About the Ovulation Calculator
An ovulation calculator predicts when you are likely to ovulate and identifies your fertile window. Because the egg survives less than a day while sperm can survive several, the fertile window opens before ovulation rather than on it.
The formula
Ovulation ≈ LMP + (cycle length − 14) daysThe fertile window runs from roughly five days before ovulation to one day after. The luteal phase after ovulation is consistently around 14 days, which is what makes the prediction possible.
How to use this calculator
- 1Enter your First Day of Last Period (Year). The field starts at
2026, which you can overwrite. - 2Enter your Month (1–12). The field starts at
8, which you can overwrite. - 3Enter your Day. The field starts at
1, which you can overwrite. - 4Enter your Average Cycle Length (Days). The field starts at
28, which you can overwrite. - 5Read the result straight away — it recalculates as you type, so there is no button to press. Use Share to copy a link that reopens the page with your exact numbers filled in.
Worked example
| Input | Value |
|---|---|
| First Day of Last Period (Year) | 2026 |
| Month (1–12) | 8 |
| Day | 1 |
| Average Cycle Length (Days) | 28 |
Result
Ovulation Day: Aug 15
Fertile Window: Aug 10 – Aug 16
Next Period: Aug 29
Best Conception Days: Aug 13 – Aug 15
Those are the values the page loads with, so you can reproduce this result yourself and then change one field at a time to see what drives the outcome.
Understanding your result
Conception is most likely in the two days before ovulation and on the day itself. Sperm can survive up to five days in the reproductive tract, so intercourse several days ahead can still result in pregnancy, whereas waiting until after ovulation usually misses the window entirely.
Calendar prediction assumes regular cycles, which many people do not have. Ovulation can shift by several days between cycles in response to stress, illness, travel, or weight change. LH urine tests detect the hormone surge 24 to 36 hours before ovulation and are considerably more reliable, as is tracking basal body temperature and cervical mucus.
Things worth knowing
- LH test strips detect the surge one to two days before ovulation, which is more accurate than calendar prediction.
- Cervical mucus becomes clear, slippery, and stretchy near ovulation — a useful free indicator.
- Basal body temperature rises slightly after ovulation, confirming it happened but too late to time intercourse.
- Intercourse every one to two days through the fertile window is the usual recommendation.
- Most couples conceive within a year. Seek advice after 12 months of trying, or 6 months if over 35.
Frequently asked questions
When do I ovulate?+
About 14 days before your next period. With a 28-day cycle that is around day 14, but with a 35-day cycle it is closer to day 21. The luteal phase length is more consistent than the follicular one.
How long is the fertile window?+
About six days: the five days before ovulation plus the day itself. Sperm survive up to five days, while the egg is viable only 12 to 24 hours after release.
How reliable is calendar-based prediction?+
Moderately, if your cycles are regular. Ovulation timing varies between cycles, so LH testing or cervical mucus tracking gives substantially better accuracy.
Can I ovulate more than once in a cycle?+
Multiple eggs can be released within a short window of the same ovulation event, which is how fraternal twins occur. A second, separate ovulation later in the same cycle does not happen.