Ovulation Calculator — Fertile Window Planner
Enter your last period date and cycle length to calculate your ovulation date and most fertile days.
Quick Answer
Ovulation typically occurs 14 days before your next period, not necessarily on day 14 of your cycle. For a 28-day cycle, ovulation is day 14. For a 35-day cycle, ovulation is around day 21. Your fertile window is the 5 days before ovulation plus ovulation day itself — 6 days total.
Understanding Your Menstrual Cycle and Ovulation
The menstrual cycle has two phases: the follicular phase (from menstruation to ovulation) and the luteal phase (from ovulation to the next period). The luteal phase is relatively fixed at approximately 14 days for most women. The follicular phase, however, varies — and this is what determines when ovulation occurs and why cycle length differs between women and month to month.
Ovulation is triggered by a surge in luteinising hormone (LH), which causes the dominant follicle in the ovary to rupture and release a mature egg. The egg then travels through the fallopian tube toward the uterus. It remains viable for fertilisation for only 12–24 hours. If sperm are present in the fallopian tube during this window — having survived from intercourse up to 5 days prior — fertilisation can occur.
How the Fertile Window Works
The fertile window is the approximately 6-day period during which conception is possible. Because sperm survive for up to 5 days and the egg survives for only 1 day, the window opens 5 days before ovulation and closes on ovulation day itself. The most fertile days are the 2–3 days immediately before ovulation, when a live sperm is most likely to be waiting in the fallopian tube when the egg arrives.
This biology explains why timing intercourse every 1–2 days throughout the fertile window — rather than targeting only the estimated ovulation day — maximises monthly conception probability. It also explains why using only an ovulation calculator (which estimates based on averages) has limited precision compared to ovulation predictor kits (OPKs), which detect the actual LH surge.
Improving Accuracy With Cycle Tracking
An ovulation calculator based on cycle length alone is a starting point. For better accuracy: track basal body temperature (BBT) daily — a temperature rise of 0.2–0.5°C the morning after ovulation confirms it occurred. Use LH-detecting ovulation predictor kits — a positive test indicates ovulation will occur within 24–48 hours. Monitor cervical mucus — the transition to clear, stretchy, egg-white consistency indicates peak fertility.
Fertility tracking apps that combine cycle history, BBT, and LH testing can predict ovulation with substantially higher accuracy than calendar calculations alone, particularly for women with variable cycle lengths.
Medical Note
If you have been trying to conceive for 12 months without success (6 months if aged 35+), consult your GP for a fertility assessment. This calculator provides estimates only.