Ovulation Calculator
Predict your ovulation day and fertile window from the first day of your last period and your cycle length — free online. — free, fast and private.
How to use Ovulation Calculator
- Choose the first day of your last period.
- Enter your usual cycle length in days (28 is average).
- Click Calculate Ovulation to see your fertile window.
Note: this tool runs in your browser. Nothing is uploaded.
Disclaimer: this is an estimator only — please see a qualified professional for advice about your specific situation. This is an estimate only and cannot confirm ovulation or guarantee pregnancy or prevent it. Cycles vary and ovulation can shift. For conception help or contraception, see a doctor or fertility specialist.
Frequently Asked Questions
How is the fertile window calculated?
Ovulation usually happens about 14 days before the next period. The fertile window is the 5 days before ovulation plus ovulation day itself, because sperm can survive up to 5 days. Cycle length and ovulation timing vary, so this is only an estimate.
Is this tool free to use?
Yes, it is 100% free with no sign-up. Your data stays in your browser.
Is my data uploaded or stored?
No. This tool runs entirely in your browser — nothing you enter is uploaded to a server.
Can I use it on my phone?
Yes, it is fully mobile-friendly and works in any modern browser on phone, tablet or computer.