About ovulation and the fertile window
This ovulation calculator estimates your most fertile days from the first day of your last period and your average cycle length. It works out your likely ovulation day, the fertile window around it, your next period, and an estimated due date if you conceive. It is a planning guide based on a regular cycle, not medical advice or contraception.
How Ovulation works
How to use it
- Enter the first day of your last period.
- Enter your average cycle length (commonly 28 days).
- Read your estimated ovulation day and fertile window.
How it estimates
Ovulation usually happens about 14 days before your next period starts. The fertile window covers roughly the five days before ovulation plus ovulation day, because sperm can survive several days.
Use as a guide
Cycles vary month to month, so treat these dates as estimates. For trying to conceive, avoiding pregnancy or any concerns, speak to a healthcare professional.
Common uses
- Estimate your fertile window
- Find your likely ovulation day
- Plan for conception
- Predict your next period
- Track your cycle
- Estimate a due date if conceiving
- Understand cycle timing
- Plan around your cycle
Frequently asked questions
How do I calculate ovulation?
Enter your last period date and cycle length; ovulation is estimated about 14 days before your next period.
What is the fertile window?
Roughly the five days before ovulation plus ovulation day — the days you are most likely to conceive.
Is this reliable for birth control?
No — it is an estimate and should not be used as contraception.
What cycle length should I use?
Your own average; 28 days is typical but anywhere from 21–35 is common.
Does it predict my due date?
Yes — it estimates a due date about 266 days after ovulation if you conceive.
Is my data uploaded?
No — it calculates entirely in your browser.
Why might it be off?
Cycles vary; stress, health and other factors shift ovulation.
Is it free?
Yes — completely free with no sign-up.