Ovulation Calculator
This free ovulation calculator estimates your next period, ovulation date, and fertile window from the first day of your last period and your average cycle length. It also lists your next few predicted periods. Nothing you enter is sent anywhere or stored.
How to Use It
- Enter the first day of your last period.
- Enter your average cycle length in days.
- Your estimated next period, ovulation date, and fertile window update instantly.
How It Works
Your next period is estimated as your last period's start date plus your average cycle length. Ovulation is then estimated backward from that predicted next period by 14 days, the luteal phase, since that phase stays relatively consistent even when overall cycle length varies. The fertile window spans from 5 days before the estimated ovulation date through 1 day after it, covering the days sperm can survive and the roughly 24 hours an egg remains viable after release.
Formulas
next period date = last period start + average cycle length
ovulation date = next period date − 14 days
fertile window = ovulation date − 5 days through ovulation date + 1 day
A worked example: a last period starting January 1 with a 28 day average cycle predicts the next period on January 29, ovulation around January 15, and a fertile window from January 10 through January 16. A longer, 30 day cycle from the same start date shifts everything later: next period January 31, ovulation January 17, fertile window January 12 through January 18.
This is a calendar estimate based on average cycle timing, not a measurement of your actual ovulation. Cycles vary from person to person and month to month, and this tool does not diagnose fertility issues or serve as a method of contraception. Everything runs client-side in your browser. Nothing you enter is sent anywhere or stored.
Frequently Asked Questions
Why is ovulation calculated backward from the next period instead of forward from the last one?
The luteal phase, the time from ovulation to the start of the next period, is relatively consistent at about 14 days across most cycle lengths. The time before ovulation, the follicular phase, varies much more from person to person and cycle to cycle. Working backward from the predicted next period gives a more reliable estimate than assuming ovulation always falls on the same day count after the last period.
Why does the fertile window extend before ovulation instead of being a single day?
An egg is generally viable for about 24 hours after ovulation, but sperm can survive in the reproductive tract for up to about 5 days. That combination is why the commonly cited fertile window starts roughly 5 days before ovulation and ends about a day after it, not just on the day of ovulation itself.
How is this different from the Pregnancy Calculator?
The Pregnancy Calculator dates an already-confirmed pregnancy: due date, current week, and trimester, starting from a known reference point like a last period or an ultrasound. This tool predicts an upcoming cycle, before conception, estimating when the next period and fertile window will fall. If you already know you're pregnant, the Pregnancy Calculator is the right tool.
Does this account for irregular cycles?
Not directly. This tool assumes a consistent average cycle length, and the predictions get less reliable the more a person's actual cycles vary month to month. If your cycles are irregular, treat the results as a rough estimate rather than a precise prediction, and consider tracking a few real cycles to find your own average.
What cycle length should I enter?
Your average cycle length, counted from the first day of one period to the first day of the next. A typical cycle falls between 22 and 44 days, though most commonly around 28. If you are not sure of your average, a recent cycle length is a reasonable starting point.
Is this a form of birth control?
No. This tool estimates fertile days from average cycle timing for informational purposes only. It is not a reliable method of contraception and does not account for cycle irregularity, ovulation timing variation, or any individual medical factors.