Due Date Formula Explained

This reference explains the Due Date formula used by our free Due Date Calculator. Use it to understand variables, units, and common mistakes before you calculate.

The formula

EDD = LMP + 280 days (Naegele's rule)

Variables and units

• LMP — First day of last menstrual period (date)
• 280 — Standard gestation length (Naegele's rule) (days)

Worked example

Inputs: LMP March 15, 2026

1. Add 280 days to LMP: March 15 + 280 days
2. 280 days = 40 weeks exactly
3. Due date = December 20, 2026

Result: Estimated due date: December 20, 2026

When to use this formula

Use this formula when you need a quick due date estimate with values you already know. Our Due Date Calculator applies the same relationship automatically so you can verify handwritten work.

Common mistakes

• Using conception date without adding 14 days
• Forgetting to adjust for cycle length ≠ 28 days
• Treating due date as exact — only ~5% deliver on that day

Frequently asked questions