Health & Fitness
Ovulation Calculator
Ovulation — ovulation Calculator estimates ovulation as roughly 14 days before your next expected period. This page runs that same calculation on your numbers, in your browser: no signup, no uploads, nothing stored.
Ovulation Calculator lets you estimate your ovulation date and fertile window in seconds: enter your last period start date and typical cycle length for your estimated ovulation date, fertile window and next period, all worked out locally in your browser.
About this calculator
Ovulation Calculator estimates ovulation as roughly 14 days before your next expected period. It matters when you want a quick screening number before deciding whether a habit, plan or measurement deserves a closer look. Here is the calculation on real numbers: In a 28-day cycle, ovulation is around day 14, with the fertile window roughly days 10–15. Cycle length shifts this estimate. For your own figures, use the calculator above — same method, your inputs, computed on-device.
How to use
- Enter the first day of your last period.
- Enter your usual cycle length in days.
- Press Calculate to see your fertile window.
- Remember this estimate is not a form of contraception.
Why use Ovulation Calculator
Fast
Calculations run in your browser. No round trip to a server.
Private
Your inputs never leave your device. Nothing is uploaded.
Free
No signup, no paywall, no ads-in-results, no watermarks.
Educational
Uses common calendar-based estimates for educational purposes only. Confirm medical questions with a qualified professional.
Mobile-ready
Optimised for phones, tablets, and desktops alike.
Transparent
Clear about the method used and its limits, with disclaimers where they matter.
Common uses
Use it when discussing timing with a doctor
Use it when planning around ovulation
Use it when comparing cycles month to month
Use it for identifying your most fertile days
Use it when trying to conceive
Use it when noting your next expected period
Technical notes
Ovulation Calculator estimates ovulation as roughly 14 days before your next expected period.
These are general-population estimates and not a substitute for advice from a qualified clinician.
Because it runs locally, results appear instantly with no server round-trip.
FAQ
Is this safe for contraception?
No. Calendar ovulation estimates are not reliable birth control. Do not use this tool to avoid pregnancy; choose a proven contraceptive method instead.
When is the most fertile window?
Typically the day of ovulation and the few days before it. This tool highlights roughly five days before ovulation through one day after.
Why is ovulation about 14 days before the period?
The luteal phase after ovulation is fairly constant at about 14 days, so ovulation is estimated by counting back 14 days from the next expected period.
Does the Ovulation Calculator send my data to a server?
No. The Ovulation Calculator runs entirely in your browser — your inputs never leave your device, and nothing is uploaded or stored.
Worked example
In a 28-day cycle, ovulation is around day 14, with the fertile window roughly days 10–15. Cycle length shifts this estimate.