Period Calculator
Estimate when your next six periods start from the first day of your last one and your average cycle length.
Result
Next period expected to start: September 29, 2026Estimate
Assumes every cycle is your average length. Real cycles vary, so treat each date as the middle of a few likely days.
- Second period from now starts
- October 27, 2026Estimate
- Third period from now starts
- November 24, 2026Estimate
- Fourth period from now starts
- December 22, 2026Estimate
- Fifth period from now starts
- January 19, 2027Estimate
- Sixth period from now starts
- February 16, 2027Estimate
https://calcgallery.com/pregnancy/period-calculator/
How to use the Period Calculator
- Enter the first day of your last period, meaning the first day of bleeding rather than spotting.
- Enter your average cycle length in days, counted from the first day of one period to the first day of the next.
- Optionally enter how many days your period usually lasts to see when the next one is expected to end.
- Read the next six start dates as estimates. The further ahead a date is, the more real cycles can drift from it.
How the dates are worked out
A menstrual cycle is counted from the first day of one period to the first day of the next. So if you know when your last period started and how long your cycles usually are, each future start date is one more cycle on:
next period = first day of last period + cycle lengthk-th period ahead = first day of last period + k × cycle lengthperiod end = that start + period length − 1 dayThe minus one in the last line is because the first day of bleeding is day 1. A five-day period that starts on a Monday ends on the Friday, not the Saturday.
The page lists six upcoming start dates, about half a year for most cycles. The calendar arithmetic handles month lengths, leap days and year ends, so a cycle that crosses the end of February or the end of December lands on the right day.
Why the dates get less certain further out
Every date assumes every cycle is exactly your average. If your cycles actually run a couple of days either side of that average, the uncertainty adds up: the next period might be a day or two off, while the sixth could be a week off. Treat the first date as a reasonable estimate and the later ones as a rough plan.
If you track your periods, updating the first date each time a period starts keeps the next prediction close. Recomputing from your latest period is always better than reading far down the list.
How it relates to ovulation and a due date
Three pages use the same starting point, the first day of your last period:
- This page adds whole cycles to it. It uses no assumption beyond your own average.
- The ovulation calculator takes the next expected period and counts back a luteal phase, usually taken as about fourteen days, to estimate ovulation and the fertile window.
- The due date calculator adds 280 days to it, the obstetric convention for dating a pregnancy.
A missed period is often the first reason people reach for the other two. If your period is late and you could be pregnant, a test answers the question that a calendar cannot.
What this cannot do
This is an estimate from averages, not a medical assessment. It cannot tell you why a period is late, early, heavy or missing, and it is not contraception. Very short or very long cycles, cycles that vary a lot from month to month, bleeding between periods and periods that stop are all things to discuss with a clinician rather than enter into a calculator.
With your numbers
- Each later period is another 30 days on: the sixth is expected to start on February 28, 2027
- A 5-day period that starts then is expected to end on October 5, 2026, counting the first day as day 1
- September 1, 2026 plus one 30-day cycle is October 1, 2026
Quick reference
Assumes every cycle is your average length. Real cycles vary, so treat each date as the middle of a few likely days.
| Example | Next period expected to start |
|---|---|
| 21-day cycle | September 22, 2026 |
| 24-day cycle | September 25, 2026 |
| 28-day cycle | September 29, 2026 |
| 30-day cycle | October 1, 2026 |
| 32-day cycle | October 3, 2026 |
| 35-day cycle | October 6, 2026 |
| 28-day cycle across a leap day | March 9, 2028 |
| 28-day cycle across a year end | January 17, 2027 |
Common questions about the Period Calculator
- How do I work out my cycle length?
- Count the days from the first day of one period up to, but not including, the first day of the next. If your last three periods started on 1 March, 29 March and 27 April, your cycles were 28 and 29 days: 1 March to 29 March is 28 days, and 29 March to 27 April is 29 days, because March has 31 days. Averaging a few cycles gives a better input than a single one or a remembered figure.
- Why does my period not arrive on the predicted day?
- Because cycles are not all the same length. The page assumes every cycle equals the average you entered, and a few days either side of that is common even in regular cycles. Stress, illness, travel, weight change, breastfeeding and hormonal contraception can all move a period. A prediction is a middle guess, not an appointment.
- My period is late. What does that mean?
- A late period has many ordinary causes, and a calendar cannot tell them apart. If you could be pregnant, a home pregnancy test is the next step, and the due date calculator estimates a due date from the same first day of your last period. If periods stop or change a lot without an obvious reason, that is worth raising with a clinician.
- How is this different from the ovulation calculator?
- This page only adds cycles together, so it needs nothing but your dates. The ovulation calculator goes a step further and counts back a luteal phase from the next expected period to estimate ovulation and the fertile window. That extra step relies on an assumed luteal length, so its dates are less certain than the period dates here.
- Can this tell me when it is safe to have sex without contraception?
- No. Predicting periods is not contraception, and neither is estimating ovulation from a calendar. Cycles vary and ovulation can come earlier or later than an average predicts. Use a contraceptive method, or fertility awareness taught properly with daily signs, not a date calculation.