Menstrual Cycle Calendar

First day of the last period

Cycle length, days

Period length, days

Result

Next period2026-08-17
Ovulation2026-08-03
Fertile window2026-07-29 — 2026-08-04
Todayday 14 of the cycle
The two after that2026-09-14, 2026-10-12
The next one ends2026-08-21

This result is informational only and is not professional advice.

Counted backwards, not forwards

Works out the dates of the next periods, the likely day of ovulation and the fertile window from the last period and the cycle length.

Ovulation does not happen in the middle of the cycle but about fourteen days before the next period. The luteal phase — from ovulation to menstruation — is stable and barely varies with cycle length; the first phase is the one that stretches and shrinks.

The practical consequence: on a 32-day cycle, ovulation falls on day 18 rather than day 16. Calendar arithmetic gives an orientation and no more — stress, illness, travel and a change of routine all move the dates.

What this calculator shows

The dates of the next three periods
The likely day of ovulation and the fertile window
Which day of the cycle today is

What to keep in mind

The calculation assumes a regular cycle of constant length; with an irregular cycle the calendar method carries a very wide error.
The fertile window is taken as five days before ovulation and one after: sperm survive up to five days, the egg about one.

FAQs

How do I work out the day of ovulation?

Subtract fourteen days from the date of the next period. On a 28-day cycle that is day 14; on a 32-day cycle it is day 18.

How long is the fertile window?

About six days: five before ovulation and one after. Sperm remain capable of fertilisation for up to five days and the egg lives around a day.

How reliable is the calendar method?

As contraception, poorly: even in women with regular cycles ovulation shifts by several days. For planning a pregnancy it is perfectly serviceable.

What cycle length is normal?

Twenty-one to thirty-five days, and variation of a few days is normal. Consistently outside that range is worth discussing with a doctor.

Worked example

A 28-day cycle

Input: Last period 2026-07-20, 28-day cycle

Output: Next on 2026-08-17, ovulation on 2026-08-03

Note: The fertile window runs 29 July to 4 August. On a 32-day cycle ovulation would move to 7 August and the window to 2–8 August: the first phase is longer, the second unchanged.

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Menstrual Cycle Calendar

Result

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