Ovulation Calculator
Estimate fertile window
Quick guide
What can Ovulation Calculator help you do?
Free ovulation calculator. Estimate ovulation day and fertile window from last period date, cycle length, and luteal phase.
Best for
Explore a date projection based on the cycle and luteal-phase lengths you provide, as an estimated entry in cycle notes. This widget does not read temperature, ovulation tests, or previous cycles, confirm that ovulation occurred, or calculate an individual probability of conception.
How to use it
Enter the first day of the last period, cycle length, and luteal-phase length. Defaults are 2026-05-10, 28 days, and 14 days. Both lengths must be positive and the luteal phase shorter than the cycle. Use whole days and replace the sample date with your intended input.
How to read the result
Estimated ovulation is the period start plus cycle length minus luteal-phase length. The displayed fertile window runs from five days before that estimate through one day after it. This is a fixed arithmetic range, not a physiologically confirmed ovulation date or a safe-day classification.
Privacy and limits
Period and cycle inputs are handled by date functions in the page, without a cycle account, cloud diary, or external prediction API. Copying writes the result to the device clipboard. The site separately loads analytics resources; consider screen and clipboard visibility on shared devices.
The output is not contraception advice, ovulation confirmation, or a fertility diagnosis; cycle-to-cycle variation is not modeled. Blank and fractional lengths are rejected, but future period dates can still be entered. Producing a range does not establish that inputs reflect reality or assure safety outside the range.
Frequently Asked Questions
Is this for medical diagnosis?
No. It is a general cycle estimate and cannot replace medical advice.
Can cycles vary?
Yes. Stress, health, and irregular cycles can change timing.