Ovulation Calculator
Estimate ovulation day and fertile window from cycle length and current cycle day.
Use the calculator below, then review the formula, a numeric example, and the reference table to understand how the ovulation calculator result is produced.
Calculator inputs
Result
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What is this calculator?
Estimate ovulation day and fertile window from cycle length and current cycle day. The formula, a worked example, and a reference table are below. Results are for education only, not medical advice.
How it works
Ovulation day ≈ cycle length − 14; fertile window often cycle days (ovulation−5) through (ovulation+1).
Example
28-day cycle, currently day 12 ovulation ~day 14, fertile window ~days 9–15.
Reference Table
| Value | Meaning |
|---|---|
| 28 days | Day 14 |
| 30 days | Day 16 |
| 26 days | Day 12 |
FAQ
How reliable is calendar timing?
Cycle length and ovulation day vary.
Irregular cycles?
Use longer tracking or LH tests for better timing.
LH kits?
Urine tests detect the surge before ovulation.
Fertility treatment?
Protocols use different dating rules.