Ovulation Calculator
The Ovulation Calculator estimates your most fertile days, ovulation date, next period, and an estimated due date if you conceive — based on the first day of your last period and your average cycle length. The 14-day gap between ovulation and the next period reflects the luteal phase — the second half of the cycle — which stays comparatively fixed near 14 days even when total cycle length varies, so nearly all of that variability happens earlier, in the follicular phase before ovulation. Because of this, a longer or shorter cycle shifts the ovulation date earlier or later while the 14-day countdown to the next period stays constant. Whatever cycle length you enter is clamped between 21 and 45 days, since cycles far outside that range fall outside the average assumptions the formula is built on and would produce misleading estimates.
Enter the first day of your last period to see your fertile window.
Ovulation is estimated as roughly 14 days before your next period, with the fertile window covering the 5 days before through the day after. Cycles vary, so this is an estimate, not medical advice or contraception. Nothing is uploaded.
How This Tool Works
Pick the first day of your last period and enter your average cycle length. You’ll see your fertile window, estimated ovulation day, next period date, and an estimated due date if you conceive.
Formula & Method
Ovulation is estimated at about 14 days before your next expected period (cycle length − 14). The fertile window spans the 5 days before ovulation through the day after, when conception is most likely.
Example Calculation
With a last period starting March 3, 2026 and an average cycle length of 30 days, the calculator estimates ovulation at 30 − 14 = 16 days after the start date, landing on March 19, 2026. The fertile window spans 5 days before ovulation through the day after: March 14 through March 20, 2026. The next expected period adds the full cycle length to the start date: March 3 + 30 days = April 2, 2026. And if conception happens this cycle, the estimated due date adds 266 days to the ovulation date: March 19, 2026 + 266 days = December 10, 2026.
Frequently Asked Questions
How is the fertile window calculated?+
It’s the 5 days before ovulation plus the day after — sperm can survive several days, so intercourse in this window gives the best chance of conception.
Can I use this to avoid pregnancy?+
No — cycle timing is only an estimate and ovulation can shift, so this is not a reliable form of contraception. Talk to a healthcare provider about birth control.
Is my information private?+
Yes — all dates are calculated in your browser and nothing is uploaded or stored.
What if my cycle length is shorter than 21 days or longer than 45 days?+
The calculator automatically limits whatever you enter to a minimum of 21 and maximum of 45 days, since the underlying formula is built on typical cycle-length assumptions and becomes unreliable further outside that range. If your true cycle falls outside this window, treat the estimate with extra caution and consider talking to a healthcare provider about a more personalized assessment.
How is the estimated due date calculated if I conceive this cycle?+
The calculator adds 266 days to your estimated ovulation date — the same conception-based method Naegele's rule uses in the standalone Pregnancy Due Date Calculator — treating ovulation as a stand-in for conception, since the two typically happen within about a day of each other.
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<a href="https://everyfix.net/tools/ovulation-calculator/">Ovulation Calculator</a> by <a href="https://everyfix.net">EveryFix</a>Related Tools
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