Period Calculator
One cycle, three numbers — see your next 6 periods, ovulation day and fertile window.
Estimates only, based on your average cycle — real cycles vary from month to month. This tool is not a method of contraception. For medical concerns, talk to a doctor or midwife.
The first day of actual bleeding, not the day it ended.
From the first day of one period to the first day of the next. Most cycles run 21–35 days, but longer ones up to 60 are accepted; 28 is the default.
How many days your bleeding usually lasts. Most periods run 2–10 days; 5 is the default.
Days from ovulation to the next period. Most people are 12–14; leave 14 unless a doctor or a basal-temperature chart told you otherwise.
Next 6 cycles
Calculated in your browser. Your dates stay on this device and are never sent to a server.
Predicting your next periods
A period calculator projects the start dates of your upcoming menstrual periods from a single recent cycle. Where an ovulation estimate zooms in on one fertile window, this tool looks further ahead — mapping the next several cycles so you can see, at a glance, which weeks a period is likely to land on. That is useful for planning travel and events, or simply for keeping enough supplies on hand.
The three inputs
- First day of your last period — the first day of actual bleeding, which anchors every prediction that follows.
- Average cycle length — the number of days from the start of one period to the start of the next, usually between 21 and 45 days.
- Period length — how many days your bleeding typically lasts, most often between 2 and 10 days.
How the projection is built
Each predicted period simply adds your average cycle length to the start of the one before it, repeated forward for several cycles. The last day of bleeding for any cycle is its start date plus the period length, minus one — because the first day already counts as day one.
Example 1 — mapping the next cycle. Last period started March 3, cycle length 30 days, period length 5 days. The next period starts March 3 + 30 = April 2 and runs April 2–6. The one after that starts May 2, then June 1, marching down the calendar.
Example 2 — why predictions drift. Predictions stack, so any error in your average multiplies. If your true cycle is 28 days but you entered 30, each prediction is 2 days late, and by the sixth cycle the projected start is about 6 × 2 = 12 days off the real one.
Example 3 — an irregular range. If your cycles swing between 26 and 32 days, a period predicted for a given day could realistically arrive anywhere across a roughly six-day span. The tool shows the average path; treat each date as the centre of a window, not a fixed appointment.
How cycle length moves the dates
| Average cycle | 2nd period | 3rd period | 4th period |
|---|---|---|---|
| 26 days | +26 days | +52 days | +78 days |
| 28 days | +28 days | +56 days | +84 days |
| 30 days | +30 days | +60 days | +90 days |
| 32 days | +32 days | +64 days | +96 days |
Offsets are counted from the first day of the period you entered.
Getting a sharper prediction
- Use a real average. Tracking three or four cycles and averaging their lengths beats guessing 28, especially if your cycles are naturally short or long.
- Re-anchor each month. Because errors accumulate, updating the "last period" date whenever a new period actually arrives keeps the next prediction accurate.
- Expect variation. Illness, stress, travel, and hormonal changes routinely shift a cycle by a few days; a calendar cannot account for them.
Important limits
These are estimates based on your own averages, not a diagnosis and not a method of contraception. Real cycles vary, and a missed or very irregular period can have many causes. For contraception, for trying to conceive or to avoid pregnancy, or for any concern about changes in your cycle, talk to a doctor or midwife. Everything here is calculated on your device and never sent to a server.
Sources & further reading
- ACOG — patient guidance on menstrual cycles, ovulation and reproductive health
- U.S. Office on Women's Health — how the menstrual cycle works and what counts as irregular
- NIH NICHD — research-based overview of menstruation and cycle disorders
- NHS — periods, cycle length variation and when to see a clinician
Frequently asked questions
How do you calculate my next 6 period dates?
This period calculator adds your average cycle length to the first day of your last period, then repeats that six times to project the start of your next 6 periods. Each period's end date is the start plus your period length minus one day. For example, a last period starting January 1 with a 28-day cycle and a 5-day period gives a next period from January 29 to February 2, then February 26 to March 2, and so on. The further out a projection is, the more small cycle variations can shift the real date.
How is my ovulation day and fertile window estimated?
Each cycle's ovulation day is estimated as that cycle's period start minus 14 days, using the standard luteal-phase method — the time from ovulation to the next period is the most stable part of the menstrual cycle at about 14 days, while the days before ovulation vary more. The fertile window then runs from 5 days before ovulation to 1 day after, a 7-day span, because sperm can survive a few days in fertile cervical mucus while the egg lives only about a day after release.
Can I use this period calculator instead of birth control?
No. This tool estimates dates from averages, and real cycles shift with stress, illness, travel, sleep and many other factors, so calendar-based predictions are not a reliable form of contraception — with typical use, calendar or rhythm methods fail for roughly 12–24 out of every 100 women in a year. Use it for planning and awareness only, and ask a doctor about a proven contraceptive method if you need one.
What if my cycle length or period length isn't always the same?
Enter the average of your last 3–6 cycles and treat every date below as a rough estimate rather than a guarantee. If your cycle length swings by more than a few days, falls outside 21–45 days, or your period length falls outside 2–10 days, these projections become less reliable — an ovulation test kit (LH strips), basal body temperature tracking, or a doctor's advice will serve you better than the calendar alone.
Is my period data stored or sent anywhere?
No. Every date is calculated locally in your browser with JavaScript — your last period date, cycle length and period length are saved only in this browser's local storage so the form is ready next time, and "Clear saved data" removes them immediately. Nothing is uploaded, there's no account, and no one else can see what you enter.