Ovulation Calculator
An ovulation calculator estimates the fertile window by counting back 14 days (the typical luteal phase length) from the start of the next expected period, using the last menstrual period and average cycle length. The luteal phase, the time from ovulation to the next period, is far more consistent across cycles than the follicular phase before it, which is exactly why counting backward 14 days from the next expected period is a more reliable estimate method than counting forward from the last period for people with variable cycle lengths.
Educational estimate only. Results are not medical, legal, or financial advice. Confirm important decisions with a qualified professional.
How it works
- Enter the first day of your last period.
- Enter your average cycle length (21-35 days).
- Read the estimated ovulation day, fertile window, and next period date.
Formula Ovulation ≈ LMP + (cycle length − 14) days; fertile window = ovulation day − 5 to +1
Frequently asked questions
How accurate is this estimate?
It is a population-average estimate. Actual ovulation timing varies cycle to cycle, especially with irregular cycles — ovulation predictor kits or a doctor’s guidance give a more precise picture.
Why is the luteal phase assumed to be 14 days?
The luteal phase (ovulation to next period) is much more consistent across people and cycles than the follicular phase (period to ovulation), which is why counting backward from the next period is more reliable than counting forward from the last one.
What if my cycles are irregular?
This estimate is less reliable for irregular cycles — consider ovulation predictor kits or basal body temperature tracking for a more accurate picture.
Is the fertile window exactly the days shown?
It is a standard estimate (5 days before ovulation, when sperm can survive, through 1 day after, when the egg is viable) — individual variation exists.
Is this medical advice?
No. It is an educational estimate — consult a healthcare provider for fertility planning or concerns.