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Ovulation timing varies from cycle to cycle even with regular periods, so treat these dates as a guide. Ovulation predictor kits, cervical mucus changes and basal body temperature give a more reliable picture. This is not medical advice or a method of contraception.
What The Ovulation Calculator Works Out
- Estimated ovulation: ovulation usually happens about 14 days before the next period starts, whatever the cycle length, because the second half of the cycle (the luteal phase) is fairly constant. The calculator adds your cycle length to the first day of your last period and counts back 14 days: day 14 of a 28-day cycle, day 18 of a 32-day cycle.
- Fertile window: from five days before ovulation to one day after. Sperm survive up to five days and the egg about 12 to 24 hours, which is why the days just before ovulation matter most when trying to conceive.
- Next period due: the last period's start date plus the cycle length. The table lists the next few cycles so you can plan ahead.
- Luteal phase length (advanced): 14 days is the average, but 10 to 16 is normal. If temperature charting or test strips have shown yours, entering it moves the ovulation estimate accordingly.
- Cycle variation (advanced): most cycles vary by a few days month to month. Entering the variation widens the fertile window either side, which is the realistic planning range rather than a single date.
- Where you are today (advanced): the current cycle day and either the days until expected ovulation or the days past ovulation (dpo), the figure fertility forums count in.
- Implantation window (advanced): a fertilised egg implants 6 to 12 days after ovulation, most often around day 9. A test cannot be positive before implantation.
- Earliest pregnancy test (advanced): the first day of a missed period is the most reliable time to test. Early-result tests can detect a pregnancy up to four days before, but a negative that early may still turn positive.
- Start ovulation test strips (advanced): ovulation predictor kits detect the LH surge 24 to 36 hours before ovulation, so testing from a few days before the expected date catches it. Cycle counting is only an estimate; stress, illness and natural variation shift ovulation, and irregular cycles make the prediction much less reliable. If you have been trying for a year (six months if over 36), the NHS recommends speaking to your GP. This tool is for guidance only, is not medical advice, and must not be relied on as contraception.
Source and further reading: NHS: trying to get pregnant.