Calculate your ovulation date and 6-day fertile window from your last period and cycle length — with peak fertility days and days until ovulation.
Enter your figures and click Calculate to see your results.
Enter the first day of your last period and your average cycle length — ovulation date and fertile window are calculated instantly.
Press the Calculate button. Results appear instantly using standard clinical and scientific formulas.
Results are displayed with all key values clearly labelled. Use the Copy button to grab your results or Download to save a text file. For health decisions, always consult a healthcare professional.
Ovulation typically occurs 14 days before the next period — not 14 days after the last period (unless the cycle is exactly 28 days). For a 35-day cycle, ovulation occurs around day 21; for a 24-day cycle, around day 10. The luteal phase (ovulation to next period) is relatively constant at 14 ± 2 days, while the follicular phase varies.
The fertile window spans 6 days: the 5 days before ovulation and the day of ovulation itself. Sperm can survive in the female reproductive tract for 3–5 days, but the egg lives only 12–24 hours after release. The most fertile days are the 2 days before ovulation and the day of ovulation — having intercourse on these days maximises the chance of conception.
Methods to confirm ovulation: ovulation predictor kits (OPKs) detect the LH surge 24–36 hours before ovulation; basal body temperature (BBT) rises 0.2–0.5°C after ovulation; cervical mucus becomes clear, slippery and "egg-white" texture at peak fertility. Combining multiple methods gives the most reliable confirmation.
Yes — cycle length variation of up to 7 days is considered normal. Stress, illness, travel, significant weight changes, intense exercise and hormonal fluctuations can all shift ovulation timing. This is why calendar-only methods of contraception are unreliable and why combining tracking methods is important for fertility awareness.
This calculator provides estimates based on average cycle patterns and the assumption that ovulation occurs 14 days before the next period. It is most accurate for women with regular cycles. Women with irregular cycles, PCOS or those who have recently started/stopped hormonal contraception may experience significantly different timing than predicted.
