Health & Fitness
Fertility Calculator
Fertility: fertility Calculator estimates the fertile window from your cycle length and last period, centred on ovulation. Worked through below with real figures, and the calculator repeats the working on your own inputs — free, on-device, instant.
Fertility Calculator is a browser-based way to find your fertile window. Feed it your first day of last period and average cycle length and it computes your fertile days, estimated ovulation and next period on the spot, keeping your data local.
About this calculator
Fertility is easiest to get wrong in the setup, not the arithmetic. Fertility Calculator estimates the fertile window from your cycle length and last period, centred on ovulation. It matters when you want a quick screening number before deciding whether a habit, plan or measurement deserves a closer look. The worked example on this page (For a 30-day cycle, ovulation is near day 16 (cycle length − 14), so the most fertile days are about days 12–17 .) shows the full substitution; the calculator above runs the same working on your own numbers, in your browser, with nothing uploaded.
How to use
- Enter the start date of your last period.
- Enter your typical cycle length in days.
- Press Calculate to see your most fertile days.
- Treat the window as an estimate, not a guarantee.
Why use Fertility 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 estimating your fertile window
Use it when trying to conceive
Use it when planning around ovulation
Use it when noting your next expected period
Use it for identifying your most fertile days
Use it for a quick ovulation estimate
Technical notes
Fertility Calculator estimates the fertile window from your cycle length and last period, centred on ovulation.
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
Which days are the most fertile?
The fertile window spans roughly the five days before ovulation plus ovulation day, with the highest chance in the two to three days just before ovulation.
Can a fertility calculator help me conceive?
It can highlight likely fertile days to focus on, but it cannot guarantee conception. Persistent difficulty conceiving is worth discussing with a doctor.
Is the fertile window the same every cycle?
No. It shifts with cycle length and ovulation timing, which can vary month to month, so recalculate each cycle.
Does the Fertility Calculator send my data to a server?
No. The Fertility Calculator runs entirely in your browser — your inputs never leave your device, and nothing is uploaded or stored.
Worked example
For a 30-day cycle, ovulation is near day 16 (cycle length − 14), so the most fertile days are about days 12–17.