Due Date Calculator
Estimate your baby's due date based on your last menstrual period.
Due Date Calculator
Calculate your estimated due date instantly.
What This Due Date Calculator Does
This tool applies Naegele's rule, the standard obstetric formula for estimating due date, adding 280 days, or 40 weeks, to the first day of the last menstrual period. If conception date is used instead as the starting reference point, the calculator adjusts by adding approximately 266 days to reach the same estimated delivery window.
How to Use the Due Date Calculator
Select whether you know your last menstrual period date or your estimated conception date, and enter that specific date. Your estimated due date appears instantly once calculated, based on the standard 280-day pregnancy length used across most clinical due date estimation.
What Naegele's Rule Is and Why It Remains the Clinical Standard
Naegele's rule, developed by a German obstetrician in the early 19th century, remains the standard method used across most modern clinical settings for estimating due date, despite its long history, largely because it provides a simple, consistent, reasonably reliable starting estimate that can later be refined with ultrasound measurements as pregnancy progresses.
Pro Tip: Research consistently shows only a small percentage of babies actually arrive on their exact calculated due date. Most full-term deliveries occur within roughly two weeks before or after the estimated date.
Why Only About 5 Percent of Babies Arrive on Their Exact Due Date
A calculated due date represents a statistical estimate, not a guaranteed delivery date, and research consistently shows only a small percentage of babies actually arrive on that exact calculated date. Most full-term deliveries occur within a window of roughly two weeks before or after the estimated due date, which is why due dates are best understood as a helpful planning reference rather than a precise prediction.
How Ultrasound Dating Can Refine This Initial Estimate
An early pregnancy ultrasound, particularly one performed in the first trimester, can provide a more precise due date estimate than the last menstrual period method alone, particularly for women with irregular menstrual cycles where the standard calculation method is inherently less reliable. Most healthcare providers combine both methods for the most accurate overall estimate.
Frequently Asked Questions
The standard method, called Naegele's rule, adds 280 days to the first day of the last menstrual period, representing the standard 40-week pregnancy length used in most clinical settings.
No, only a small percentage of babies arrive on their exact calculated due date, with most full-term deliveries occurring within roughly two weeks before or after that estimated date.
An early first-trimester ultrasound often provides a more precise estimate, particularly for women with irregular cycles, which is why many providers combine both methods for the most accurate overall due date.
Yes, this calculator supports either reference point, adjusting the calculation accordingly since conception typically occurs about two weeks after the last menstrual period begins.