Ovulation Calculator
Adjust the inputs below. Results update as you type.
How it works
Ovulation typically occurs 14 days before the next expected period: ovulation day = LMP + (cycle length − 14). Fertile window spans 5 days before ovulation through 1 day after (sperm can survive 3–5 days; egg lives 12–24 hours). Cycle length varies by 2–3 days month-to-month for most women. Basal body temperature (BBT) charting and LH ovulation tests provide more accurate timing than calendar methods.
Input guidance
- Use measured values (recent weight, height, activity level) rather than guesses.
- Recalculate every few weeks as your routine and body metrics change.
- Pair screening metrics with context such as sleep, stress, and medical history.
The formula
Ovulation occurs about 14 days before the next period: ovulation day = cycle length − 14. The fertile window spans roughly the 5 days before ovulation plus the day itself.
Worked example
On a 30-day cycle, ovulation is around day 16 (30 − 14), with the fertile window roughly days 11–16.
More examples to test
- Maintenance case: keep activity constant and observe expected baseline ranges.
- Adjustment case: change only one major variable to understand sensitivity.
How to interpret results
Use results for awareness and planning, then validate with qualified medical or nutrition professionals for individualized decisions.
When this can be inaccurate
Population formulas cannot account for all biological factors such as medical conditions, medication effects, body composition differences, and genetics.
Change history
- July 2026: Quality-reviewed for publication with formula checks and explanatory copy updates.
Site-wide corrections also appear on the corrections log.
Track fitness goals with a top app
General wellness resource.
Frequently asked questions
When am I most fertile?+
In the few days leading up to and including ovulation, because sperm can survive several days while the egg lives about 24 hours.
How reliable is the calendar method?+
It works best with regular cycles and is only an estimate. Ovulation tests or tracking basal temperature improve accuracy.
Does cycle length change ovulation?+
Yes. Ovulation is roughly 14 days before the next period, so longer or shorter cycles shift the fertile window accordingly.
Can this calculator diagnose a condition?+
No. It is a screening and education tool only, not a clinical diagnosis.
When should I seek professional advice?+
Seek professional advice when results are extreme, persistent, or conflict with symptoms, goals, or prior medical guidance.