Ovulation Calculator
Find your estimated ovulation date and 6-day fertile window based on your last period, average cycle length, and luteal phase. Use it when trying to conceive or better understand your menstrual cycle timing.
About this calculator
Ovulation timing is calculated from two key inputs: cycle length and luteal phase length. The luteal phase — the interval from ovulation to the next period — is relatively fixed for each person, typically 12–16 days. Ovulation day within the cycle is therefore: ovulationDay = cycleLength − lutealPhase. Adding that many days to the first day of your last period gives the estimated ovulation date. The fertile window spans 5 days before ovulation through 1 day after, because sperm can survive in the reproductive tract for up to 5 days while the egg is viable for 12–24 hours after release: fertileWindowStart = ovulationDate − 5 days; fertileWindowEnd = ovulationDate + 1 day. For a 28-day cycle with a 14-day luteal phase, ovulation falls on day 14 — the textbook case, though individual cycles vary considerably.
How to use
Suppose your last period started on June 1, your average cycle length is 30 days, and your luteal phase is 13 days. Ovulation day = 30 − 13 = day 17 of the cycle. Ovulation date = June 1 + 17 days = June 18. Fertile window: June 13 (18 − 5) through June 19 (18 + 1). Days until ovulation from today (say June 10) = 8 days. To maximise conception chances, plan intercourse every 1–2 days from June 13 to June 19, with peak probability on June 17–18.
Frequently asked questions
How do I know my luteal phase length to use the ovulation calculator?
The luteal phase is the number of days between ovulation and the first day of your next period. The most accurate way to determine it is to track basal body temperature (BBT) daily — a sustained temperature rise of ~0.2°C marks ovulation, and counting from that day to your period gives the luteal phase length. Ovulation predictor kits (OPKs) can also help identify the LH surge that precedes ovulation by 24–36 hours. If you have not tracked your cycle, the default assumption of 14 days is a reasonable starting point for a 28-day cycle, but it may be off by 1–3 days.
Why does the fertile window start 5 days before ovulation?
Sperm can survive in cervical mucus and the fallopian tubes for up to 5 days under favourable conditions, meaning intercourse well before ovulation can still result in fertilisation. The egg, by contrast, is only viable for 12–24 hours after release. This asymmetry creates a 6-day fertile window — the 5 days before ovulation plus ovulation day itself — with the two days immediately before and on ovulation being the highest-probability days for conception. Having intercourse every 1–2 days throughout the window maximises the chance that live sperm are present when the egg is released.
Can irregular cycles affect the accuracy of an ovulation calculator?
Yes — this calculator assumes a consistent cycle length and luteal phase, so irregular cycles reduce its accuracy. Conditions such as polycystic ovary syndrome (PCOS), thyroid disorders, and significant weight changes can cause cycles to vary by a week or more, making date-based predictions unreliable. In those cases, combining this calculator with daily OPK testing or BBT charting provides a much better picture of actual ovulation timing. If cycles are highly irregular, a gynaecologist or reproductive endocrinologist can offer hormone testing to identify the underlying cause and guide conception timing.