Period Calculator
Enter the first day of your last period plus your usual cycle and period length to forecast your next three periods and fertile windows.
Example
If your last period started on June 1, 2026 with a 28-day cycle and a 5-day period:
Next period 1: Jun 29 - Jul 3 fertile Jun 10 - Jun 15
Next period 2: Jul 27 - Jul 31 fertile Jul 08 - Jul 13
Next period 3: Aug 24 - Aug 28 fertile Aug 05 - Aug 10
Est. ovulation (cycle 1): Jun 15 (Jun 29 minus 14 days)
Each start is 28 days after the previous; ovulation is 14 days before the next start, and the fertile window is the 5 days leading up to it plus ovulation day.
How it works
Each next period start is the previous start plus your cycle length; the period end adds your period length, and the fertile window is the five days before plus the day of estimated ovulation (cycle length minus 14 days).
Good to know
This Period Calculator forecasts your next three menstrual cycles from three inputs: the first day of your last period (LMP), your typical cycle length, and how many days your period usually lasts. For each upcoming cycle it shows the predicted period start and end dates, an estimated ovulation day, and a fertile window, so you can see roughly a quarter-year of cycles at a glance. It's aimed at anyone who menstruates and wants a quick, no-account way to anticipate their next period or know which days they're most likely to conceive.
You'd reach for it when planning around your cycle: scheduling travel or events, packing supplies ahead of time, or timing intimacy if you're trying to conceive. Because everything runs in your browser, it's also a sensible choice when you'd rather not log sensitive reproductive data into a tracking app or account.
To read the result, start with the large date and the countdown badge, which tell you when the next period is expected and how many days away that is. The three "Period" stats give consecutive cycles, while "Est. ovulation" and "Fertile window" mark your most fertile days; the table below repeats this cycle by cycle. The math is fixed: each start is one cycle length after the last, ovulation is set at the next start minus 14 days, and the fertile window spans the five days before ovulation plus ovulation day itself.
A key caveat: the tool assumes every cycle is the same length, and it always counts back 14 days for ovulation regardless of your actual cycle. Real luteal phases and ovulation timing vary between people and from month to month, so the predictions drift further out the more cycles ahead you look. Treat the fertile window as a rough guide rather than a reliable form of contraception, and re-enter your LMP each new cycle to keep the forecast accurate.
Frequently asked questions
How is the fertile window calculated?
Ovulation is estimated as the next period start minus 14 days (the luteal phase is fairly constant). The fertile window is the 5 days before ovulation plus ovulation day itself, since sperm can survive several days and the egg lives about 24 hours.
Why might the predicted dates be wrong?
This tool assumes your cycles are regular and the same length each month. If your cycle length varies, you are stressed, ill, breastfeeding, or approaching menopause, ovulation and period timing can shift, so treat these as estimates only.
Is my data uploaded anywhere?
No — this calculator runs entirely in your browser; nothing is uploaded.
Is this a substitute for medical advice?
No. These are educational estimates — consult a qualified health professional for medical decisions.
People also ask
What is LMP and why does the period calculator ask for it?
LMP stands for the first day of your last menstrual period, meaning the first day you actually started bleeding rather than when it ended. The calculator counts forward from that date in cycle-length increments to project future periods, so an accurate LMP is the most important input.
Does this calculator track ovulation if I have an irregular cycle?
It estimates ovulation by subtracting 14 days from each predicted period start, which assumes regular, consistent cycles. If your cycle length varies month to month, the estimated ovulation and fertile dates can be off, so they are best treated as rough approximations.
How many days before my period does ovulation usually happen in this tool?
This tool always places estimated ovulation 14 days before the next predicted period start, based on the luteal phase being relatively constant. The fertile window then covers the five days before that estimated ovulation day plus ovulation day itself.
What cycle length should I enter if I don't know mine?
The calculator defaults to a 28-day cycle, which is a common average, and accepts values from 20 to 45 days. If you are unsure, you can count the days from the start of one period to the start of the next over a few cycles to find your typical length.
Can I use a period calculator as birth control?
Calendar-based predictions like this are not designed to be used as contraception, because they assume regular cycles and cannot account for early or late ovulation. The tool itself notes it is not a substitute for medical advice or contraception.
Why does the calculator show three future periods instead of just one?
Showing three consecutive cycles lets you plan further ahead, such as around trips or events that fall a month or two away. Keep in mind that accuracy decreases for cycles further in the future, since small variations in cycle length compound over time.
Is a period calculator accurate for predicting pregnancy or due dates?
This tool predicts period start dates, ovulation estimates, and fertile windows, not pregnancy due dates. A separate due-date calculation is typically based on LMP and a standard gestation length, and any pregnancy questions are best confirmed with a health professional.
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