When Do You Ovulate on a 40-Day Cycle?
On a 40-day cycle, you are most likely to ovulate around cycle day 26, counting the first day of your period as day 1. For example, if your period started on March 1, ovulation would probably fall around March 26.
| Cycle length | 40 days |
|---|---|
| Likely ovulation day | Cycle day 26 (range 24–28) |
| Fertile window | Cycle days 21–26 |
| Most fertile days | Cycle days 24–26 |
| Next period due | Cycle day 41 |
Why day 26?
Ovulation is counted backwards from the next period because the second half of the cycle, the luteal phase, is fairly steady at about 12 to 14 days for most people. The first half, before ovulation, is what changes from person to person.
So for a 40-day cycle, 40 minus 14 gives day 26. If your luteal phase is a little shorter or longer, ovulation could fall anywhere from about day 24 to day 28.
Confirming ovulation
Ovulation predictor kits detect the LH surge that comes 24 to 36 hours before the egg is released, and a sustained rise in basal body temperature confirms ovulation after it has happened. Changes in cervical mucus, which becomes clear and stretchy, are another useful sign.
Cycles longer than 35 days fall outside the typical adult range of 21 to 35 days. Long or unpredictable cycles can be a sign that ovulation isn't happening every month, for example with polycystic ovary syndrome (PCOS) or thyroid problems. If your cycles are regularly 40 days or longer, it's worth talking to a doctor, especially if you're trying to conceive.
Ovulation Calculator: Predict ovulation days for your next three cycles.
Open the calculatorThis article is general information, not medical advice. Talk to a doctor or midwife about your own pregnancy or fertility.