How to Use the Due Date Calculator
Knowing your estimated due date helps you plan prenatal care, prepare for your baby arrival, and track developmental milestones throughout pregnancy.
Step-by-Step Instructions
- 1 Select your calculation method (LMP or conception date).
- 2 Enter the relevant date.
- 3 Review your estimated due date and current pregnancy week.
Understanding the Inputs
First day of last menstrual period
The first day of your last menstrual period, which is the standard starting point for pregnancy dating. For example, if your last period started on March 15, 2026, you would select March 15, 2026. This date is used to calculate gestational age.
How to find it: Check your calendar, period tracking app, or the date you recorded on your menstrual cycle log. If you are unsure of the exact date, recall the approximate date as accurately as possible, as even a few days of error can shift the due date.
Why it matters: The LMP date is the foundation of Naegeles rule. An error of just a few days in the LMP date shifts the due date by the same number of days. Accurate recall is essential for a reliable estimate, especially since early ultrasound dating later depends on having an accurate LMP for comparison.
Type: date · Default:
Cycle Length (days)
The typical length of your menstrual cycle in days, counting from the first day of one period to the first day of the next. The standard 28-day cycle is the default, but cycles can range from 21 to 35 days and still be considered normal.
How to find it: Review your period tracking history over the last 3-6 months to find your average cycle length. If you have irregular cycles or are unsure, using the 28-day default is reasonable, but a personalized average is more accurate.
Why it matters: Cycle length adjusts the due date because ovulation occurs later in longer cycles. For a 35-day cycle, ovulation happens around day 21 instead of day 14, so the due date shifts approximately 7 days later. Using your actual cycle length personalizes the estimate.
Type: number · Default: 28
Tips & Best Practices
- ✓ Only ~5% of babies arrive on their exact due date. Consider it a target, not a guarantee.
- ✓ Your first ultrasound may adjust the due date based on fetal measurements.
- ✓ Full-term is considered 39-41 weeks. Babies born in this range have the best outcomes.
by CalculatorPro Tools · Updated 2026-07-29