Ovulation Calculator
Find your estimated ovulation date and fertile window for the next three cycles based on your last period date and average cycle length.
This calculator provides general estimates for informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider.
How It Works
Ovulation is the release of a mature egg from the ovary, typically occurring once per menstrual cycle. For conception to occur, the egg must be fertilised within 12–24 hours of release. Sperm can survive in the female reproductive tract for up to 5 days, which is why the fertile window extends several days before ovulation.
Ovulation timing:
Ovulation occurs approximately 14 days before the next menstrual period, regardless of cycle length. For a 28-day cycle, ovulation is on approximately day 14. For a 32-day cycle, it is on approximately day 18.
Formula:
Ovulation date = LMP + (cycle length − 14) days
Fertile window:
The fertile window spans approximately 6 days — the 5 days before ovulation and the day of ovulation itself (egg survives ~24 hours, sperm survive up to ~5 days).
Fertile window start = Ovulation date − 5 days
Fertile window end = Ovulation date + 1 day
Next cycle:
Next period = LMP + cycle length
These calculations are repeated for the next three cycles by adding the cycle length to each successive period start date.
Important: This calculator provides estimates based on average cycle patterns. Actual ovulation timing can vary due to stress, illness, travel, and other factors. Ovulation predictor kits (LH surge tests) and basal body temperature tracking provide real-time confirmation.
Frequently Asked Questions
How accurate is this ovulation calculator?
The calculator assumes regular cycles and uses the standard 14-day luteal phase estimate. For women with regular cycles, it is reasonably accurate within a few days. For irregular cycles, the estimate is less reliable. Ovulation predictor kits (OPKs) that detect the LH surge are a more accurate real-time method.
What is the luteal phase?
The luteal phase is the period from ovulation to the start of the next period. It averages 14 days and is relatively consistent across women (typically 12–16 days), unlike the follicular phase (day 1 of period to ovulation) which varies more. This consistency is why subtracting 14 from cycle length estimates ovulation timing.
Can I get pregnant outside the fertile window?
The probability is very low but not zero, because ovulation timing can shift unexpectedly. A small percentage of people also experience very early or very late ovulation in some cycles. For reliable contraception, this calculator should not be used as a birth control method.
Is any of my data stored?
No. All calculations happen in your browser using JavaScript. No dates or personal information are transmitted to or stored on any server.