Due Date Calculator

Calculate your pregnancy due date from last menstrual period (LMP) or conception date. This tool processes all data locally in your browser. No information is ever sent to any server. Completely free, no registration required.

How to Use the Due Date Calculator

1. Enter your input values above
2. Results update automatically
3. Copy or download the output

What is a Due Date Calculator?

A Due Date Calculator estimates when your baby will arrive based on your last menstrual period (LMP) or conception date. While only about 4-5% of babies are born exactly on their estimated due date, the calculation provides a crucial reference point for tracking pregnancy milestones, scheduling prenatal appointments, and planning for maternity leave and delivery. The standard pregnancy is 40 weeks (280 days) from the first day of the last menstrual period — not from the conception date, which typically occurs about 2 weeks later.

How Does It Work?

Enter the first day of your last period or your estimated conception date. Using Naegele's Rule (the standard obstetrical formula), the calculator adds 280 days (40 weeks) to the first day of your LMP. It also shows: end of first trimester (13 weeks), end of second trimester (26 weeks), key prenatal test windows, and your pregnancy week-by-week. If using conception date, 266 days (38 weeks) are added instead.

Formula

Naegele's Rule (from LMP): Due Date = LMP + 280 days\nFrom Conception: Due Date = Conception Date + 266 days\n\nFirst Trimester = LMP to LMP + 13 weeks\nSecond Trimester = LMP + 14 weeks to LMP + 26 weeks\nThird Trimester = LMP + 27 weeks to Due Date\n\nFull Term Window: 37 weeks to 42 weeks\n\nAlternative: Due Date = LMP date − 3 months + 7 days + 1 year

Who Uses This Tool?

• Expectant mothers calculating their baby's expected arrival
• OB/GYN offices providing initial due date estimates
• Pregnancy planning: understanding conception timing
• HR departments calculating maternity leave dates
• Family members planning showers, support, and visits

Pro Tips

• Only 4-5% of babies arrive on their exact due date. First-time mothers average 41 weeks + 1 day. Consider your due date a 'due month' — any time from 37-42 weeks is normal
• Ultrasound dating in the first trimester is more accurate than LMP dating. If your dating ultrasound differs from your LMP date by more than 5-7 days, doctors use the ultrasound date
• Pregnancy is counted from LMP, so you're already considered 2 weeks 'pregnant' at conception. At your missed period, you're already 4 weeks along
• The 'fourth trimester' concept: the first 3 months after birth are a critical adjustment period. Plan your support system (partner leave, family help, meal prep) for this time

Frequently Asked Questions about Due Date Calculator

How accurate is the due date calculator?

Based on LMP, it gives a reasonable estimate, but individual variation is significant. First-trimester ultrasound provides more accurate dating (±5-7 days). Only 4-5% deliver on the exact due date; 80% deliver within 2 weeks (before or after).

Why is pregnancy counted from the last period, not conception?

The LMP is an easily observable date; exact conception date is rarely known. Counting from LMP standardizes pregnancy dating across all patients. This means the first two weeks of 'pregnancy' actually occur before conception.

What's considered a full-term pregnancy?

Early term: 37 weeks 0 days to 38 weeks 6 days. Full term: 39 weeks 0 days to 40 weeks 6 days. Late term: 41 weeks 0 days to 41 weeks 6 days. Postterm: 42 weeks and beyond. Babies born before 37 weeks are preterm.

Free online Due Date Calculator — no signup, 100% client-side processing. All data stays in your browser.