Period Calculator
Predict next menstruation dates
Period Calculator tool
Period Calculator: key facts
- What it does
- Predict next menstruation dates
- Category
- Health & Fitness Calculators
- Cost
- Free, with no account, sign-up, or install.
- Your data
- Runs entirely in your browser — the files and text you enter are never uploaded to a server.
- Last reviewed
- . Report an incorrect result.
What this tool is for
Knowing roughly when your next period will arrive makes planning easier, from travel to events to simply being prepared. This calculator projects your upcoming periods from the start date of your last one and your average cycle length, looking two cycles ahead.
It is a simple tracking aid for anyone who wants a heads-up on their cycle without an app or account. By adding your cycle length to your last start date, it estimates the next period and the one after that. The projection assumes consistent cycles, so it works best when your periods are fairly regular. If yours vary noticeably from month to month, treat the dates as a rough window rather than a fixed appointment, and update the inputs each cycle to keep the estimate honest.
How to use the Period Calculator
- Pick the date your last period began.
- Enter your average cycle length in days.
- Read the predicted start date of your next period.
- Check the following cycle's estimated start date shown beneath it.
Projecting the next two cycles
The calculation adds your average cycle length to the first day of your last period to give the next expected start date, then adds the same length again for the cycle after. Cycle length is counted from the first day of one period to the first day of the next, not from when bleeding stopped — a common source of confusion that shifts the result by several days.
Projecting only two cycles ahead is deliberate rather than a limitation. Small variations compound, so each successive prediction is less reliable than the last; by the sixth cycle a couple of days of ordinary variation has accumulated into well over a week.
The default 28 days is the textbook figure, but normal adult cycles range from about 21 to 35 days. Using your own average — ideally taken across three to six recorded cycles — produces a materially better prediction than leaving the default in place.
- From a period starting 1 September on a 28-day cycle, the next is expected 29 September and the one after 27 October.
- On a 31-day cycle from the same date those become 2 October and 2 November.
- If your cycles range between 26 and 30 days, treat any prediction as a window of several days rather than a specific date.
What the Period Calculator gets right
- It projects two cycles ahead, not just one, so you can plan a little further out.
- It uses your own cycle length rather than assuming a standard 28 days, tailoring the estimate to you.
- It needs only two inputs and shows results instantly — no app install or account.
- Your dates are calculated locally and never leave your browser.
Normal variation, and what is worth a doctor's attention
Some cycle-to-cycle variation is entirely usual, and a few days either side of a prediction is not a sign of anything wrong. Cycles are also naturally more irregular in the years after periods begin and in the run-up to menopause. Stress, illness, travel across time zones, significant weight change, intense training, and starting or stopping hormonal contraception can all delay or bring forward a period.
Recording actual start dates for a few months is the single most useful thing you can do, because your own average is a far better input than any default. Once tracked, prediction accuracy improves noticeably.
Several patterns do warrant medical advice rather than tracking: cycles consistently shorter than 21 or longer than 35 days, bleeding that is unusually heavy or lasts beyond seven days, bleeding between periods or after sex, periods that stop for three months or more without pregnancy, severe pain that interferes with daily life, or any bleeding after menopause. Any of these is worth raising with a doctor, since conditions including fibroids, endometriosis, PCOS, and thyroid disorders are common and treatable — and a calendar tool cannot assess them.
Frequently Asked Questions
How does the calculator predict my next period?
It adds your average cycle length to the start date of your last period to estimate the next start date, then repeats once more for the following cycle. It assumes your cycle length stays consistent from month to month.
Why is my actual period different from the prediction?
Cycle length naturally varies with stress, illness, travel, hormonal changes, and other factors, so a calendar projection is an estimate rather than a certainty. Tracking several cycles helps you find a more accurate average to enter.
What counts as a normal cycle length?
Cycle length is commonly anywhere from about 21 to 35 days, measured from the first day of one period to the first day of the next. Use your own observed average for the most accurate prediction rather than a generic figure.
Should I see a doctor about irregular cycles?
Occasional variation is normal, but consistently irregular, very long, very short, or absent cycles can have medical causes worth discussing with a healthcare provider. This tool is for general planning, not medical diagnosis.
