About this tool
The Ovulation Calculator is a free, in-browser tool that estimates your ovulation day and fertile window from the first day of your last menstrual period (LMP) and your average cycle length. It projects the next three cycles at once, showing the estimated 6-day fertile window, ovulation day and next period date for each.
The estimate uses the standard calendar method: ovulation is assumed to happen about 14 days before the next period starts (the luteal phase), and the fertile window spans the five days before ovulation plus the ovulation day itself. Real cycles vary from month to month, so treat these dates as a rough guide — it is an estimate, not a contraception method. Nothing you enter leaves your device.
This tool is informational only — it is not medical advice.
Frequently asked questions
How is the ovulation day estimated?
Each projected period date is your LMP plus one, two or three cycle lengths; ovulation is estimated 14 days before each period (the average luteal phase), and the fertile window runs from 5 days before ovulation through the ovulation day itself.
Can I use this as birth control?
No. This is a calendar estimate only — sperm can survive several days and ovulation timing shifts between cycles, so the calculator must not be relied on as a contraception method.
What if my cycles are irregular?
The calendar method assumes a steady cycle length between 21 and 45 days; if your cycles vary a lot, the projected dates can be off by several days, and ovulation test kits or temperature tracking give better estimates.
Is my data private, and is this medical advice?
All dates are calculated locally in your browser and never uploaded. The results are informational only and not medical advice — talk to a healthcare professional about fertility planning.
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