Ovulation Calculator
Knowing your ovulation date is valuable whether you are trying to conceive or simply want to understand your menstrual cycle better. Ovulation is the release of a mature egg from the ovary and marks the most fertile point of your cycle. It occurs approximately 14 days before the start of your next period. The six-day fertile window includes the five days before ovulation, when sperm deposited in the reproductive tract can survive to fertilise the egg, and the day of ovulation itself, when the egg is available. Conception is most likely on the day of ovulation and the two days immediately before it. This calculator estimates your next ovulation date and fertile window from your last menstrual period (LMP) start date and average cycle length. These are population-average estimates. Individual ovulation timing can shift by several days due to stress, illness, breastfeeding, medications, and other factors. For precision tracking, combine this estimate with basal body temperature charting and ovulation predictor tests.
Formula
Next period = LMP + cycle length
Ovulation = Next period - 14 days
Fertile window start = Ovulation - 5 days
Fertile window end = Ovulation + 1 day
Ovulation Calculator: frequently asked questions
When does ovulation occur?
Ovulation typically occurs 14 days before the start of the next menstrual period, not 14 days after the last period began. For a standard 28-day cycle this means ovulation on day 14. For a 35-day cycle, ovulation would occur around day 21. The 14-day figure before the next period is more reliable than counting forward from the LMP because the length of the follicular phase (before ovulation) varies more than the luteal phase (after ovulation).
What is the fertile window?
The fertile window is the period during which conception is possible. Sperm can survive in the female reproductive tract for up to 5 days. The egg survives for approximately 12 to 24 hours after ovulation. The fertile window therefore spans from 5 days before ovulation to 1 day after ovulation: a total of approximately 6 days per cycle.
How accurate is this estimate?
The estimate is based on average timing and assumes a regular cycle. Ovulation can vary by several days even in women with regular cycles due to stress, illness, travel, and other factors. For more precise ovulation detection, use ovulation predictor kits (OPKs) that detect the LH surge, or track basal body temperature over multiple cycles.
Can I use this calculator to avoid pregnancy?
No. This calculator is not a reliable method of contraception. Cycle length and ovulation timing can vary unpredictably. For contraception, consult a healthcare provider about evidence-based methods. Natural family planning methods require training, consistent cycle tracking, and professional guidance.
What if my cycle length varies from month to month?
Use your shortest cycle length to find the earliest possible ovulation date and your longest cycle length for the latest. The fertile window then spans from 5 days before the earliest ovulation to 1 day after the latest ovulation. This broader window is used in natural family planning methods such as the Standard Days Method.
Official sources
- NIH Eunice Kennedy Shriver NICHD: Ovulation Tracking
- Wilcox AJ, Weinberg CR, Baird DD. (1995). "Timing of sexual intercourse in relation to ovulation." New England Journal of Medicine, 333(23):1517-1521.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.