Ovulation Calculator
An ovulation calculator estimates the fertile window based on the last menstrual period (LMP) and cycle length. Ovulation typically occurs 14 days before the next period (not 14 days after the LMP). The fertile window spans approximately 6 days — 5 days before ovulation plus ovulation day itself.
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Tip: Basal body temperature (BBT) rises 0.2–0.5°C after ovulation due to progesterone. Tracking BBT over several months can confirm ovulation timing, though it tells you after ovulation has already occurred.
- 1Ovulation day = LMP + (cycle length − 14)
- 2Fertile window = ovulation day − 5 through ovulation day + 1
- 3Sperm can survive in the reproductive tract for up to 5 days
- 4The egg is only viable for 12–24 hours after ovulation
LMP Jan 1, cycle length 28 days=Ovulation Jan 15, fertile Jan 10–1628 − 14 = day 14
LMP Jan 1, cycle length 35 days=Ovulation Jan 22, fertile Jan 17–2335 − 14 = day 21
| Cycle Length | Ovulation Day | Fertile Window Start |
|---|---|---|
| 21 days | Day 7 | Day 2 |
| 25 days | Day 11 | Day 6 |
| 28 days | Day 14 | Day 9 |
| 30 days | Day 16 | Day 11 |
| 35 days | Day 21 | Day 16 |
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Fun Fact
Only about 30% of women have a 28-day cycle. Cycle lengths from 21 to 35 days are considered normal. The pre-ovulation phase varies most; the post-ovulation (luteal) phase is usually a consistent 14 days.
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