How this calculator predicts your next period
This tool needs only two figures to build its forecast: the first day of your last period, and how long your cycles usually run from one period's first day to the next. Add the two together and you get the start date of your next period. From there, the duration slider tells the calculator how many days that period is expected to run, so it can mark both the start and the end date rather than leaving you to guess where a period actually finishes.
The same sum then repeats for each cycle after that, one cycle length at a time, to build the 6-period look-ahead in the table below. That repetition is exactly why a single guessed cycle length matters more than it looks: a figure that is a couple of days off your true average will drift further from reality with every cycle it is applied to, which is one reason this calculator keeps the sliders adjustable rather than locking in the 28-day textbook figure.
Choosing your average cycle length and period duration
Cycle length is counted from the first day of bleeding in one period to the first day of bleeding in the next, not from when a period ends to when the next one begins. A cycle anywhere from 21 to 35 days, averaging around 28, is considered typical for adults, though very few people sit on exactly 28 every month. If you track your dates in an app or a diary, use the average of your last three to six cycles rather than a single one; a single unusually short or long cycle will skew the estimate more than it should.
Period duration works the same way: most periods run somewhere between 2 and 7 days, with some running a day or so longer, which is why the slider here extends to 8. If you are not sure, leave it on the 5-day default; it sits close to the middle of the typical range and will not skew the forecast much either way.
Reading the band, the table and the "days away" count
The headline figure above the working always answers one question: when does your next period start, and how many days away is that. If the predicted start date has already passed without a fresh date entered, the headline switches to say your period may have started instead of counting down, since a negative day count reads oddly for something that is meant to already be underway.
The instrument band underneath shows exactly where today sits inside the cycle you are currently in, with the period days marked at the start of the bar and the remaining, non-bleeding days filling out the rest. It is the same chart shape used across our date-based calculators, just re-purposed here to mark bleeding days instead of a single milestone. The table below that projects the same start-plus-duration sum across your next 6 cycles, so you can plan travel, events or supplies several months out rather than one prediction at a time.
Why calendar predictions drift, and why tracking beats them
Every date on this page assumes your next cycle repeats the average you entered, and that assumption gets shakier the further into the future a row in the table sits. Cycle length genuinely varies month to month for most people, nudged by stress, travel, illness, sleep changes, exercise load and, for some, hormonal contraception changes; a calculator working from one flat average cannot see any of that coming. This is not a flaw specific to this tool; it applies to every calendar-based period predictor, including the ones built into period-tracking apps, whenever a cycle departs from its usual pattern.
That is also why real tracking consistently beats a calendar guess: logging your actual start and end dates over a few cycles builds a personal average from lived data rather than a textbook figure, and it catches a genuine shift in your pattern (running consistently longer, shorter or heavier) far sooner than a static prediction ever could. Treat this calculator as the planning tool it is, useful for a rough date to pencil in, and treat your own recent history as the better source whenever the two start to disagree.
When to see a GP about your cycle
Most month-to-month variation is normal and not something to worry about. It is worth speaking to a GP if any of the following apply: your cycles are consistently shorter than 21 days or longer than 35 days, your usual pattern changes suddenly rather than drifting gradually, or your bleeding is very heavy, meaning you are soaking through a pad or tampon roughly every hour for several hours in a row, passing large clots, or bleeding for longer than seven or eight days at a time. Many health guidelines, including the NHS, list these as the situations where an irregular or heavy period is worth a proper check rather than another calendar estimate.
How this differs from our ovulation calculator
This page is deliberately built around one question: when does your next period arrive, and how long will it last. It does not estimate ovulation day or a fertile window, and none of the dates shown here should be read as fertility guidance either way. If you are trying to work out your most fertile days instead, our Ovulation Calculator uses the same last-period-plus-cycle-length inputs but counts backward from your next predicted period to estimate ovulation and the days around it, which is a different calculation built for a different question than the one this page answers.
Questions people ask
How accurate is a period calculator?
It is only as accurate as the average cycle length and duration you enter, and both hide real month-to-month variation. For someone with fairly regular cycles the next date or two in the table usually lands close to reality; predictions further down the 6-period list carry more uncertainty, since any drift in your actual cycle compounds with each repeat of the sum.
Why does my predicted date keep changing when I adjust the sliders?
Every date on this page is built from your last period plus a multiple of your cycle length, so moving the cycle-length slider shifts every predicted start date by that same number of days, cycle after cycle. The duration slider only affects each period's end date and does not move the start dates at all.
What counts as a normal period length?
Most periods last somewhere between 2 and 7 days, with some running a little longer, which is why this calculator's duration slider extends to 8. A period that regularly runs longer than 7 to 8 days, or is unusually heavy, is one of the patterns worth mentioning to a GP.
What if my cycle length varies a lot from month to month?
The more your cycles vary, the less a single average has to work with, and predictions several cycles ahead will drift furthest from what actually happens. Logging your own recent cycles in a tracking app or diary and using that personal average, updated every few months, will always beat a one-off estimate from this calculator.
Can I use this to plan travel or events around my period?
Yes, that is exactly what the 6-period table is for: a rough date range to pencil in supplies, timing or bookings around. Treat it as a planning aid rather than a fixed date, and lean on your own recent cycle history the closer the event actually gets.
Why does the headline say my period "may have started" instead of giving a days-away count?
Once the predicted start date has passed, counting days away the same way would show a negative number for something that should already be underway, which reads oddly. The headline switches wording at that point instead; if your period genuinely has not arrived yet, your current cycle may simply be running longer than the average you entered.
How is this different from an ovulation calculator?
This page only predicts when a period arrives and how long it runs; it never shows an ovulation date or fertile window. Our Ovulation Calculator takes the same last-period-plus-cycle-length inputs but counts backward from your next predicted period to estimate ovulation instead, which answers a fertility question rather than a period-planning one.
This calculator gives a calendar estimate from standard cycle-tracking assumptions; it is not a medical assessment and cannot diagnose a menstrual or fertility condition. Always speak to a GP or other qualified clinician about irregular, heavy or otherwise concerning periods.