Ovulation Calendar

How to use it

1. Enter the First day of your last period. Leave it empty to start 14 days before today.
2. Set the Cycle length and the Luteal phase, or keep the usual 28 and 14 days.
3. Choose the Months to show in the table.
4. Read the Next ovulation and the Fertile window, then the cycles in the table.

How the dates are estimated

Ovulation is estimated as the luteal phase before the next period: the cycle length minus the luteal length, counted from the first day of the period. The luteal phase is about 14 days in most women. In a study of normal cycles by Lenton and colleagues (1984), the mean luteal phase was 14.13 days, with a standard deviation of 1.41 days.

These dates are estimates and not medical advice. Talk to a doctor if your cycles are very irregular or if you are trying to conceive and it is not working.

Ovulation day  = first day of the period + (cycle length - luteal length)
Fertile window = the 5 days before ovulation and the day of ovulation
Next period    = first day of the period + cycle length

The fertile window

In a study of couples by Wilcox and colleagues (1995), conception happened only after intercourse in a six-day period that ended on the estimated day of ovulation. The probability of conception was 0.10 when intercourse was five days before ovulation and 0.33 on the day of ovulation itself.

The ovulation day is itself an estimate, so the fertile window shown here is an estimate too.

What this calendar does not do

• It is not contraception. The NHS says fertility awareness methods are between 91% and 99% effective when used correctly all the time, and only 76% effective if the instructions are not followed exactly.
• It assumes every cycle has the length you entered. Illness, stress, travel, breastfeeding and some medicines can change the timing, and the rows further down the table are projections.
• It gives dates, not a diagnosis. Ask a doctor about irregular or missed periods.

Frequently asked questions