What it does
This calendar estimates when you are most likely to ovulate, your fertile window and when your next periods should start. Enter the first day of your last period, your usual cycle length and how long your period lasts. It lists the next six cycles, marks the days on a month calendar, and makes an .ics file you can add to your phone or computer calendar.
It is an estimate from your usual cycle, not medical advice and not a form of contraception. If you become pregnant, the due date estimates the due date, and the sleep calculator helps plan bedtimes.
How to use it
- Enter the first day of your last period.
- Set your usual cycle length (first day of one period to the first day of the next) and period length.
- Read the ovulation date and fertile window, and move through the months with the arrows.
- Tap Download calendar (.ics) to save the estimates to a calendar app.
What each setting does
- First day of last period: day 1 of the cycle.
- Usual cycle length: 21 to 45 days. Ovulation is estimated 14 days before the next period.
- Period length: 1 to 10 days, shown as P on the calendar.
- Remember these three numbers: off by default. When ticked, the three values are kept in this browser only. Clear saved deletes them.
- Reset: back to the example.
On the calendar, P is a period day, F the fertile window and O the estimated ovulation day. The window runs from 5 days before ovulation to 1 day after, because sperm can live several days and the egg about a day.
Worked example
Your last period started on 1 November 2026 and your cycle is 28 days. Ovulation is estimated 14 days before the next period, on 15 November. The fertile window runs from 10 to 16 November, and the next period is due about 29 November 2026.
FAQs
How accurate is this? It is a rough guide. Even regular cycles vary, and stress or illness can move ovulation. Ovulation tests or tracking temperature give a closer picture.
Can I use it to avoid pregnancy? No. A calendar estimate is not reliable contraception.
Is my data saved? Only if you tick the box, and then only in this browser. Nothing is sent anywhere.
Where the numbers come from
- NHS, Periods and fertility in the menstrual cycle: ovulation is usually 10 to 16 days before the next period; sperm can live up to 7 days (checked 2026-11-10).
- Wilcox A.J., Weinberg C.R., Baird D.D. (1995), Timing of sexual intercourse in relation to ovulation, New England Journal of Medicine 333: the fertile window is the 6 days ending on ovulation day (checked 2026-11-10).
- Ovulation 14 days before the next period and a window of 5 days before to 1 day after are the common calendar-method estimates; fibub's own working.
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Last updated 2026-11-10.