Ovulation typically occurs about 14 days before your next period, and the fertile window runs from about 4 days before ovulation to 1 day after. Example: last period started 1.7 with a 28-day cycle — next period expected 29.7, estimated ovulation 15.7, fertile window 11–16.7.
How is it calculated?
The calculation is simple: add your cycle length to the first day of your last period to get the next period date, then subtract 14 days — that is the estimated ovulation date, because the phase after ovulation (the luteal phase) is relatively stable at about two weeks. The fertile window is marked from 4 days before ovulation to 1 day after, since sperm survive up to about 5 days while the egg survives only about 24 hours. A normal cycle length is anywhere from 21 to 35 days.
Example: last period on 1.7 with a 28-day cycle — the next period is expected on 29.7 (1.7 + 28), estimated ovulation on 15.7 (29.7 minus 14 days), and the fertile window is 11–16.7. Important: this is a statistical estimate only. With irregular cycles the result is less accurate, and home LH ovulation kits, basal body temperature tracking or ultrasound monitoring are far more reliable. This calculator is not a contraceptive method.
Frequently asked questions
When do you ovulate on a 28-day cycle?
On a 28-day cycle, ovulation usually happens around day 14 — that is, 14 days before your next period. For example, if your last period started on 1.7, estimated ovulation is 15.7 and the next period is expected on 29.7.
How many days does the fertile window last?
The fertile window lasts about 6 days: from 4–5 days before ovulation until about 1 day after. Sperm can survive in the reproductive tract for up to about 5 days, while the egg survives only about 24 hours after ovulation. The calculator marks the window from 4 days before ovulation to 1 day after.
Is the calculation accurate with irregular cycles?
No. The calculation assumes a fixed cycle length, so with irregular cycles the estimate is weak. In that case home LH ovulation kits, basal body temperature tracking or ultrasound monitoring are much more accurate. Remember this is an estimate only and not a method of contraception.
The information on this site is a general estimate for informational purposes only, is not medical advice and is no substitute for consulting a doctor. Your data is never stored.
