Ovulation Calculator
Estimate your ovulation date and fertile window from your last period and cycle length.
Formula last reviewed 4 August 2026 · How we verify our calculators
Estimated ovulation date
2024-01-15
- Fertile window starts
- 2024-01-10
- Fertile window ends
- 2024-01-16
- Next period (estimate)
- 2024-01-29
Updates live as you type
Frequently asked questions
Ovulation typically occurs about (cycle length − luteal phase) days after your period starts. For a 28-day cycle with a 14-day luteal phase, that is around day 14.
It spans roughly five days before ovulation through one day after, because sperm can survive several days and the egg is viable for about 24 hours.
Ovulation day is not fixed at 14 for everyone. Longer cycles ovulate later and shorter cycles earlier, so entering your real cycle length improves the estimate.
It is an estimate based on averages and assumes regular cycles. It should not be relied on alone for contraception; ovulation tests and medical advice are more reliable.
A six-day window, ending on the day itself
With a last period starting 1 January 2024, a 28-day cycle and a 14-day luteal phase, estimated ovulation is 15 January, the fertile window runs 10 to 16 January, and the next period is expected around 29 January.
The fertile window is roughly six days long and it ends on the day of ovulation, not after it. That asymmetry surprises many people and it is the single most useful fact here.
Why the window is shaped that way
Sperm survive in the female reproductive tract for up to five days in favourable cervical mucus.
An egg survives only about 12 to 24 hours after release.
So intercourse five days before ovulation can result in conception, while intercourse two days after generally cannot. The window opens five days before ovulation and closes within a day of it, with the highest probability in the two to three days immediately before.
The calculation
Ovulation day = LMP + (cycle length − luteal phase) 1 January + (28 − 14) = 1 January + 14 days = 15 January
Fertile window = five days before ovulation through the day of ovulation.
Next period = LMP + cycle length = 1 January + 28 = 29 January.
The key insight behind this method is that the luteal phase — from ovulation to the next period — is relatively constant at around 12 to 16 days for a given person, while the follicular phase — from period to ovulation — varies considerably. So ovulation is better estimated by counting backwards from the expected next period than forwards from the last one.
This is why someone with a 35-day cycle ovulates around day 21, not day 17.
Where calendar prediction falls short
Calendar-based estimation assumes cycles are regular. For many people they are not, and the method is correspondingly unreliable.
Several factors shift ovulation: stress, illness, significant weight change, travel and disrupted sleep, intensive exercise, and conditions such as PCOS or thyroid disorders, which can make cycles highly irregular or anovulatory.
More reliable indicators, roughly in order of usefulness:
Ovulation predictor kits, which detect the luteinising hormone surge occurring 24–36 hours before ovulation. These identify the window prospectively.
Cervical mucus, which becomes clear, stretchy and slippery — often compared to raw egg white — in the fertile days.
Basal body temperature, which rises about 0.3–0.5°C after ovulation. This confirms ovulation retrospectively rather than predicting it, so it is useful for establishing a pattern over several cycles but not for timing within the current one.
Combining a predictor kit with mucus observation is considerably more reliable than calendar counting alone.
Using this for contraception
Calendar-based methods are a poor contraceptive. Typical-use failure rates for rhythm methods run around 12–24% per year — meaning roughly one in five to one in eight users experiences an unintended pregnancy annually.
The reason is precisely the variability above: an unexpectedly early ovulation combined with five-day sperm survival can place a supposedly safe day well inside the fertile window. Anyone relying on fertility awareness for contraception should use a properly taught symptothermal method with instruction, not a calendar estimate.
Reading the result
Treat the dates as a rough guide for a regular cycle, best used to know approximately when to start testing with a predictor kit rather than as a definitive answer.
This is general information, not medical advice. If you have been trying to conceive for twelve months without success — or six months if the female partner is over 35 — that is the point at which medical guidance is usually recommended. Very irregular cycles, absent periods, or cycles consistently shorter than 21 or longer than 35 days warrant a conversation with a doctor regardless of how long you have been trying.
Common mistakes
Assuming ovulation is always day 14. It is cycle length minus luteal phase.
Timing intercourse after ovulation. The window closes within a day of it.
Relying on temperature to predict. It confirms after the fact.
Using calendar methods as contraception. Failure rates are high.
Assuming regularity. Illness, stress and travel all shift ovulation.