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Pregnancy Due Date Calculator.
Calculate your estimated due date based on your last period
Set your values
Results update as you type.
Last Menstrual Period
Enter the first day of your last period. The estimate assumes a regular 28-day cycle and a 280-day pregnancy.
FAQs
How is a due date calculated?
Most healthcare providers use the first day of your last menstrual period (LMP) and add 280 days (40 weeks) to estimate the due date. This is known as Naegele's rule.
How accurate is the due date?
An estimated due date is just that—an estimate. Only about 4% of babies are born on their actual due date. Most are born between 37 and 42 weeks.
What is my conception date?
Conception typically occurs about two weeks after the first day of your last period, assuming a 28-day cycle.
How to Use the Pregnancy Due Date Calculator
Estimate your due date and track pregnancy milestones from your last period or conception date.
- Enter the first day of your last menstrual period (LMP).
- Adjust your average cycle length if it isn't 28 days.
- See your estimated due date, current week, and trimester.
Due Date Formula
Naegele's Rule
Add 40 weeks to the first day of your last period (adjust for cycle length).
Example:
Input: LMP = Jan 1
Calculation: Jan 1 + 280 days
Result: ≈ Oct 8
Real-World Use Cases
Prenatal Planning
Schedule appointments and prepare for each trimester.
Milestone Tracking
Know your current gestational week.
Tips
Tips
- Only about 4% of babies arrive on the exact due date.
- An early ultrasound gives the most accurate dating.
Related Calculators
Cite this calculator
Canonical URL: https://mathify.one/en/health/due-date
Cite as: Mathify. (2026). Pregnancy Due Date Calculator. https://mathify.one/en/health/due-date
Use Cases
Plan for your baby's arrival
Know your estimated due date to plan maternity leave, prepare the nursery, and schedule prenatal appointments.
Example: Enter your LMP to see your due date and current pregnancy week.
Track pregnancy progress
Monitor how far along you are and what to expect each week, helping you stay informed about your baby's development.
Example: At 20 weeks, you're halfway through your pregnancy.
Frequently Asked Questions
- How is my due date calculated?
- The calculator uses the first day of your last menstrual period (LMP) and adds 280 days (40 weeks) to estimate your due date. This is the standard Naegele's rule, assuming a regular 28-day cycle.
- Can I find out my conception date?
- Yes, the calculator estimates your conception date by subtracting 266 days (38 weeks) from your due date, which is the average time from conception to birth. This is an estimate, as conception can vary by a few days.
- What if my cycle is irregular?
- The calculator assumes a regular 28-day cycle. If your cycle is longer or shorter, your due date may differ. For the most accurate estimate, consult your healthcare provider, who may use ultrasound measurements.
Tips & Common Mistakes
Tips
- Enter the first day of your last menstrual period accurately for the best estimate.
- Remember that only about 5% of babies are born on their exact due date; it's normal to give birth a week or two before or after.
- Use the conception date estimate to understand your pregnancy timeline, but rely on ultrasound for precise dating.
- If your cycle is longer than 28 days, your due date may be later; consider discussing with your doctor.
Common Mistakes to Avoid
- Using the date your period ended instead of the first day of your last period.
- Assuming the due date is exact; it's an estimate and can change based on ultrasound findings.
- Forgetting to account for irregular cycles, which can affect the accuracy of the calculation.
Last updated: August 13, 2026