How Ovulation Timing Works
Ovulation typically occurs 14 days before the start of the next period — not 14 days after the LMP. For a 28-day cycle, that means Day 14. For a 30-day cycle, Day 16. For a 25-day cycle, Day 11.
Fertile Window = Ovulation Day − 5 to Ovulation Day + 1
(Sperm survive 3–5 days; egg viable 12–24 hours)
Ovulation by Cycle Length
| Cycle Length | Ovulation Day | Fertile Window |
|---|---|---|
| 24 days | Day 10 | Days 5–11 |
| 26 days | Day 12 | Days 7–13 |
| 28 days | Day 14 | Days 9–15 |
| 30 days | Day 16 | Days 11–17 |
| 32 days | Day 18 | Days 13–19 |
| 35 days | Day 21 | Days 16–22 |
Signs of Ovulation
- Cervical mucus changes: Becomes clear, slippery, and stretchy (like raw egg white) — the most reliable physical sign
- Basal body temperature (BBT): Rises 0.2–0.5°C after ovulation and stays elevated until next period
- LH surge: Detectable with ovulation predictor kits (OPKs) 24–36 hours before ovulation
- Mild pelvic pain: Called Mittelschmerz — brief cramping on one side, felt by some women
- Breast tenderness and slight increase in sex drive near ovulation
Frequently Asked Questions
How accurate is this calculator?
This calculator estimates ovulation based on average cycle patterns. Actual ovulation varies by individual, stress, illness, travel, and other factors. It is most useful for regular cycles (25–35 days). Irregular cycles require additional tracking methods such as BBT charting or OPK tests.
What is the fertile window?
The fertile window is the period when conception is possible — typically 5 days before ovulation through the day of ovulation (and up to 1 day after). Sperm can survive up to 5 days in the female reproductive tract; the egg is viable for only 12–24 hours after release. Having intercourse in the 2–3 days before ovulation gives the highest probability of conception.
When should I take a pregnancy test?
Wait until the first day of your missed period for the most accurate result. Testing too early (before implantation, which occurs 6–12 days after ovulation) can give false negatives. Some sensitive tests can detect hCG as early as 10 days after ovulation, but results are less reliable that early.