Period Calculator

One date in — your next six periods out, each with start, end, ovulation day and fertile window.

Your cycle

Cycle log

Log each period’s start date — after two logs, forecasts use your real average cycle. Saves on this device and syncs with your ListCalc account.

Next period expected

—

Enter your last period to begin.

Days until it starts—
It should last until—
Ovulation before it (est.)—
Fertile window before it—

Forecasts assume your average cycle repeats. Cycles naturally wobble a few days — update the start date each month and the forecast corrects itself.

Your next six cycles

#PeriodFertile windowOvulation

Related calculators

Pregnancy Due Date Pregnancy Weight Gain Ovulation Conception Pregnancy Test

How the forecast is built

The maths is honest and simple: your next period is projected one average cycle length after the last one, then the one after that, six cycles out. Each cycle also gets its estimated ovulation day (14 days before the period that follows it, reflecting the stable luteal phase) and a six-day fertile window around it.

What counts as "normal"

Adult cycles commonly run 21–35 days, with 28 the textbook average, and bleeding typically lasts 2–7 days. Teenage cycles range wider (about 21–45 days) while regulating. Variation of a few days month to month is completely normal — which is exactly why a forecast built on averages should be refreshed with each real start date.

Making the forecast useful

Six months of projected dates is enough to plan around holidays, exams, competitions and travel. Save each month's result to your ListCalc history and you'll build a personal record of how your cycles actually behave — the average that drives this forecast then gets better over time.

When the pattern breaks

A missed period with pregnancy possible is worth a test from the day the period was due. Persistently irregular cycles, cycles shorter than 21 or longer than 40 days, very heavy bleeding, or severe pain are all worth raising with a clinician — the forecast can only describe a pattern, not explain a change in one.

ListCalc health tools are educational estimates, not medical advice. Dates and ranges here are population averages — your midwife, OB-GYN or doctor confirms what applies to you. See our disclaimer.

Frequently asked questions

How accurate is a six-cycle forecast?
The first projected period is usually within a few days for regular cycles; accuracy fades the further out you look, because small monthly wobbles compound. Re-enter each real start date and the forecast re-anchors.
My cycle varies between 27 and 31 days. What should I enter?
Enter the average (29) and read every projected date as ±2 days. If your variation is bigger than about a week, calendar forecasts get rough — tracking actual dates matters more.
Is my period late or is this normal wobble?
A few days either side of the projection is normal. If you're sexually active and more than a few days past the projected start, a pregnancy test from the missed-period day gives a reliable answer.
Why does the table show fertile windows too?
Because ovulation sits ~14 days before each projected period, the same forecast doubles as a conception or caution calendar — whichever direction you need it for. For contraception, calendar estimates alone are not reliable.
Do periods sync with the moon or with other people?
Careful studies haven't supported either idea — apparent syncing is what random overlap between cycles of different lengths looks like. Your average cycle length is the only driver this calculator needs.
When should I talk to a clinician about my cycle?
Persistent cycles outside 21–40 days, bleeding lasting beyond about 7 days, soaking through protection hourly, severe pain, or a sudden change from your normal pattern all merit a conversation.

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Guides & articles

healthHow to Predict Your Next Period4 min readhealthWhen Are You Most Fertile? The Fertile Window Explained5 min read