EDD Calculator (Estimated Due Date)
Calculate your baby’s due date and key pregnancy milestones with our easy-to-use EDD Calculator.
Select the first day of your last period.
The typical cycle is 28 days. Adjust if your cycle is consistently shorter or longer.
What is an EDD Calculator?
An EDD (Estimated Due Date) Calculator is a digital tool used to predict the date a pregnant person is expected to give birth. It is one of the first and most common calculations made after confirming a pregnancy. While it’s called a “due date,” it’s important to remember this is an estimate; only about 4-5% of babies are born on their exact due date. The primary purpose of an EDD calculator is to establish a timeline for prenatal care, track the baby’s growth, and plan for the delivery.
The calculation is most commonly based on the first day of the woman’s last menstrual period (LMP), assuming a standard 280-day (40-week) gestation. However, modern calculators can also use data from an ultrasound, which is considered the most accurate method, especially when performed in the first trimester.
The EDD Calculator Formula and Explanation
There are two primary methods used by this EDD calculator to estimate your due date:
1. Last Menstrual Period (LMP) Method
This is the most traditional method, often called Naegele’s Rule. It works by adding 280 days (40 weeks) to the first day of your last period. This formula assumes a regular 28-day menstrual cycle where ovulation occurs on day 14. Our calculator adjusts this formula based on your actual cycle length for a more personalized estimate.
Formula: EDD = (First Day of LMP) + 280 days + (Your Cycle Length - 28 days)
2. Ultrasound Method
A first-trimester ultrasound is the most accurate way to date a pregnancy. During the scan, a technician measures the fetus (usually crown-rump length) to determine its gestational age. This EDD calculator uses that data to project the full 40-week gestation period.
Formula: EDD = (Date of Ultrasound) - (Gestational Age at Ultrasound) + 280 days
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| LMP Date | The first day of your last menstrual period. | Date | A valid past date |
| Cycle Length | The average number of days in your menstrual cycle. | Days | 21 – 40 |
| Ultrasound Date | The date the measurement was taken. | Date | A valid past date |
| Gestational Age | The age of the fetus at the time of the ultrasound. | Weeks & Days | 5 – 20 weeks (for dating) |
Practical Examples
Example 1: Calculation via LMP
Let’s say a person’s last menstrual period started on June 1, 2025, and they have a regular 28-day cycle.
- Inputs: LMP Date = June 1, 2025; Cycle Length = 28 days.
- Calculation: June 1, 2025 + 280 days.
- Results: The EDD would be approximately March 8, 2026. The current gestational age would also be calculated based on the time elapsed since June 1, 2025.
Example 2: Calculation via Ultrasound
Imagine a person has an ultrasound on October 15, 2025. The ultrasound determines the fetus is at a gestational age of 12 weeks and 3 days.
- Inputs: Ultrasound Date = October 15, 2025; Gestational Age = 12 weeks, 3 days.
- Calculation: First, calculate total days of gestation (12 * 7 + 3 = 87 days). Then, project the due date: October 15, 2025 – 87 days + 280 days.
- Results: The EDD would be approximately May 7, 2026.
How to Use This EDD Calculator
- Select Your Method: Choose whether you want to calculate your due date using your Last Menstrual Period (LMP) or an Ultrasound.
- Enter LMP Details: If using the LMP method, pick the date corresponding to the *first day* of your last period. Then, adjust the ‘Average Cycle Length’ if it’s different from the standard 28 days.
- Enter Ultrasound Details: If using the ultrasound method, enter the date the scan was performed and the gestational age (in weeks and days) provided by your doctor at that time.
- Calculate: Click the “Calculate Due Date” button.
- Interpret Results: The calculator will display your Estimated Due Date, your current gestational age, your approximate conception date, and a timeline of key milestones including trimester end dates. The progress bar and table provide a visual guide to your pregnancy milestones.
Key Factors That Affect Your Due Date
While an EDD calculator provides a great estimate, several factors can influence the actual delivery date:
- Irregular Cycles: If your menstrual cycles are irregular, the LMP method can be inaccurate. An early ultrasound is recommended for a more precise date.
- LMP Recall: Forgetting the exact date of your last period can shift the calculation by days or even weeks.
- Ovulation Timing: Even with a regular cycle, ovulation doesn’t always happen on day 14. Late or early ovulation will affect the conception date.
- First vs. Subsequent Pregnancies: First-time mothers, on average, tend to deliver slightly later than their estimated due date.
- Carrying Multiples: Pregnancies with twins, triplets, or more often result in earlier, often preterm, delivery.
- Maternal Health: Conditions like high blood pressure or diabetes can necessitate an earlier delivery. A useful tool to monitor this is an ovulation calculator to better understand your cycle.
Frequently Asked Questions (FAQ)
An EDD calculator provides an estimate, not a guarantee. The LMP method is a good starting point but can be off by more than a week. A first-trimester ultrasound provides the most accurate possible due date.
Yes. Your healthcare provider may adjust your due date after an early ultrasound if it differs significantly from your LMP-calculated date. This is done to ensure the most accurate timeline for prenatal care.
If you don’t know your LMP or have very irregular periods, you should use the Ultrasound method in the calculator once you have that information from your doctor. Until then, any calculation would be a rough guess.
A 30-day cycle means you likely ovulate around day 16 instead of day 14. Our EDD calculator automatically adds those extra 2 days to your 280-day gestation period, pushing your due date back by 2 days for a more accurate estimate.
For IVF, it’s more accurate to use a specific IVF calculator. However, you can approximate it here by using the “Ultrasound” method. Use your transfer date as the “Ultrasound Date” and enter the embryo’s age (e.g., 0 weeks, 3 days for a 3-day transfer or 0 weeks, 5 days for a 5-day transfer).
A full-term pregnancy is one that lasts between 39 weeks, 0 days and 40 weeks, 6 days. Deliveries before 37 weeks are considered preterm. Our calculator highlights the date you reach 37 weeks.
The conception date is an estimate of when fertilization likely occurred, which is typically around the time of ovulation (about 14 days after the LMP starts, adjusted for cycle length). This is different from the implantation calculator which estimates when the embryo attaches to the uterus.
If you have had a dating ultrasound in your first trimester (before 14 weeks), that date is considered more reliable than the LMP date. For a full overview, see our guide to pregnancy week by week.
Related Tools and Internal Resources
Continue your journey with more of our specialized health calculators and resources:
- Ovulation Calculator: Pinpoint your most fertile days to understand your cycle better.
- Pregnancy Week by Week: Get detailed information on your baby’s development at each stage.
- Implantation Symptoms Guide: Learn about the early signs of pregnancy.
- Understanding Gestational Age: A deep dive into how pregnancy is dated.
- Pregnancy Milestones: Track the key moments and developments in your pregnancy.
- Baby Development Guide: Explore what to expect in your baby’s growth.