Find your most fertile days. This free ovulation calculator estimates your ovulation date and fertile window from your last period and cycle length, to help when you’re trying to conceive.
These are estimates based on average cycles, useful for understanding your fertile window when trying to conceive. They are not a reliable method of birth control — ovulation timing varies, especially with irregular cycles. For contraception or fertility concerns, speak with a healthcare professional.
Your cycle
days
Enter the first day of your last period and your usual cycle length (the number of days from one period’s start to the next — typically 21–35). The fertile window is the days you’re most likely to conceive.
Estimated ovulation
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Enter your details
Most fertile window
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Next period (est.)
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If conceived, due date
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Estimates only, not medical advice or contraception. Cycles vary month to month; ovulation predictor kits and tracking with your doctor give more precise timing.
Use this free ovulation calculator to estimate your most fertile days. Enter your last period and cycle length to see your likely ovulation date and fertile window for trying to conceive.
What this ovulation calculator does
An ovulation calculator estimates when you ovulate and your fertile window — the days you’re most likely to conceive. It works from your last period and average cycle length, assuming ovulation about 14 days before your next period.
How ovulation timing is estimated
Ovulation ≈ first day of last period + (cycle length − 14)
Fertile window ≈ 5 days before ovulation to 1 day after
On a 28-day cycle, ovulation is around day 14; on a 32-day cycle it’s around day 18. The fertile window spans about six days because sperm can survive several days.
The fertile window
Day relative to ovulation
Chance to conceive
5–3 days before
Lower but possible
2 days before to ovulation
Highest
1 day after
Decreasing
Important: not a contraception method
These estimates help when trying to conceive, but they are not reliable for preventing pregnancy. Ovulation shifts from cycle to cycle, and pregnancy is possible outside the predicted window. For birth control, use a proven method and speak with a healthcare professional.
How to use the ovulation calculator
Enter the first day of your last period.
Enter your average cycle length.
Read your estimated ovulation date and fertile window.
Ovulation FAQ
How do I calculate my ovulation date?
Ovulation usually happens about 14 days before your next period — so cycle length minus 14 days from the start of your last period. On a 28-day cycle that's around day 14.
What is the fertile window?
The roughly six days ending on ovulation day when conception is most likely, because sperm can survive several days and the egg about a day.
When am I most likely to conceive?
In the two to three days leading up to and including ovulation. The calculator highlights this fertile window for you.
Can I use this for birth control?
No. Ovulation timing varies between cycles and pregnancy is possible outside the predicted window, so this isn't reliable contraception. Use a proven method and consult a professional.
What if my cycles are irregular?
Predictions are less accurate with irregular cycles. Ovulation predictor kits, tracking basal body temperature, or guidance from your doctor give more precise timing.
Is the ovulation calculator free?
Yes, this ovulation calculator is completely free, needs no sign-up, and gives instant results in your browser.
