Ovulation calculator: estimate your fertile window and ovulation date
This ovulation calculator estimates your fertile window, the day you are most likely to ovulate and the date your next period is due, from the first day of your last period and your usual cycle length.
How this is calculated
Ovulation = first day of last period + (cycle length − luteal phase length)
Fertile window = the 5 days before ovulation, and the day of ovulation
Next period = first day of last period + cycle length
The luteal phase, the days from ovulation to the next period, lasts 14 days on average and 11 to 17 days in a normal cycle. Sperm survive up to 5 days in the reproductive tract and an egg about 12 to 24 hours after ovulation, which is why the fertile window ends on the day of ovulation.
A calendar cannot detect ovulation in the current cycle. In a study of 221 women, the fertile window fell entirely within days 10 to 17 of the cycle for only about 30 in 100, and its timing was highly unpredictable even in regular cycles.
Sources: ACOG, on the fertile window; Wilcox, Weinberg and Baird, Timing of sexual intercourse in relation to ovulation (New England Journal of Medicine, 1995; 221 women, 625 cycles); ASRM Practice Committee, Diagnosis and treatment of luteal phase deficiency: a committee opinion (Fertility and Sterility, 2021); Wilcox, Dunson and Baird, The timing of the fertile window in the menstrual cycle (BMJ, 2000; 221 women, 696 cycles).
The fertile window is the six days that end on the day of ovulation
Conception is possible only from intercourse on a few days of each cycle. In a study of 221 women who were trying to conceive, every pregnancy came from intercourse during the six days that ended on the estimated day of ovulation (Wilcox, Weinberg and Baird, New England Journal of Medicine, 1995). Sperm can live in the body for as long as 5 days, while an egg survives for about 12 to 24 hours after ovulation, so the window closes with ovulation (ACOG).
The most fertile days are the ones closest to ovulation
In the same study, the chance of conception from intercourse on a single day rose from about 10 in 100 five days before ovulation to about 33 in 100 on the day of ovulation itself (Wilcox, Weinberg and Baird, 1995).
Ovulation predictor kits detect the hormone surge that comes before ovulation
An ovulation calculator works from the length of your cycle, so it has no information about the cycle you are in. A study of 949 volunteers, cited by the American Society for Reproductive Medicine (ASRM), compared urinary LH testing with several calendar apps and found a maximum accuracy of 21% in predicting the day of ovulation (ASRM, 2022). Ovulation predictor kits do use the current cycle: they detect the rise in luteinizing hormone (LH) that precedes ovulation, and once LH appears in the urine, ovulation usually follows within 12 to 24 hours (ASRM, Am I ovulating?, 2023). Changes in cervical mucus also indicate that ovulation is approaching: the chance of conception is highest when the mucus is slippery and clear, although pregnancy can occur without it (ASRM, 2022). Basal body temperature, your temperature at rest, can confirm that ovulation has happened but cannot predict when it will occur (ASRM, 2023).
With irregular periods, the estimated ovulation date moves with each cycle
The calculation assumes that every cycle is the length you enter, and that the luteal phase, the days between ovulation and the next period, stays close to its usual length. ASRM describes a typical luteal phase as relatively fixed at 12 to 14 days but ranging from 11 to 17 (ASRM, 2021). If your cycle length varies, the estimated ovulation date moves by the same number of days. Even in women whose cycles are usually regular, the timing of the fertile window can be highly unpredictable (Wilcox, Dunson and Baird, BMJ, 2000). With irregular cycles, a predictor kit or cervical mucus gives information about the current cycle that a calendar estimate cannot.
Sources
- Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation: effects on the probability of conception, survival of the pregnancy, and sex of the baby. New England Journal of Medicine. 1995;333(23):1517–1521. doi:10.1056/NEJM199512073332301. PMID 7477165.
- American College of Obstetricians and Gynecologists. Trying to get pregnant? Here’s when to have sex. Last updated August 2023, last reviewed August 2025.
- Wilcox AJ, Dunson D, Baird DD. The timing of the “fertile window” in the menstrual cycle: day specific estimates from a prospective study. BMJ. 2000;321(7271):1259–1262. doi:10.1136/bmj.321.7271.1259. PMID 11082086.
- Practice Committees of the American Society for Reproductive Medicine and the Society for Reproductive Endocrinology and Infertility. Diagnosis and treatment of luteal phase deficiency: a committee opinion. Fertility and Sterility. 2021;115(6):1416–1423. doi:10.1016/j.fertnstert.2021.02.010.
- Practice Committee of the American Society for Reproductive Medicine. Optimizing natural fertility: a committee opinion. Fertility and Sterility. 2022;117:53–63.
- American Society for Reproductive Medicine. Am I ovulating? Patient fact sheet, revised 2023. ReproductiveFacts.org.