Estimate your likely ovulation day and fertile window locally from the first day of your last period and your cycle length. No upload, not medical advice.
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This calculator gives a rough orientation only and does not replace medical advice or examination. More in the disclaimer
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The calendar method estimates ovulation as the cycle length minus the average 14-day luteal phase after the first day of the last period. The fertile window sits around that day: 5 days before, because sperm can survive for several days, and 1 day after, while the egg is still viable.
The calculation runs entirely locally in your browser, in pure JavaScript - nothing is uploaded and nothing is stored. From the first day of the last period and the entered cycle length, the calculator works out the likely ovulation day, the fertile window (start and end) and the estimated next period. Change an input and everything updates instantly.
An honest note: this is a model-based estimate using the calendar method, not medical advice and not a contraceptive method. The luteal phase varies slightly from person to person, and irregular cycles make the estimate noticeably less certain. For a reliable assessment, talk to a doctor or midwife.
Specifications
Specifications
Input formats
Form inputs (no file)
Processing
Locally in your browser (JavaScript)
File upload
None
In 3 steps
Enter the first day of your last period.
Adjust the cycle length (default: 28 days).
Read off the likely ovulation day, fertile window and next period.
Limitations: A model-based estimate using the calendar method (cycle length minus a 14-day luteal phase), not medical advice and not a contraceptive method. Irregular cycles and individual variation in the luteal phase are not accounted for. Talk to a doctor or midwife.
FAQ
Are my inputs uploaded?
No. The calculation runs entirely locally in the browser (pure JavaScript); nothing is sent or stored.
Is this a contraceptive method?
No. This calculator only gives a rough, formula-based orientation and does not replace medical advice or a reliable contraceptive method.
How accurate is the calendar method?
It assumes a fixed 14-day luteal phase. For regular cycles it gives a reasonable orientation, but is noticeably less certain for irregular cycles.
Why is the fertile window 6 days long?
Because sperm can survive in the body for up to 5 days and the egg stays viable for about 1 day after ovulation - together that gives a 6-day window around ovulation.