Get quick, accurate estimates for your fertile days and due date — simple, mobile-friendly, and easy to use. For information only · Not medical advice.
Ovulation date is estimated as cycle length − 14 days after your last period. Due date uses Naegele’s rule: LMP + 280 days (or Conception + 266 days).
Ovulation is when an ovary releases a mature egg. It usually happens once per menstrual cycle, about 14 days before your next period (not always day 14 of the cycle). The egg can be fertilized for roughly 12–24 hours. Because sperm can live in the reproductive tract for up to five days, the most fertile time spans the few days before ovulation and the day of ovulation.
The most common method is Naegele’s rule: add 280 days (40 weeks) to the first day of your last menstrual period. This assumes a regular 28-day cycle with ovulation around day 14.
If you know the likely conception date (e.g., IVF or IUI), add 266 days (38 weeks). This roughly aligns with LMP + 280 days because conception typically occurs ~14 days after LMP in a 28-day cycle.
An early ultrasound (often 7–12 weeks) measures the embryo’s crown–rump length to estimate gestational age. If ultrasound differs from your LMP estimate, clinicians often rely on the ultrasound because it can be more precise early in pregnancy.
See a healthcare professional if:
This site is for education only and is not a substitute for professional medical advice, diagnosis, or treatment.
We estimate your fertile window using your cycle length and the date of your last period. Ovulation typically happens about 14 days before your next period. By counting backward from your average cycle length, we estimate ovulation and your most fertile days.
Irregular cycles reduce calculator accuracy. To improve targeting:
It’s possible but less likely.
They’re based on averages and can shift due to:
For more precision, pair with OPKs and BBT tracking.
No. An ovulation calculator is not birth control.
Yes—stress can delay ovulation or lead to a skipped cycle, shifting fertile days.
Most tests are most accurate from the first day of your missed period. Testing earlier can lead to false negatives.