Pregnancy Due Date Calculator
Calculate your estimated due date (EDD) from the first day of your last menstrual period (LMP).
How to use this tool
- Enter first day of last period in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your estimated due date and the full breakdown beneath it.
Add 280 days (40 weeks) to the first day of your last menstrual period to get your estimated due date. This is the Naegele's rule calculation.
⚠ This tool is for general information and education only and is not medical advice. Consult a qualified healthcare professional before making health decisions.
Formula
EDD = LMP + 280 days (40 weeks)
Where LMP is the first day of the last menstrual period.
How it works
The estimated due date is calculated by adding 280 days (40 weeks) to the first day of the last menstrual period, a method known as Naegele's rule. This assumes a regular 28-day cycle with ovulation on day 14, making the actual gestational age from conception approximately 38 weeks. For cycles that differ significantly from 28 days, or when an early ultrasound is available, clinicians may adjust the EDD accordingly.
Worked example
- LMP = 1 January 2026.
- Add 280 days: 1 January 2026 + 280 days.
- Count forward: 31 days remaining in January + 28 Feb + 31 Mar + 30 Apr + 31 May + 30 Jun + 31 Jul + 31 Aug + 30 Sep = 273 days by 30 Sep; 8 more days reaches 8 October 2026.
Estimated due date: 2026-10-08
Common mistakes to avoid
- Entering the date of conception rather than the first day of the last menstrual period — Naegele's rule adds 280 days from LMP, not from conception.
- Treating the EDD as a definite delivery date — only about 5% of babies are born exactly on their due date; a range of two weeks either side is normal.
- Using the wrong month length when counting 40 weeks manually — always calculate in days (280) rather than months to avoid calendar errors.
Key terms
- LMP (Last Menstrual Period)
- The first day of the most recent menstrual period, used as the standard starting point for calculating gestational age and due dates.
- EDD (Estimated Due Date)
- The date on which delivery is estimated to occur, calculated as 40 weeks after the LMP.
- Naegele's rule
- The standard obstetric formula for estimating the due date: add one year, subtract three months, and add seven days to the LMP (equivalent to LMP + 280 days).
- Gestational age
- The age of the pregnancy measured in weeks from the first day of the LMP, not from conception.
- Trimester
- One of three roughly 13-week periods of pregnancy: first (weeks 1–13), second (weeks 14–26), and third (weeks 27–40).
Frequently asked questions
- How accurate is this due date calculation?
- Only about 5% of babies are born on their exact due date. Most arrive within two weeks either side. An ultrasound gives a more precise estimate.
- What if I don't know my LMP?
- An early ultrasound (ideally at 8–12 weeks) can establish gestational age and give a reliable due date even without a known LMP.