Estimate your ovulation day and fertile window from your last period and average cycle length.
Enter your cycle details
How ovulation is estimated
Ovulation typically happens a set number of days — the luteal phase, usually around 14 days — before your next period, not a fixed number of days after your last period starts. This calculator works backward from your expected next period (last period date plus your cycle length) by subtracting your luteal phase length, which gives a more personalized estimate than assuming everyone ovulates on day 14.
The fertile window
Sperm can survive in the body for up to five days, and the egg is viable for about 24 hours after ovulation. That’s why the fertile window shown here spans the five days before estimated ovulation through the day after it — the days with the highest chance of conception.
Frequently asked questions
What if my cycle length varies from month to month?
Use your average cycle length over the last few months. If your cycles vary a lot, the estimate will be less precise — ovulation predictor kits or tracking basal body temperature can help narrow it down further.
Why does this calculator ask for luteal phase length?
The luteal phase (time from ovulation to your next period) is fairly consistent for most people, usually 12–14 days, while the time before ovulation can vary more. Working backward from your next expected period gives a more accurate ovulation estimate than counting forward from your last period.
Is this a reliable way to avoid or achieve pregnancy?
This gives a general estimate for planning purposes only. It isn’t a substitute for a medically reviewed fertility awareness method, and shouldn’t be relied on alone for contraception.
Medical note: This calculator gives a general estimate based on average cycle patterns. It is not medical advice and cannot predict ovulation with certainty, especially with irregular cycles. Talk to your doctor or midwife about fertility, contraception or any concerns.