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Period Calculator

Estimate upcoming period days and the most probable ovulation window from the first day of your last period, how long it lasted, and your average cycle length.

ExampleSample values — edit any field to see your result.

Leave blank to use today's date.

Days of bleeding this period (default 5).

Days from the start of one period to the next (default 28).

Results update as you type.

Next period start

October 7, 2026

Estimated result

Next period days
October 7, 2026 – October 11, 2026
Most probable ovulation date
September 23, 2026
Most probable ovulation days
September 21, 2026 – September 25, 2026

Important dates for the next 6 cycles

PeriodMost probable ovulation days
September 9, 2026 – September 13, 2026September 21, 2026 – September 25, 2026
October 7, 2026 – October 11, 2026October 19, 2026 – October 23, 2026
November 4, 2026 – November 8, 2026November 16, 2026 – November 20, 2026
December 2, 2026 – December 6, 2026December 14, 2026 – December 18, 2026
December 30, 2026 – January 3, 2027January 11, 2027 – January 15, 2027
January 27, 2027 – January 31, 2027February 8, 2027 – February 12, 2027
Next period startOctober 7, 2026

Enter the first day of your last period, how long it lasted, and your average cycle length to estimate upcoming period days and the most probable ovulation window. Leave the last-period date blank to use today.

Formula

Period days and ovulation dates are derived from the last-period start D, period length L, and cycle length C. The luteal phase is treated as 14 days:

periodStart(i)    = D + i × C
periodDays(i)     = periodStart(i)  …  periodStart(i) + L − 1
nextPeriod        = D + C
ovulation         = nextPeriod − 14
ovulationWindow   = ovulation − 2  …  ovulation + 2

The six-cycle table uses i from 0 through 5. A 1-day period is still shown as a range (start and end on the same day).

These dates are estimates for planning, not a form of birth control and not a medical diagnosis. Irregular cycles, illness, and stress can all move ovulation and bleeding.

Typical days on a 28-day cycle with a 5-day period

EventCycle dayDates if last period starts September 8
Period daysDays 1–5September 8–12
Ovulation window opensDay 13September 20
Most probable ovulationDay 15September 22
Ovulation window closesDay 17September 24
Next periodDay 29October 6–10

Examples

Last period on September 8, 2026 (5-day period, 28-day cycle)

Next period = September 8 + 28 = October 6, 2026, lasting through October 10. Ovulation = October 6 − 14 = September 22, 2026, with a most-probable window of September 20–24.

Same last period with a 22-day cycle

A shorter cycle moves everything earlier: next period is September 30, 2026, and the ovulation window is September 14–18, 2026.

Frequently asked questions

How are upcoming period days calculated?
Each cycle starts on the first day of bleeding. The next period starts that date plus your average cycle length. Bleeding is then counted for as many days as you entered (default 5), so a period that starts on October 6 and lasts 5 days runs October 6–10. The table repeats this for the next six cycles.
How are the most probable ovulation days estimated?
Ovulation is estimated as 14 days before the next period would start, because the luteal phase after ovulation is fairly constant. The most probable window is two days on either side of that date (five days in total). Cycle length mainly changes the days before ovulation.
What counts as a regular menstrual cycle?
A cycle is counted from the first day of one period to the first day of the next. The average is about 28 days. A cycle is often called regular when the longest and shortest recent cycles differ by fewer than 8 days. This calculator accepts cycle lengths from 22 through 44 days.
Can I use this as birth control?
No. Calendar estimates are not reliable enough to prevent pregnancy. Cycle length can shift from month to month, ovulation can move with illness or stress, and sperm can survive for several days. Use a method designed for contraception if you are trying to avoid pregnancy.
Why might my period not match these dates?
These dates assume a steady cycle length and period length. Travel, illness, stress, new medications, pregnancy, and the years around menarche or menopause can all move bleeding and ovulation. Treat the calendar as a planning estimate, not a diagnosis.

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