How to use the ovulation calculator
Prepare the values for this calculation
Estimate ovulation and fertile-window dates from the first day of the last menstrual period and usual cycle length. Use the formula below to check that it answers your question. Read the input units before transferring figures from a statement, measurement or dataset.
Enter and check each input
- First day of last menstrual period: choose a real calendar date. The input uses year, month and day; confirm the date rather than entering an elapsed duration.
- Usual cycle length (days): enter a value at least 21 and no more than 45 and a whole number. Use the unit named in the label or selected above. Do not leave this required field blank.
The input table gives a complete example. Keep units and time periods consistent across related fields.
Calculate and resolve any input error
Choose Calculate after entering the values. Changes also update the answer after a short pause. If an error appears, follow its message, correct the field and calculate again. Reset restores the example values.
Interpret the answer
Check Estimated ovulation date first. With the unchanged example inputs, it is 2026-09-15. Compare the additional metrics with the primary result; each has its own label and unit.
Compare and keep a useful result
For a controlled comparison, keep every other example input unchanged and change Usual cycle length (days) from 28 to 31. The resulting Estimated ovulation date is 2026-09-18. This illustrates the equation’s response to that input, rather than a forecast of real events.
The result diagram shows the inputs, method and output. A numeric chart is shown only when the calculation supports a meaningful numeric relationship.
Use Copy result or Download CSV to keep the answer. Share tool shares the page address without placing your input values in the URL. Recheck inputs and assumptions before using the result for a decision.
Ovulation Calculator formula
Estimated next period = LMP + cycle length; estimated ovulation = next period − 14 days. The fertile window is shown as an estimate around ovulation.
Inputs and units
| Input | Format / range | Example |
|---|---|---|
| First day of last menstrual period | YYYY-MM-DD | 2026-09-01 |
| Usual cycle length (days) | minimum 21; maximum 45; whole number | 28 |
Worked example
Using the example inputs above, the estimated ovulation date is 2026-09-15.
- Estimated ovulation date
- 2026-09-15
- Estimated fertile window
- 2026-09-10 to 2026-09-16
- Estimated next period
- 2026-09-29
Results and assumptions
Cycle timing varies and this estimate should not be relied on as contraception or as a diagnosis of fertility.
Fix an input or unexpected result
Usual cycle length (days) is required. Enter usual cycle length (days) using the format and bounds in the input table. Zero and an empty field are different: use zero only if it is allowed and represents your data.
If your answer differs from the example, choose Reset and compare every field, including selected units and options. Check percentage versus decimal entry and whether values describe the same period. Round only the final displayed answer when reproducing the formula.
If a discrepancy remains, report the calculator issue with the page address, non-sensitive example inputs, actual result, expected result and a reference for your calculation.