Period Calculator

Enter the first day of your last period, your average cycle length, and how long your periods last to see when your next period is due — plus a six-cycle forecast with estimated ovulation days.

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How the period calculator works

A menstrual cycle is counted from the first day of one period to the day before the next one starts — so predicting the next period is a matter of adding your average cycle length to your last start date, then repeating. This calculator projects six cycles ahead, marks the expected start and end of bleeding for each, and estimates the ovulation day in each cycle (about 14 days before the following period, since the post-ovulation luteal phase is the steadiest part of the cycle). If your last recorded period was a while ago, it fast-forwards through the elapsed cycles and starts the forecast from the next expected one.

The formula

next start = last start + cycle length  (repeated)
period end ≈ start + period length − 1   |   ovulation ≈ next start − 14 days

Cycle length is day 1 of one period to the day before the next (21–35 days is typical); period length is days of bleeding (2–7 is typical). The "−1" is because a 5-day period starting July 30 runs July 30 through August 3, not August 4.

Worked example

Last period started July 2, 2026, with 28-day cycles and 5-day periods (taking today as July 14, 2026):

Next period: Thursday, July 30, 2026, running July 30 – August 3. The following ones land August 27, September 24, October 22, November 19, and December 17. Estimated ovulation before that first predicted period: around July 16 (July 30 minus 14 days).

Predictions wobble — and that's normal

Even textbook-regular cycles drift a few days month to month, because cycle length is set by when ovulation happens and ovulation responds to stress, illness, travel, weight change, and hard training. So read the forecast as a window (a couple of days either side) rather than an appointment, and expect the sixth predicted period to be fuzzier than the first. The forecast is most useful as a baseline: when you track actual start dates against it for a few months, your real average emerges quickly, and genuine pattern changes become visible. Sudden changes in a previously steady pattern, cycles consistently outside 21–35 days, bleeding between periods, or very heavy bleeding (soaking through protection every hour, or periods beyond 7 days) are the things worth raising with a provider — not because they always mean trouble, but because they're the signals that deserve a real answer instead of a calendar estimate.

Frequently asked questions

How do I calculate when my next period will come?

Add your average cycle length to the first day of your last period. A cycle is counted from day 1 of bleeding to the day before the next period starts — so with a period that began July 2 and 28-day cycles, the next one is expected around July 30. This calculator repeats that addition to forecast six cycles ahead.

What is a normal menstrual cycle length?

Anywhere from about 21 to 35 days is considered typical for adults, and 28 is just the average, not a target. It's also normal for the same person's cycles to vary by a few days month to month. Period bleeding itself usually lasts 2 to 7 days, with around 5 being common.

Why is my period sometimes early or late?

Cycle length is set mostly by when you ovulate, and ovulation shifts with stress, illness, travel, weight changes, intense exercise, and sometimes nothing identifiable. A few days of wobble is normal biology, not a malfunction — which is also why these predictions get fuzzier the further ahead they reach.

When should I talk to a provider about my cycle?

Worth a conversation: cycles that suddenly change pattern, cycles consistently shorter than 21 or longer than 35 days, missed periods when pregnancy isn't the reason, bleeding between periods, or periods heavy enough to soak through protection hourly or lasting more than 7 days. None of these automatically mean something is wrong, but they're all worth checking.

How do I work out my average cycle length?

Track the first day of bleeding for at least three cycles (six is better), count the days from each start to the day before the next start, and average them. A calendar, notes app, or cycle-tracking app all work — the method matters less than writing down day 1 consistently.

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