Fertile Window Calculator
Result
Fertile Window Calculator — free online calculator.
Formula
- Fertile Window Calculator calculates its result from First day of last period, Typical cycle length (days). It uses the formula Estimated ovulation ≈ cycle length − 14 days after LMP; fertile window spans roughly five days before through one day after.. Change any input to recalculate the result instantly. This is an informational calculation and is not medical advice.
- The main inputs are First day of last period, Typical cycle length. Enter them in the units and format shown in the calculator.
- Use the measurement units shown beside each field. The result is an estimate and should be interpreted in context.
- Changing any input above recalculates the result with the same calculation engine used by this page.
Example: Fertile Window
- First day of last period: 2026-01-01
- Typical cycle length: 28 days
Using First day of last period = 2026-01-01, Typical cycle length (days) = 28, the calculated result is Estimated ovulation: 2026-01-15 | Window start: 2026-01-10 | Window end: 2026-01-16.
Sample result: Calculating…
Frequently asked questions
What does the Fertile Window calculate?
Fertile Window Calculator — free online calculator.
Which values should I enter?
Enter First day of last period, Typical cycle length using the units and format shown beside each field.
What formula does this calculator use?
The calculator uses the following relationship: Estimated ovulation ≈ cycle length − 14 days after LMP; fertile window spans roughly five days before through one day after..
Why might my result differ from another source?
Results can differ because measurement methods, rounding and assumptions vary. This calculator is an estimate and is not a medical diagnosis.
Does the result update automatically?
Yes. Changing an input updates the result using the same calculation engine shown on this page.