Ovulation Calculator
Estimate your ovulation day and fertile window from your cycle.
How to use the Ovulation Calculator
- Enter the first day of your last period.
- Enter your average cycle length in days.
- See your estimated ovulation day and fertile window.
About the Ovulation Calculator
This calculator estimates your ovulation day from two inputs: the first day of your last period and your average cycle length. It works back from the next expected period using a typical 14-day luteal phase, placing ovulation at your cycle length minus 14 days. Around that day it marks a fertile window from four days before estimated ovulation to the day after, shows when your next period is expected (last period plus your cycle length), and gives a possible due date if conception occurs, using the standard 280-day estimate.
Please read these results as a rough guide, not a certainty. They assume a regular cycle and an average luteal phase, but in real life ovulation shifts from cycle to cycle and from person to person, so your true fertile days may fall earlier or later than shown. This tool does not diagnose anything, is not a form of contraception, and is not a substitute for medical advice.
If you are trying to conceive, trying to avoid pregnancy, or your cycles are irregular, please speak with a doctor or use a clinically validated method such as ovulation test kits. To explore related dates, see the pregnancy due date calculator or count days between dates with the date difference calculator.
Frequently asked questions
How is the ovulation day worked out?
It assumes a luteal phase of about 14 days between ovulation and the next period, so it places ovulation at your cycle length minus 14 days after your last period began. For a 28-day cycle that lands around day 14, but it is only an estimate.
Can I rely on this to avoid pregnancy?
No. This is not a contraceptive method. Ovulation timing varies from cycle to cycle, so the fertile window shown can be wrong. If you are trying to avoid pregnancy, use a proven contraceptive method and speak to a healthcare professional.
What if my cycles are irregular?
The estimate assumes a fairly regular cycle, so irregular cycles make it much less reliable. If your cycle length changes a lot from month to month, ovulation test kits or advice from a doctor will give you a far more accurate picture.
How accurate is the fertile window?
It is an informed estimate, not a measurement. Real ovulation can happen earlier or later than predicted, so the actual fertile days may differ from the window shown. Please treat it as a guide rather than a guarantee.
Is the due date reliable?
It is a preliminary estimate that adds 280 days to your last period, the same starting point clinics use. If you do conceive, an early ultrasound gives a more accurate date, and you can also try the pregnancy due date calculator.