Ovulation Calculator

Work out your likely ovulation day and fertile window from the first day of your last period and your usual cycle length.

Ovulation typically happens about 14 days before the next period starts — around day 14 of a 28-day cycle, later if your cycle is longer. The fertile window is the handful of days ending on ovulation day, because sperm can survive for up to about five days while an egg survives roughly a day.

First day of your last period

Your usual cycle length (days)

Estimated ovulation

Fertile window

Next period (estimate)

If you conceive, due date

Please read: This calculator gives estimates only — it is not medical advice and cannot diagnose or date a pregnancy with certainty. Only around 5% of babies arrive on their estimated due date, and most arrive within a week or so either side. Ultrasound dating performed by a clinician takes precedence over any calculator result. Always confirm your dates, screening windows and care plan with your healthcare provider. All dates are processed locally in your browser; nothing you enter is uploaded or stored.

How this estimate works

The calculator counts your cycle length from the first day of your last period to estimate when the next period would start, then counts back 14 days to ovulation — the luteal phase (the stretch after ovulation) is fairly consistent at around 14 days for many people, while the first part of the cycle varies more. The fertile window shown is the five days before ovulation plus ovulation day itself.

Cycle-based estimates are a guide, not a measurement. Stress, illness, travel and many other factors can shift ovulation by days. If timing matters — for conception or for pregnancy dating — ovulation test kits, temperature tracking or a clinician's assessment may give a clearer picture than dates alone.

Trying to date a pregnancy instead? The due date calculator can start from a conception date, and the conception calculator works in both directions.