How the ovulation date is estimated
This calculator estimates your most fertile days from the first day of your last period and your average cycle length. Ovulation typically happens about 14 days before your next period starts, and the fertile window covers the few days leading up to and including it. It is an estimate based on averages, not a precise prediction.
Worked example
With a last period starting 1 January and a 28-day cycle, ovulation is estimated around 15 January, with the most fertile days in the several days just before.
Good to know
Cycle length varies from person to person and month to month, so the fertile window shifts too. For irregular cycles this estimate is less reliable. It is a planning guide, not contraception or medical advice.
Frequently asked questions
When am I most fertile?
Usually in the few days leading up to and including ovulation, which is roughly 14 days before your next period. The estimate shifts with your cycle length.
Is this reliable for irregular cycles?
Less so. The estimate assumes a fairly regular cycle. If yours varies a lot, the fertile window is harder to predict, so use it only as a rough guide.
How accurate is a calendar estimate?
It is a good starting point for regular cycles but can be off by a few days if your cycle length varies.
Is this contraception?
No. Calendar methods are not reliable contraception on their own. Speak to a healthcare provider about family planning.