Ovulation Calculator: Fertile Window and Ovulation Day

About the Ovulation Calculator

The Ovulation Calculator estimates your fertile window, the likely day of ovulation and the date of your next period. Enter the first day of your last period, your usual cycle length, how many days your period lasts and, if you know it, the length of your luteal phase. The calculator then lists the current cycle and the five after it, so you can see six months of fertile days and period dates at a glance.

All dates are whole calendar days, so the result is the same wherever you are and never shifts with a time zone or a change to daylight saving time. If the date you enter is a few cycles ago, the calculator rolls forward to the cycle you are in today and tells you which cycle day it is.

This is an estimate, not medical advice, and it is not a method of contraception. Ovulation moves from month to month, even in regular cycles, so treat the dates as a guide. If you are trying to conceive, ovulation test kits, basal body temperature or cervical mucus checks give a direct sign that a calendar cannot.

How to use it

The formula

For each cycle the calculator works out:

$$\begin{aligned} \text{Next period} &= \text{period start} + \text{cycle length} \\ \text{Ovulation} &= \text{next period} - \text{luteal phase} \\ \text{Fertile window} &= \text{ovulation} - 5 \text{ days to ovulation} \end{aligned}$$

The six-day fertile window comes from Wilcox, Weinberg and Baird's 1995 study, which followed 221 women and found that pregnancy happened only when intercourse fell in the five days before ovulation or on the day itself. Sperm can survive for several days, while the egg lives for about a day.

Worked example

With a last period on 1 January 2026, a 28-day cycle, a 5-day period and a 14-day luteal phase, the period runs from 1 to 5 January, the next period is due on 29 January, ovulation falls on 15 January (29 January minus 14 days) and the fertile window runs from 10 to 15 January. The sixth cycle in the table starts on 21 May 2026.

Privacy

Nothing is uploaded; every calculation runs in your browser and never leaves your device. The dates you enter are never sent to a server, are not stored, and disappear when you close the page.

Related tools

If you think you are pregnant, the Due Date Calculator estimates your due date from the same last-period date, and the Pregnancy Calculator shows how many weeks along you are. To focus on period dates alone, use the Period Calculator.

Sources

• Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995;333:1517-1521.
• Lenton EA, Landgren BM, Sexton L. Normal variation in the length of the luteal phase of the menstrual cycle. Br J Obstet Gynaecol 1984;91:685-689.

Open-source credits

The calculations are implemented in-house in TypeScript, with no third-party libraries. They follow the published method above: ovulation one luteal phase before the next period and the six-day fertile window of Wilcox et al.

Frequently asked questions