Predict your next period, ovulation, and fertile window. Enter your last period and cycle length to see upcoming period dates and a cycle calendar.
Pick a typical cycle to see an example, then set your own last-period date.
These are estimates based on the calendar method and your average cycle — not a form of birth control or medical advice. Cycles vary from month to month, so predictions are less accurate for irregular cycles. See a healthcare provider about missed or irregular periods. Nothing you enter is stored.
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Knowing when your next period is due makes it easier to plan around travel, events, and everyday life — and to understand your body. This period calculator uses the standard calendar method: enter the first day of your last period and your average cycle length, and it predicts your upcoming period dates, estimates your ovulation day, and maps your fertile window. You'll also get a visual cycle calendar and a multi-cycle forecast so you can see the months ahead at a glance. It's free, works instantly, and nothing you enter is saved. Note that this is an estimate, not a contraceptive method or medical advice.
A period calculator predicts your next period from two things: the first day of your last period and your average cycle length (the number of days from the first day of one period to the first day of the next). The next period is simply your last period plus your cycle length, so a 28-day cycle that started on the 1st predicts the next period around the 29th. Ovulation is estimated using the luteal-phase rule: the luteal phase — the time from ovulation to your next period — is relatively constant at about 14 days regardless of cycle length, so ovulation lands roughly 14 days before your next period (around day 14 of a 28-day cycle). Your fertile window is the six days ending on ovulation day (the five days before, plus ovulation itself), because sperm can survive up to five days. A normal cycle is anywhere from 21 to 35 days, and a typical period lasts 2 to 7 days. For the most accurate prediction, average your last three cycles — for example, (29 + 31 + 28) ÷ 3 ≈ 29 days — to smooth out natural month-to-month variation. The 21-to-35-day normal cycle range and luteal-phase timing used here follow guidance from the American College of Obstetricians and Gynecologists (ACOG).
Period Prediction Formula
Schedule travel, events, or a beach trip around when your period is likely to arrive.
Find your estimated ovulation and fertile window to time intercourse.
Compare predicted dates with what actually happens to spot irregular cycles.
Bring your cycle dates and average length to a doctor or gynecologist visit.
See how many days past your predicted date you are before deciding on next steps.
Visualize the menstrual, follicular, fertile, and luteal phases of your cycle.
See your next period date and a countdown, so you can plan around it with confidence.
A multi-cycle forecast predicts your next 3, 6, or 12 periods, not just the next one.
Estimates your ovulation day and the six-day fertile window around it.
A color-coded calendar marks period days, your fertile window, and ovulation across the months ahead.
Shows which day of your cycle you're on today and the phase you're in.
Works instantly in your browser with no signup, and nothing you enter is stored.
Your next period is predicted by adding your average cycle length to the first day of your last period. If your last period started on the 1st and your cycle is 28 days, your next period is predicted around the 29th. The calculator repeats this to forecast several cycles ahead.
It's most accurate for regular cycles. The calendar method assumes your cycle length stays roughly the same each month, so the more consistent your cycles, the better the prediction. For irregular cycles it gives a reasonable estimate but real dates may shift by several days.
Count the days from the first day of one period to the first day of the next — that's one cycle. Do this for your last three cycles and average them, for example (29 + 31 + 28) ÷ 3 ≈ 29 days. A normal cycle is 21 to 35 days.
Ovulation usually happens about 14 days before your next period, because the luteal phase is fairly constant. Your fertile window is the six days ending on ovulation day — the five days before it plus ovulation itself — since sperm can survive up to five days.
Most cycles are between 21 and 35 days, with 28 days often used as an average. Periods themselves typically last 2 to 7 days. Cycles outside this range, or that vary a lot, are worth discussing with a healthcare provider.
Periods can be late for many reasons, including stress, travel, illness, changes in weight or exercise, hormonal birth control, perimenopause, or pregnancy. A predicted date is only an estimate; if your period is significantly late or you're concerned, take a pregnancy test or consult a healthcare provider.
No. A period calculator is for planning and awareness only — it is not a reliable form of contraception. The fertile window is an estimate and ovulation can shift, so do not rely on it to prevent pregnancy. Talk to a healthcare provider about contraception options.
It still gives an estimate, but predictions are less reliable when your cycle length changes a lot from month to month. If your cycles are irregular, use a recent average, treat the dates as approximate, and consider speaking with a healthcare provider.