Predict your next period and the months ahead from your cycle — so you can plan with confidence and know when to expect your period.
Good to know: These dates are estimates that assume regular cycles — a calendar prediction is not a form of contraception. Nothing you enter leaves your browser.
Your menstrual cycle is measured from the first day of one period to the day before your next period begins — not from the day bleeding stops. It is easy to miscount, so this is the single most useful habit to build: mark day one as the first day of real flow, every time. Once you have that anchor, the rest of the math falls into place.
For most people, a healthy cycle runs somewhere between 21 and 35 days. A 28-day cycle gets talked about as the "standard," but it is really just a common average, not a target you need to hit. Plenty of people have cycles that settle naturally at 26 days or 32 days and are perfectly healthy. What matters more than the exact number is whether your own cycles are reasonably consistent from month to month.
The bleeding itself usually lasts about 2 to 7 days, and the flow typically starts heavier in the first day or two before tapering off. Some months feel lighter, some heavier, and the color can range from bright red to brown near the end — all of that is within the normal range. Cycles also tend to be less predictable in the first few years after periods begin and again in the years approaching menopause, when hormone patterns naturally shift.
Behind the calendar, your body moves through four phases each cycle, driven by a rise and fall of hormones. Understanding them makes the predictions above feel less like magic and more like biology. The days below assume a roughly 28-day cycle; yours may run a little shorter or longer, with most of the variation landing in the follicular phase.
| Phase | Typical days | What happens |
|---|---|---|
| Menstrual | Days 1–5 | The uterine lining sheds and you bleed. Hormone levels are at their lowest, which is why energy can dip. |
| Follicular | Days 1–13 | Overlaps with your period, then continues. Rising estrogen rebuilds the lining and matures an egg-containing follicle. |
| Ovulation | Around day 14 | A surge of luteinizing hormone releases a mature egg. This is the most fertile point of the cycle. |
| Luteal | Days 15–28 | Progesterone rises to support a possible pregnancy. If none occurs, hormones fall and the next period begins. |
The tool at the top of this page uses the calendar method — the same approach people have used with a paper diary for generations, just with the arithmetic done for you. It needs only three things: the first day of your last period, your average cycle length, and how many days you typically bleed.
Next period start = first day of last period + average cycle length
From that first predicted date, the calculator simply repeats the step — adding your cycle length again and again — to map out the next six periods. For each one it shows a date range from the likely start through the last expected day of bleeding, so you can see the whole window rather than a single day. It also counts the days between today and your next predicted period, giving you an at-a-glance countdown.
For context, the calculator adds an estimate of ovulation and your fertile window. Because the luteal phase (the stretch between ovulation and your next period) tends to hold steady at about 14 days, ovulation is estimated as your next period date minus 14 days. The fertile window covers the five days before ovulation through the day after — the span when conception is most likely. Everything is calculated in your browser, and nothing you type is stored or sent anywhere.
If your period arrives a few days off from the estimate, you have not done anything wrong — and neither has the calculator. Calendar math assumes your next cycle will match your average, but real bodies rarely run on perfect schedules. The timing of ovulation is the part that moves most, and when ovulation shifts, your period shifts with it.
A lot of everyday things can nudge a cycle earlier or later. Stress, illness, travel across time zones, disrupted sleep, and big changes in exercise or weight are among the most common. These are usually temporary — one unusual cycle often gives way to a normal one the next month.
Some causes are more persistent. Polycystic ovary syndrome (PCOS) can make ovulation happen unpredictably or skip some cycles entirely, so periods become hard to forecast. Perimenopause, the years leading up to menopause, brings its own natural irregularity as hormone levels fluctuate. Hormonal birth control changes the picture completely — on the pill, patch, ring, injection, implant, or hormonal IUD, any bleeding you have is a response to the medication, not a natural cycle, so calendar predictions do not apply. If any of these describe you, tracking each cycle as it actually happens will tell you far more than a calendar projection can.
A period calculator is a planning tool, not a diagnosis. It cannot tell you whether something is wrong, and it is not a substitute for medical care. Most cycle variation is harmless, but a few patterns are worth raising with a doctor, midwife, or nurse — not to alarm you, but because they can point to something that is often simple to treat.
Consider checking in with your provider if you notice any of the following:
If pregnancy is possible and your period is late, a home test is a reasonable next step. And any time something about your cycle simply feels off to you, that is reason enough to ask. You know your own body best, and your provider would far rather answer a question early than have you wait.
A period calculator uses simple calendar math. It takes the first day of your last period and adds your average cycle length to find the likely start of your next period. It then repeats that step to project several cycles ahead. The prediction assumes your cycles stay close to your average length, so it works best when your periods are fairly regular.
For most people, a menstrual cycle runs about 21 to 35 days, counted from the first day of one period to the day before the next one starts. A cycle around 28 days is common but not a rule — anywhere in that range can be perfectly healthy. Bleeding usually lasts about 2 to 7 days. Cycles that are consistently shorter than 21 days or longer than 35 are worth mentioning to your provider.
No. A calendar prediction is an estimate, not contraception. Ovulation can shift from cycle to cycle, and sperm can survive for several days, so the days that seem "safe" on a calendar are not reliable for preventing pregnancy. If you want to avoid pregnancy, talk with your provider about contraception that fits you.
Predictions drift because real cycles vary. Stress, illness, travel, poor sleep, big changes in exercise or weight, and hormonal shifts can all push ovulation — and therefore your period — earlier or later. A period that is a few days off from the estimate is common and usually nothing to worry about. If a period is very late and pregnancy is possible, consider taking a test.
Not reliably. Calendar math assumes your next cycle will look like your average, and with irregular cycles that assumption often breaks down. If your cycle length swings by more than a few days, or you have a condition like PCOS, the predicted dates may miss by a wide margin. Tracking each cycle over several months, or talking with your provider, will give you a clearer picture.