Pregnancy Due Date Calculator
Enter the first day of your last menstrual period (LMP) and your average cycle length to estimate your due date.
Reviewed by the ToolNestr Editorial Team — July 2026
Select your LMP date and cycle length to see your due date.
This is an estimate only. Confirm with your healthcare provider via ultrasound.
About due date estimation
Knowing your estimated due date helps you plan prenatal care, track foetal development milestones and prepare for birth. The most common method is Naegele rule, named after the German obstetrician Franz Naegele, who developed it in the 19th century. It assumes a 28-day menstrual cycle with ovulation on day 14. While simple, it remains the standard for due date calculation worldwide. Early ultrasound is more accurate for dating, especially for irregular cycles. This calculator can be used alongside the Ovulation Calculator and Pregnancy Weight Gain Calculator for a complete pregnancy tracking toolkit.
For informational purposes only. Always consult your midwife or obstetrician for clinical pregnancy care.
How it works
Naegele rule is straightforward: add 280 days (40 weeks) to the first day of your last menstrual period. However, because the formula assumes a 28-day cycle, the calculator adjusts for your actual cycle length. If your cycle is 32 days, ovulation typically occurs on day 18 (instead of day 14), so the due date is moved 4 days later. Conversely, a 24-day cycle moves ovulation to day 10 and shifts the due date 4 days earlier. The calculator also shows your current gestational week and trimester based on today's date.
The formula explained
Naegele rule: Due date = LMP + 280 days + (cycle length − 28) days
Pregnancy is divided into three trimesters of roughly 13–14 weeks each:
Trimester 1: Week 1–12 (conception, implantation, organ formation)
Trimester 2: Week 13–26 (rapid growth, movements felt, anatomy scan)
Trimester 3: Week 27–40 (final growth, lung maturation, positioning for birth)
Worked example
A woman with a 30-day cycle had her LMP on 1 March 2026. What is her due date?
Expectant mother
Know your estimated due date to plan prenatal appointments, maternity leave and prepare for the arrival of your baby.
Birth partner
Track the pregnancy timeline so you can support your partner through each trimester and be prepared for key appointments.
Midwife
Quickly calculate due dates for patients and discuss how cycle length affects the estimate before ultrasound confirmation.
Pregnancy tracker
Log the due date and monitor your current week. Use the timeline to know when to schedule each prenatal screening.
How to use this calculator
Enter your LMP date
Select the first day of your last menstrual period from the date picker. This is the starting point for the calculation.
Set your cycle length
Choose your average cycle length from the dropdown. The standard is 28 days, but many women have shorter or longer cycles.
Read your results
See your estimated due date, current week of pregnancy, days remaining and current trimester. Confirm with your provider via ultrasound.
Tips for due date tracking
Use ultrasound for confirmation
Naegele rule is a starting estimate. Early ultrasound (8–14 weeks) measures the baby's crown-rump length and provides the most accurate dating, especially for irregular cycles.
Track your cycle before pregnancy
Knowing your typical cycle length before conception makes the due date estimate more accurate. Track for at least a few months if you are planning pregnancy.
Be flexible with the date
Fewer than 5% of babies arrive on their exact due date. Full-term is 37–42 weeks. Use the due date as a planning guide, not a fixed deadline.
Related tools
Frequently asked questions
How is the due date calculated?
Naegele rule adds 280 days (40 weeks) from the first day of your LMP, adjusted for cycle length. This is the standard obstetric method used by healthcare providers worldwide.
How accurate is the due date?
Only about 5% of babies are born on their exact due date. A normal full-term pregnancy can range from 37 to 42 weeks. The due date is best thought of as a target midpoint, not a fixed prediction.
Why does cycle length matter?
The standard Naegele rule assumes a 28-day cycle. If your cycle is longer or shorter, ovulation and conception happen on a different day, so the due date shifts accordingly.
What if I have irregular cycles?
For irregular cycles, early ultrasound measurements (crown-rump length between 8–14 weeks) give the most accurate dating. Use a 28-day average here and consult your provider for ultrasound confirmation.
When is the baby considered full term?
Early term: 37–38 weeks. Full term: 39–40 weeks. Late term: 41–42 weeks. Post-term: beyond 42 weeks. Most providers recommend induction by 41–42 weeks.
Sources & references
This tool uses standard formulas and reference values from:
- • NIH / National Institutes of Health and CDC reference values and formulas. nih.gov
- • Peer-reviewed equations (e.g. Mifflin-St Jeor, Harris-Benedict) as originally published.
- • ACOG for pregnancy, ovulation and due-date methodology. acog.org
Not medical advice. Estimates only — consult a qualified healthcare professional for any personal health decision.