Period Calculator
Find out when your next period is due, how many days to go and what day of your cycle you're on, with a calendar of the months ahead.
Your cycle runs from the first day of one period to the day before the next. 28 days is a common average; this calculator accepts 21 to 45.
Add the first day of 2 or more earlier periods and we'll use your average cycle instead of the usual cycle length above, with your shortest and longest cycles.
The luteal phase is the time from ovulation to your next period, usually 12 to 14 days. It only moves the estimated fertile days; leave it at 14 unless you track ovulation.
Opens in Google Calendar, Apple Calendar or Outlook. The file is made in your browser.
Nothing is sent to us. Saving keeps your dates only in this browser, so they're filled in next time.
How your next period is worked out
Add your cycle to your last period, mark the period days, count down from today, then repeat.
- 1
Add your cycle
9 October + 28 d
= 6 November - 2
Mark the period days
6 November +4
to 10 November - 3
Count from today
18 days to go
cycle day 11 - 4
Repeat each cycle
+28 d = 4 December
ovulation ~20 November
Read more: the formula, a worked example, irregular and late periods, and when to see a doctor
A period calculator uses the calendar: your next period should start one cycle length after your last one began. Counting the first day of bleeding as day 1, it adds your cycle length to find the next start date, adds your period length to find when it should end, and repeats that for the months ahead. Real cycles move around by a few days, so treat every date as an estimate.
The formula
next period starts = first day of last period + cycle length
next period ends = next period start + period length − 1 day
days to go = next period start − today
cycle day today = today − first day of this cycle + 1
ovulation (estimate) = next period start − luteal phase (14 days)
fertile window (estimate) = the 5 days before ovulation + ovulation day
With earlier start dates, each gap between two period starts is one cycle. The average gap, rounded to a whole day, becomes your cycle length, and the shortest and longest gaps give the range your next period could start in. If today is past the date your period was due, the calculator tells you how many days late it is, and moves the calendar on as if it had come on time.
Worked example
Say your last period started on 1 January 2026, your cycle is 28 days and your period lasts 5 days. Your next period is due on 1 January + 28 days = 29 January, and should last 5 days, from 29 January to 2 February (29, 30, 31 January, 1 and 2 February). The one after that starts 29 January + 28 = 26 February and runs to 2 March, because February 2026 has 28 days. On 5 January you'd be on cycle day 5, with 29 − 5 = 24 days to go. Your estimated ovulation is 29 January − 14 = 15 January, and the estimated fertile window is 10 to 15 January.
With past start dates of 4 December 2025, 3 January, 1 February and 1 March 2026, the gaps are 30, 29 and 28 days. The average is 87 ÷ 3 = 29 days, so the next period is due 1 March + 29 = 30 March. The shortest (28) and longest (30) cycles say it could start between 29 and 31 March. If nothing had started by 3 April, it would be 4 days late, and later than your longest recent cycle.
Cycle length and your next period
If your last period started on day 1, your next one starts on this day of the calendar count:
| Cycle length | Next period starts on | Estimated ovulation |
|---|---|---|
| 21 days | day 22 | cycle day 8 |
| 24 days | day 25 | cycle day 11 |
| 26 days | day 27 | cycle day 13 |
| 28 days | day 29 | cycle day 15 |
| 30 days | day 31 | cycle day 17 |
| 32 days | day 33 | cycle day 19 |
| 35 days | day 36 | cycle day 22 |
| 40 days | day 41 | cycle day 27 |
Irregular and late periods
It's normal for cycles to change by a few days. If your shortest and longest recent cycles are more than 7 days apart, your periods are usually called irregular, and a single predicted date is less likely to be right. Enter at least 3 to 6 past start dates in Advanced to see your own range. Stress, illness, travel, changes in weight or exercise, breastfeeding, starting or stopping hormonal contraception, and conditions such as PCOS or thyroid problems can all change your cycle.
If your period is late and you could be pregnant, a home pregnancy test is most reliable from the day your period was due. Our ovulation calculator shows your fertile days in more detail, and the due date calculator works out a due date from your last period.
When to talk to a doctor
- Your periods have stopped for 3 months or more and you're not pregnant, or you could be pregnant and aren't sure.
- Your cycles are often shorter than 21 days or longer than 35 days, or change a lot from month to month.
- Your periods are very heavy, last longer than 7 days, are very painful, or you bleed between periods or after sex.
- You're worried about anything to do with your periods. That's always a good enough reason.
Please note
- Predicted dates are estimates from the calendar, not a medical test.
- This is not a form of contraception. Calendar dates alone are not a reliable way to avoid pregnancy.
- Nothing you enter is sent to us. "Save on this device" keeps your dates in this browser only.
For information only, not medical advice. Please speak to your doctor, nurse or pharmacist about your health. Checked on 10 October 2026.
Common questions
How do I work out when my next period is due?
Add your usual cycle length to the first day of your last period. If your last period started on 1 January and your cycle is 28 days, your next period is due on 29 January. With a 5-day period it would last until 2 February. The calculator does this for the next 6 to 12 cycles.
What is a normal cycle length?
Counting from the first day of one period to the day before the next, cycles of 21 to 35 days are common in adults, and periods usually last 2 to 7 days. Your cycle can change by a few days from month to month, so a predicted date is an estimate, not a fixed day.
My period is late. What should I do?
A period a few days late is common, and stress, illness, travel or weight changes can delay it. If you could be pregnant, a home pregnancy test is most reliable from the day your period was due. If you have missed 3 periods in a row, or you are worried, talk to a doctor or nurse.
Can I use a period calculator if my periods are irregular?
Yes. In Advanced, enter the start dates of several past periods. The calculator averages the gaps between them, shows your shortest and longest cycles, and gives a range of dates your next period could fall in. If your shortest and longest cycles are more than 7 days apart, treat the dates as rough.
Can I use this to avoid getting pregnant?
No. Predicted dates are estimates and are not a form of contraception. Ovulation can move from month to month, and it can happen earlier or later than a calendar suggests. Please talk to a doctor, nurse or pharmacist about contraception that suits you.
Do you store my period dates?
No. Everything is worked out in your browser and nothing is sent to us. If you tap "Save on this device", your dates are kept only in this browser so they are filled in next time, and "Forget my dates" removes them.
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Made by Zulu Calculator, part of AlphaZulu LLC. Formulas are shown in full and rule figures are checked against official sources; see how we build calculators.