Ovulation Calculator
Estimate your ovulation date, fertile window, and next expected period based on your last period and average cycle length.
First day of your last period
Average cycle length
days
Most cycles range from 21 to 35 days. Default is 28.
Estimated ovulation date
—
Fertile window
—
Next expected period
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How the Ovulation Calculator Works
- Enter the first day of your last menstrual period (LMP).
- Enter your average cycle length — the number of days from the start of one period to the start of the next (default 28 days).
- The calculator estimates ovulation as occurring 14 days before your next expected period, then shows a fertile window of the 5 days leading up to and including that day.
Example
If your last period started on the 1st of the month and your average cycle is 28 days, ovulation is estimated around day 14 (the 15th), with a fertile window from the 10th to the 15th, and your next period expected around day 28 (the 29th).
Common Use Cases
- Identifying the days you're most likely to conceive when trying to get pregnant.
- Getting a rough estimate of when your next period is due.
- Tracking cycle patterns alongside other fertility signs like basal body temperature or cervical mucus.
FAQs
How accurate is this ovulation calculator?
It's an estimate based on average cycle statistics, not a measurement of your actual hormone levels. This calculator is not a substitute for medical advice, and it is not a reliable method of contraception — actual ovulation timing varies from cycle to cycle and can be affected by stress, illness, travel, and many other factors. If you're trying to conceive or avoid pregnancy, talk to a healthcare provider and consider tracking methods like ovulation predictor kits or basal body temperature alongside this calculator.
What is the fertile window?
The fertile window is the span of days in a cycle when pregnancy is possible — typically the 5 days before ovulation plus the day of ovulation itself, since sperm can survive in the body for several days while the egg is only viable for about 24 hours after release.
What if my cycles are irregular?
This calculator assumes a consistent cycle length, so estimates will be less reliable if your cycles vary significantly month to month. In that case, tracking multiple cycles and using additional fertility signs — or speaking with a doctor — will give a more reliable picture.