Enter the first day of your last period, your average cycle length and - if you know it - your luteal-phase length. The estimated ovulation date, the 6-day fertile window and your next expected period update as you type, with a look-ahead table for the next three cycles below the result.
Ovulation Calculator
The ovulation calculator estimates three dates from two numbers you already know - the first day of your last period and your average cycle length: the estimated ovulation day, the 6-day fertile window around it, and when your next period is expected. Everything runs in your browser; nothing you enter is stored or sent anywhere.
It uses the calendar method: ovulation is assumed to fall a luteal-phase length before the next period starts. The luteal field defaults to 14 days and accepts 10 to 16 if you know yours; cycle length accepts 21 to 45 days. The fertile window shown is the 5 days before estimated ovulation through ovulation day itself.
A look-ahead table lists the fertile window, estimated ovulation and next-period date for the current and next two cycles, so a date a few weeks out is one glance away rather than a hand count on a calendar. Every field recalculates the result as you type, and clearing the date shows a prompt instead of broken output.
The result is a planning estimate, not a diagnosis: the calendar method assumes your cycles run at a consistent length, and actual ovulation varies from cycle to cycle - the page says so right in the result. It is not a contraception method. And it answers the before question only: for a due date after a positive test, use the pregnancy due date calculator instead.
Frequently Asked Questions
What does the ovulation calculator do?
It estimates your ovulation date, the 6-day fertile window and your next expected period from the first day of your last period and your average cycle length, with a look-ahead table for the next three cycles.
How is the estimate calculated?
With the calendar method: ovulation is assumed to fall a luteal-phase length (14 days by default, adjustable 10 to 16) before the next period starts, so the tool computes it as your last period start plus cycle length minus luteal length. The fertile window is the 5 days before estimated ovulation through ovulation day.
How accurate is it?
It is a planning estimate, not a measurement - the calendar method assumes consistent cycle lengths, and actual ovulation varies from cycle to cycle. It does not measure hormones or temperature, and it must not be used as medical advice or as a contraception method.
Does anything I enter leave my browser?
No. The dates are computed locally in the page - nothing is uploaded, stored or sent, and no account is needed.
Can it give me a due date?
No - this page answers the before question (fertile window and ovulation). For a due date after a positive test, use the pregnancy due date calculator on this site.