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
Sat 12 Sept
in 28 days · through Wed 16 Sept
Fertile window
Mon 24 Aug – Sun 30 Aug
Six days, ending the day after ovulation — sperm survive several days, an egg roughly one.
Estimated ovulation
Sat 29 Aug
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 1Sat 12 Sept
- Cycle 2Sat 10 Oct
- Cycle 3Sat 7 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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