Ovulation Calculator
28 days
Ovulation Date--
Fertile Window--
Next Period--
Pinpointing ovulation is the key to timing intercourse for conception, because an egg is only viable for a day or so while sperm can survive several days beforehand. This calculator works from the date your last period started and your usual cycle length to estimate the day you ovulate, then frames a six-day fertile window around it. It assumes ovulation occurs about 14 days before your next period, the most consistent part of the cycle.
Formula
ovulation = lastPeriod + (cycleLength − 14)
- lastPeriod
- First day of your most recent period
- cycleLength
- Average number of days from one period’s start to the next
- 14
- Typical length of the luteal phase, counted back from the next period
How it works
- Enter the first day of your last menstrual period and your average cycle length in days (commonly 28).
- Ovulation is estimated as the start date plus (cycle length − 14), placing it 14 days before your next expected period.
- The fertile window is set from five days before ovulation to one day after, and your next period date is projected as the start date plus one full cycle length.
Worked example
A period started on March 1, 2026, with a 28-day cycle.
- Ovulation: March 1 + (28 − 14) = March 1 + 14 = March 15, 2026.
- Fertile window: 5 days before to 1 day after → March 10 to March 16, 2026.
- Next period: March 1 + 28 = March 29, 2026.
Estimated ovulation on March 15, with the fertile window spanning March 10–16, 2026.
Frequently asked questions
- Why is the fertile window six days long?
- Sperm can survive in the reproductive tract for up to about five days, while the egg lives roughly 24 hours. The window therefore opens a few days before ovulation and closes shortly after, which is why this tool spans five days before to one day after.
- How reliable is a calendar-based ovulation estimate?
- It is an estimate built on cycle averages and the assumption of a 14-day luteal phase. Stress, illness, and naturally irregular cycles can shift the real ovulation day, so methods like ovulation predictor kits or basal body temperature tracking can confirm it.
- Does a longer or shorter cycle change my ovulation day?
- Yes. Because ovulation is counted back 14 days from the next period, a 32-day cycle pushes ovulation to about day 18, while a 24-day cycle moves it to around day 10. Entering your true cycle length keeps the estimate accurate.
- Can I use this to avoid pregnancy?
- Calendar timing alone is not a reliable form of contraception because ovulation can vary. If you are trying to prevent pregnancy, use a proven contraceptive method rather than relying on fertile-window estimates.