Fertile window for irregular cycles.

Fertile Window Calculator for Irregular Cycles: how it works

The calendar method estimates your fertile days as running from your shortest cycle minus 18 to your longest cycle minus 11. The days are counted from day 1, the first day of a period. For example, if your cycles ranged from 26 to 35 days, the window is cycle day 8 (26 - 18) through cycle day 24 (35 - 11), which is 17 days.

This is the traditional Knaus-Ogino (rhythm) calendar rule as described in the Wikipedia article on calendar-based contraceptive methods: subtract 18 from the shortest cycle for the first fertile day, and subtract 11 from the longest cycle for the last. It is a rule of thumb, not a clinical standard, and the more your cycles vary the wider the window becomes. The Wikipedia article also gives a typical-use failure rate of 25% per year when such methods are used to avoid pregnancy, and Planned Parenthood says fertility awareness methods will not work well with an irregular cycle. Do not use this page for contraception.

For timing intercourse when trying to conceive, in 625 cycles Wilcox, Weinberg and Baird (1995) found conception occurred only when intercourse fell in the six days ending on the day of ovulation, and the NHS says sperm can survive inside the body for up to 7 days. The NHS also suggests sex every 2 or 3 days throughout the cycle when periods are irregular, since ovulation may not happen regularly. Ovulation tests or temperature charting can narrow the window; ask your clinician if you have been trying for a while.

Worked example. Cycles of 26, 29, 32, 27, 35 and 30 days. Shortest 26, so first fertile day = 26 - 18 = cycle day 8. Longest 35, so last fertile day = 35 - 11 = cycle day 24. The window is 24 - 8 + 1 = 17 days. If the current cycle started on 1 October 2026, cycle day 8 is 8 October and cycle day 24 is 24 October.