Ovulation & Fertility Window Calculator
Find your ovulation date and most fertile days.
All calculations happen locally in your browser — nothing is uploaded, stored or tracked.
Enter your last period
The first day of your most recent period.
Set cycle and luteal phase
Defaults are 28 and 14 days.
Read the window
Ovulation date, fertile days and next period.
What Is an Ovulation Calculator?
Ovulation is when an ovary releases an egg, typically about 14 days before the start of the next period — which is why the calculator works backwards from your cycle length. The fertile window is the roughly six days when intercourse can lead to conception: the five days before ovulation plus the day itself.
The calculator estimates ovulation and the fertile window from the first day of your last period and your average cycle length. It also estimates the next expected period.
The Calculation
The luteal phase (time from ovulation to the next period) is fairly constant for most people — usually 12 to 16 days — while the follicular phase varies. You can adjust the luteal phase length in the calculator.
Limitations
Your data never leaves your device.
Every calculation on this site runs in your browser using vanilla JavaScript. Nothing is uploaded, stored or tracked.
- 100% local, client-side calculations
- No data stored on any server
- No tracking, no analytics, no ads scripts
- Works offline once loaded
Frequently Asked Questions
When do I ovulate?
Typically about 14 days before your next period starts. For a 28-day cycle that is around day 14 of the cycle.
What is the fertile window?
The five days before ovulation plus ovulation day itself — sperm can survive several days, so these are the days conception is possible.
Can I get pregnant outside the fertile window?
Conception requires intercourse during the fertile window. Outside it, the chance is essentially zero for a typical cycle.
What is the luteal phase?
The days between ovulation and the next period, usually 12–16 days. It is the most stable part of the cycle.
Why is this only an estimate?
Cycle length and ovulation timing shift with stress, illness and other factors. Tracking methods or a healthcare provider give more certainty.