Ovulation Calculator
Estimate your predicted ovulation date and peak fertility window from the first day of your last period and your average cycle length.
How It's Calculated
Formula
\text{next period} = \text{LMP} + \text{cycleLength}
\text{ovulation} = \text{next period} - 14\text{ days}
\text{fertile window} = [\text{ovulation} - 5\text{ days},\ \text{ovulation}]This calculator uses the standard calendar method: it predicts your next period as your last period's first day plus your average cycle length, then estimates ovulation as 14 days before that predicted next period. This works because the luteal phase (from ovulation to the next period) is relatively constant at around 14 days, while the earlier part of the cycle is what varies with overall cycle length — so a longer or shorter cycle shifts the estimated ovulation date accordingly, rather than always landing on "day 14." The peak fertility window is shown as the 5 days before predicted ovulation through the ovulation day itself, reflecting that sperm can remain viable for up to several days while the egg is viable for a shorter window after release. These are cycle-based calendar estimates, not a confirmation that ovulation has occurred or will occur on a specific day — actual ovulation timing can vary cycle to cycle, and this calculator does not diagnose fertility, confirm ovulation, or guarantee a conception date.
Worked Examples
28-day cycle, LMP January 1
- Predicted next period: Jan 1 + 28 days = Jan 29
- Predicted ovulation: Jan 29 - 14 days = Jan 15
- Fertile window: Jan 10 through Jan 15
30-day cycle, LMP January 1
- Predicted next period: Jan 1 + 30 days = Jan 31
- Predicted ovulation: Jan 31 - 14 days = Jan 17
- Fertile window: Jan 12 through Jan 17
Frequently Asked Questions
Does ovulation always happen on day 14 of my cycle?
No. This calculator does not assume a flat day 14 — it estimates ovulation as 14 days before your predicted NEXT period, so a longer or shorter average cycle length shifts the estimated date earlier or later accordingly.
How is the fertile window calculated?
The fertile window shown is the 5 days before predicted ovulation through the ovulation day itself, based on typical sperm and egg viability windows cited by mainstream reproductive-health sources.
Can this calculator confirm I ovulated or that I'm fertile?
No. This is a calendar-based estimate only. It cannot confirm ovulation occurred, diagnose fertility, or guarantee a conception date — actual timing varies from cycle to cycle and person to person.
Why is the cycle length limited to 21-35 days?
That range covers the typical adult cycle length this calendar model is designed around. Cycles consistently outside that range are better assessed with guidance from a healthcare provider rather than this general calendar estimate.