Track fertility window, ovulation dates, and fertile days
What is the Ovulation Calculator?
The ovulation works like this: it predicts the fertile window by counting back 14 days from the projected next period (luteal-phase method) and marks ovulation day. Everything happens on your device - the input never leaves your tab unless the tool clearly states it performs a server-side lookup.
How to use the Ovulation Calculator
Enter the values the calculator asks for.
Double-check the unit or period next to each field (per month vs per year, cm vs m).
Read the result together with the breakdown shown beneath it.
Adjust one input at a time to compare scenarios side by side.
Features
Track fertility window, ovulation dates, and fertile days
Instant result, no page reload
Works in any modern browser, desktop or mobile
Free with no signup
Common mistakes to avoid
Mixing units or periods between fields
Rounding inputs before the final calculation
Treating estimates as guarantees for major decisions
Examples
Input
Result
Cycle 28 days · last period started 1 Sep
Fertile window ~10-15 Sep · ovulation ~15 Sep · next period ~29 Sep
Best used for
Getting a reliable answer for ovulation without installing an app
One-off tasks done in under a minute from any device
Checking results before committing them to something important
Working on a phone where desktop software is unavailable
Privacy
Calculations run in your browser. Enter only what the calculation needs.
Frequently asked questions
Can I use this to avoid pregnancy?
Not reliably - cycles vary, and sperm survives up to 5 days. Calendar methods alone have high failure rates; use it for understanding, not contraception.
Why count back 14 days from the next period?
The luteal phase after ovulation is the most stable part of the cycle at around 14 days, so the projected next period minus 14 is a better ovulation estimate than counting forward.
What is the fertile window shown?
The roughly six days ending on ovulation day - sperm survive up to five days inside the body while the egg lasts about one, which is why conception is possible before ovulation itself.