Ovulation & Fertile Window Calculator
Fill in the values and the answer updates as you type.
How the Ovulation & Fertile Window Calculator works
Ovulation usually happens about 14 days before the next period. The fertile window covers the 5 days before ovulation plus the day after. This is an estimate for regular cycles and is not birth control. Speak to a doctor about irregular cycles. Example: with the default values, estimated ovulation day comes to Sun Oct 04 2026.
Inputs and results
The Ovulation & Fertile Window Calculator takes 2 inputs and gives 4 results.
What you enter
- First day of last period — starts at 2026-09-20.
- Average cycle length (days) — starts at 28.
What you get
- Estimated ovulation day
- Fertile window starts
- Fertile window ends
- Next period expected
Ovulation & Fertile Window Calculator FAQ
How do I use the Ovulation & Fertile Window Calculator?
Enter first day of last period and average cycle length in the fields, then read estimated ovulation day, fertile window starts, fertile window ends and next period expected in the result panel. The answer updates as you type, and you can press Calculate or Enter to refresh it. Use “Copy link” to share your exact numbers with someone else.
What does the Ovulation & Fertile Window Calculator calculate?
Estimate your ovulation day and fertile window from your cycle. It returns estimated ovulation day, fertile window starts, fertile window ends and next period expected from the 2 values you provide.
What values does the Ovulation & Fertile Window Calculator start with?
It opens with first day of last period 2026-09-20 and average cycle length 28 days so you can see a working result straight away. Replace them with your own numbers, or press Reset to return to these starting values.
Is the Ovulation & Fertile Window Calculator free and private?
Yes. It is free, needs no account, and runs in your browser, so nothing you type is sent to or stored on our servers. See the Privacy Policy for details.
Can I rely on the Ovulation & Fertile Window Calculator for medical or health decisions?
Treat the result as a well-rounded estimate. Real situations often have details a general calculator cannot know, so confirm medical or health decisions with a qualified professional. See the Disclaimer for more.