulation: September 29 − 14 days = Tuesday, September 15, 2026 (cycle day 15).
3. Fertile window: September 15 − 5 days = September 10, so the window is September 10 to 15.
4. The following cycles start on September 29, October 27, November 24 and December 22, with estimated ovulation on October 13, November 10, December 8 and January 5, 2027.

If your cycles are 35 days instead, the next period is due on October 6 and ovulation moves a week later, to September 22, with the fertile window from September 17 to 22.

Signs that tell you more than the calendar

A calendar estimate assumes that this cycle will be like your average one. Ovulation can come earlier or later than expected in any cycle, and the calendar has no way of knowing. Direct signs from your body tell you what is happening in the current cycle:

• Ovulation predictor kits detect the surge of luteinizing hormone (LH) in urine that comes shortly before ovulation.
• Basal body temperature rises slightly after ovulation. Charted every morning, it shows afterward that ovulation has happened, which helps you learn your pattern over several cycles.
• Vaginal discharge changes through the cycle, and fertility awareness methods record it daily alongside temperature.

If you are trying to conceive, the NHS suggests regular unprotected sex every 2 or 3 days through the cycle rather than relying only on predicted dates. It reports that more than 8 in 10 couples where the woman is under 40 conceive within a year this way.

When to talk to a doctor

This calculator estimates dates; it can’t tell whether you ovulate or explain a change in your cycle. See a doctor if:

• you have been trying to conceive for a year without success; the NHS advises women aged 36 and over, and anyone who already knows of a possible fertility problem, to go sooner;
• your periods are irregular, or last longer than 7 days;
• you have bleeding between periods.

If your cycles vary a lot from month to month, an average cycle length gives a wide margin of error, and ovulation tests or temperature charting are more useful than any calendar. If you think you may already be pregnant, the due date calculator estimates when the baby would be due; to count days between any two dates, use the days between dates calculator.