How is my due date calculated?

Due dates are typically calculated by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14. If your cycle is different, the calculation adjusts accordingly.

How accurate are due date calculations?

Due date calculations are estimates. Only about 5% of babies are born on their exact due date. Most births occur within 2 weeks before or after the estimated date. Ultrasound dating in the first trimester is generally the most accurate method.

What if my cycle is longer or shorter than 28 days?

If your cycle is different from 28 days, your ovulation likely occurs at a different time. Our calculator adjusts for cycle length by modifying the estimated ovulation date, which affects the due date calculation.

When is the best time to have an ultrasound for dating?

First-trimester ultrasounds (before 13 weeks) are most accurate for dating pregnancy, typically within 5-7 days. Second-trimester ultrasounds are accurate within 1-2 weeks, while third-trimester dating can vary by 2-3 weeks.

Due Date Calculator

Calculate your estimated due date from your last period, ultrasound, conception date, or IVF transfer. Track pregnancy milestones and trimester progress.

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Your due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period. This assumes a 28-day cycle.

Enter your information below for a personalized due date calculation.

Calculate Your Due Date

First Day of Last Period

Cycle Length (days)

Average cycle is 28 days. Range: 22-44 days.

Why Choose Forge Due Date Calculator?

Unlike BabyCenter, WebMD and Pampers, Forge Due Date Calculator offers a genuinely free, private, and unlimited experience with no strings attached.

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Last updated: January 2026 • Built with care by iForge Apps

How to use this tool

1

Select your calculation method (Last Period, Ultrasound, Conception, or IVF)

2

Enter the required date and any additional information

3

Click 'Calculate Due Date' to see results

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