Period Calculator: when is my next period?
Calculate your next 12 periods, even with an irregular cycle, with a reminder in your phone calendar
Runs only in your browser, nothing is stored
Your next periods at a glance
Enter the first day of your last period. You will then see when your next 12 periods are due and when you are likely to ovulate.
- Cycle day today
- Period until about
- Pregnancy test from
Period calendar: your next periods
Trying to conceive? Fertile days in the Ovulation Calculator
Reminder on your phone
Your period in the calendar
Your phone calendar reminds you of your next periods the evening before, with no app and no sign-up. If your period comes earlier or later, just recalculate.
iPhone, Mac, Outlook, and Samsung: “Add to calendar.” Android: “Google Calendar” (periods only). For the file, only the dates are sent once to our server, and nothing is stored (Privacy policy).
Printable
Printable cycle calendar
One sheet for 12 months: mark every day you bleed, and you will see your cycle lengths at a glance. If you have used the calculator above, your next periods are already filled in.
By calculation, your next period starts on the first day of your last period plus your cycle length. If your last period started on Sunday, 1 November 2026, and your cycle lasts 28 days, the next one is due on Sunday, 29 November. This is how the Period Calculator projects your next 12 periods, and it also shows your estimated ovulation.
Calculating your period: how the Period Calculator works
Cycle day 1 is the first day with real bleeding; spotting before that does not count. The cycle ends on the day before the next period.
- Regular cycle: The calculator adds your cycle length month by month and shows each period start as a fixed date.
- Irregular cycle: Your shortest and longest cycle give a range. With 26 to 32 days and a period starting on 1 November, the next period comes between 27 November and 3 December.
- Cycle length unknown: Under “Cycle length from previous periods,” enter the start dates of your last periods. The calculator uses them to work out the average and the range and applies them.
- Ovulation: By calculation, it falls about 14 days before the next period. If you know your luteal phase, set it under “Calculate more precisely.”
The calculation runs only in your browser. Fertilio does not store any cycle data.
When does your period come after ovulation?
Your period usually starts 10 to 16 days after ovulation. This time is called the luteal phase, or corpus luteum phase. In an analysis of more than 600,000 cycles, it lasted 12.4 days on average. The time before ovulation, the follicular phase, averaged 16.9 days and varied much more.
This means that if your period comes earlier or later than usual, it is usually ovulation that has shifted, not the time after it. If you know when you ovulate from ovulation tests or your basal body temperature, you can predict your period more accurately: day of ovulation plus your luteal phase. The Ovulation Calculator shows your fertile days.
How long is a normal cycle?
A cycle of exactly 28 days is more the exception than the rule. In the same analysis, the average cycle length was 29.3 days. Between the ages of 25 and 45, cycles became shorter by an average of 0.18 days per year of age, mainly because the follicular phase gets shorter.
Cycles of 21 to 35 days are common. The bleeding itself lasts 2 to 7 days, usually about 5. If your cycles are regularly shorter or longer than that, have them checked by a gynecologist.
Irregular cycle
If your cycle varies by more than 7 days, any calendar-based estimate is rough. Common reasons include stress, major changes in weight, a lot of endurance exercise, thyroid disorders, polycystic ovary syndrome (PCOS), the first months after stopping the pill, breastfeeding, and the years before menopause.
For the prediction, it helps to enter your shortest and your longest cycle from the last 6 to 12 months. If you want to get pregnant and your cycle is irregular, it is better to determine ovulation with ovulation tests or a fertility tracker.
Late period: when to test?
A urine pregnancy test is reliable from the day your period would be due. Early tests sometimes turn positive a few days earlier, but they give a false negative result more often. If the test is negative and your period still does not come, repeat it after a few days. If your period stays away for three cycles, have the cause checked by a gynecologist.
Frequently asked questions
How do I calculate my next period?
Take the first day of your last period and add your cycle length in days. If your period started on 1 November and your cycle is 30 days long, your next period is due on 1 December. If you do not know your cycle length, you can calculate it from the gaps between the start dates of your last periods.
How accurate is a period calculator?
It gives an estimate. Even with a regular cycle, your period can shift by a few days, for example because of stress, illness, or travel. The more previous periods you enter, the better the cycle length fits.
How long after ovulation does your period come?
Usually after 10 to 16 days, on average after just over 12 days. In the same woman, this luteal phase is fairly stable from cycle to cycle.
How long does a normal period last?
The bleeding lasts 2 to 7 days, usually about 5. You can set this value in the calculator under “Calculate more precisely.” It determines how many days are marked as your period in the calendar.
Can I use the Period Calculator for birth control?
No. A pure calendar estimate is too unreliable for birth control, because ovulation can shift.
Does Fertilio store my cycle data?
No. The Period Calculator calculates only in your browser. For the calendar file, the calculated dates are sent once to our server, converted into a file there, and not stored.
Sources
- Bull JR, Rowland SP, Scherwitzl EB et al. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. npj Digit Med. 2019;2:83. doi:10.1038/s41746-019-0152-7
- NHS: Periods, accessed October 2026