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.

First day of last periodCycle length — 28 daysPeriod length — 5 days

Next period

Sat 10 Oct

in 28 days · through Wed 14 Oct

Fertile window

Mon 21 Sept – Sun 27 Sept

Six days, ending the day after ovulation — sperm survive several days, an egg roughly one.

Estimated ovulation

Sat 26 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 1Sat 10 Oct
Cycle 2Sat 7 Nov
Cycle 3Sat 5 Dec

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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