Cycle & Fertile Window Calculator
Estimate your next period, ovulation and fertile window. Everything is calculated in your browser — nothing you type is stored or sent anywhere.
Next period
Mon 21 Sept
in 28 days · through Fri 25 Sept
Fertile window
Wed 2 Sept – Tue 8 Sept
Six days, ending the day after ovulation — sperm survive several days, an egg roughly one.
Estimated ovulation
Mon 7 Sept
Counted back 14 days from the next period. The luteal phase is the stable half of the cycle, so counting backwards is steadier than counting forwards.
Next three cycles
- Cycle 1Mon 21 Sept
- Cycle 2Mon 19 Oct
- Cycle 3Mon 16 Nov
Nothing leaves this page. The dates you enter are held in the page's memory and calculated on your device. There is no account, no request to a server, and nothing written to storage — close the tab and it is gone.
Estimates only. Calendar predictions assume a regular cycle and cannot account for illness, stress, medication or perimenopause. This is not contraception and not medical advice — for anything that matters, talk to a clinician.
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