Free Pregnancy Due Date Calculator
Estimate a pregnancy due date from the first day of the last menstrual period and average cycle length. The calculator uses Naegele's Rule, then adjusts for cycle length so people with shorter or longer cycles can get a more realistic estimate than a fixed 28-day assumption.
How the Due Date Estimate Works
Naegele's Rule estimates the due date by adding 280 days, or 40 weeks, to the first day of the last menstrual period. Because ovulation does not happen on the same cycle day for everyone, this calculator adjusts the estimate when your average cycle is shorter or longer than 28 days.
- 28-day cycle: Last menstrual period plus 280 days.
- Longer cycle: Adds the extra cycle days to the estimate.
- Shorter cycle: Subtracts the difference from the estimate.
How to Use This Calculator
- Enter the first day of your last menstrual period.
- Enter your average cycle length if it differs from 28 days.
- Review the estimated due date, pregnancy week, days remaining, and trimester milestones.
- Use the result as a conversation starter with your healthcare provider, not as a replacement for clinical dating.
Why Due Dates Are Estimates
A due date is not a guaranteed birth date. Many healthy pregnancies deliver before or after the estimate. Dating can also change after an early ultrasound, irregular cycles, uncertain last-period dates, assisted reproduction timing, or medical factors that affect pregnancy monitoring.
When to Ask a Clinician
Contact a qualified healthcare provider for personalized pregnancy dating, prenatal care, and medical advice. Clinical teams may use ultrasound measurements, ovulation timing, embryo transfer dates, or other medical information that this browser calculator cannot evaluate.
Privacy and Medical Disclaimer
Your dates are processed locally in your browser and are not uploaded. This page is for informational planning only and does not diagnose pregnancy status, fetal health, or delivery timing.
Last Reviewed and References
Last reviewed: July 2026. The estimate is based on Naegele's Rule, which counts approximately 280 days from the first day of the last menstrual period and adjusts for cycle length. For patient-facing context about pregnancy dating, see guidance from organizations such as the American College of Obstetricians and Gynecologists.
Medical Disclaimer
This calculator cannot account for ultrasound dating, IVF transfer dates, irregular cycles, uncertain period dates, or clinical factors that affect pregnancy care. Use it for general planning only and follow your healthcare provider's dating and prenatal-care guidance.