Ovulation Calculator
Find your fertile window and ovulation date instantly.
Enter your last menstrual period date and average cycle length to find your estimated ovulation day and 6-day fertile window.
How the Fertile Window Is Calculated
Ovulation timing
Ovulation typically occurs about 14 days before the start of your next period, regardless of how long your cycle is. For a 28-day cycle this falls on day 14; for a 35-day cycle it falls around day 21. This calculator subtracts 14 from your cycle length and adds that number of days to your last period date.
The 6-day fertile window
Sperm can survive in the reproductive tract for up to 5 days, so the fertile window spans the 5 days before ovulation plus ovulation day itself — 6 days total. Having intercourse on any of these days can result in pregnancy, with peak chances on the day before and the day of ovulation.
Important disclaimer
Cycle lengths between 21 and 35 days are considered normal, and actual ovulation can vary from cycle to cycle due to stress, illness, or hormonal changes. This tool provides estimates based on average patterns and is not a substitute for medical advice. For fertility concerns or family planning, consult a licensed healthcare provider.
Ovulation ≈ LMP + (cycle length − 14 days) Frequently Asked Questions
How accurate is this ovulation calculator?
The calculator gives a good estimate for people with regular cycles, but actual ovulation can shift by a few days due to stress, illness, travel, or hormonal changes. For the most accurate tracking, combine this estimate with basal body temperature charting or ovulation predictor kits.
What cycle length should I enter?
Enter your average cycle length — the number of days from the first day of one period to the first day of the next. If your cycles vary, use an average over the last 3–6 months. A typical range is 21 to 35 days.
When is the best time to get pregnant?
Your chances are highest in the 2–3 days right before ovulation and on ovulation day itself. The full 6-day fertile window shown by this calculator covers all days where pregnancy is possible in that cycle.
Can I use this calculator to avoid pregnancy?
This calculator is not a contraceptive method. Ovulation timing varies and cannot be predicted with certainty. For contraception, speak with a healthcare provider about proven methods.
Why does cycle length affect the ovulation date?
The second half of the cycle (after ovulation) is fairly fixed at around 14 days for most people. So a longer cycle simply means a longer first phase before ovulation, pushing the ovulation date later. A 35-day cycle ovulates around day 21; a 21-day cycle ovulates around day 7.
What is the formula for the Ovulation Calculator?
Ovulation date = LMP + (cycle length − 14 days). The fertile window starts 5 days before ovulation and ends on ovulation day. Next period = LMP + cycle length.