Ovulation Calculator

This works out an estimated fertile window from the fact that a cycle is not the same length every month. Enter the first day of your last period together with your shortest and longest recent cycle, or type your last few period start dates and let it measure the gaps for you. It places ovulation one luteal phase before the next period is due, once for the short cycle and once for the long one, then opens the window five days earlier for how long sperm survive and closes it a day after ovulation. The dates are a projection from numbers you typed rather than an observation of your body, so they are not a method of contraception and they diagnose nothing.

Calculator Type

Give the range your cycles actually run over rather than one average, and the window comes out as wide as your cycles really are.

First Day of Your Last Period (YYYY-MM-DD) *

The day bleeding started, not the day it stopped.

Shortest Cycle in the Last Six Months (days) *

First day of one period to the first day of the next.

Longest Cycle in the Last Six Months (days) *

Put the same number in both boxes if your cycles barely move.

Luteal Phase Length (days)

Ovulation to the first day of the next period. Fourteen days is the textbook average; only change it if you have tracked your own.

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