Period Calculator (next period, ovulation & fertile window)

Understanding the period calculator — concept and principle

This calculator predicts your next period, the following one, your likely ovulation day and fertile window from the first day of your last period and your usual cycle length. It also shows which day of your cycle you are on today. People use it to plan ahead, to track irregularities and when trying to conceive or avoid pregnancy.

How it is calculated

Next period = first day of last period + cycle length (moved forward by whole cycles if that date has passed)

Next period ends ≈ next period start + period length − 1

Ovulation ≈ next period − 14 days

Fertile window = 5 days before ovulation to ovulation day (next period − 19 to − 14)

Period after that = next period + cycle length

Cycle day today = (days since last period mod cycle length) + 1

The principle

A cycle is counted from the first day of one period to the first day of the next. The phase after ovulation (luteal phase) is fairly constant at about 14 days, while the phase before it varies, so ovulation is placed by counting back from the next expected period. Sperm can survive up to about 5 days and an egg about a day, which gives a six-day fertile window ending on ovulation day (Wilcox and colleagues, 1995).

Worked example

Default inputs: last period 31 Oct 2026, 28-day cycle, 5-day period (today 10 Nov 2026).

1. Next period = 31 Oct + 28 = Sat 28 Nov 2026; ends about Wed 2 Dec.
2. Ovulation = 28 Nov − 14 = Sat 14 Nov.
3. Fertile window = Mon 9 Nov to Sat 14 Nov.
4. Period after that = 28 Nov + 28 = Sat 26 Dec.
5. Cycle day today = 10 + 1 = 11.

How to read the result

Predictions are most accurate for regular cycles. Cycles of 21–35 days are typical for adults, and up to 45 days in the first years after periods start. Ovulation can shift by several days from month to month, so the dates are estimates, not guarantees.

Common mistakes

• Using the last day of a period instead of the first.
• Relying on the calendar method alone for contraception.
• Ignoring big changes in cycle length, which can have medical causes.

Frequently asked questions

How long is a normal cycle?

21–35 days in adults; a few days' variation from month to month is normal.

Can I get pregnant outside the fertile window?

It is less likely, but ovulation timing varies, so it is possible.

Why count back 14 days?

Because the luteal phase is usually about 14 days long.

Sources: Wilcox AJ, Weinberg CR, Baird DD, "Timing of sexual intercourse in relation to ovulation", N Engl J Med 1995;333:1517–1521 · ACOG Committee Opinion No. 651, "Menstruation in Girls and Adolescents: Using the Menstrual Cycle as a Vital Sign" (2015)