Due Date Calculator
Estimates your baby's arrival date using your last menstrual period and cycle length. Adjusts for non-standard cycles and ultrasound findings. Ideal for first-trimester planning.
About this calculator
The standard method for estimating a due date is Naegele's Rule, which adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). However, this assumes a 28-day cycle. For longer or shorter cycles, the formula adjusts: Due Date = LMP + 280 + (cycleLength − 28) + adjustmentDays. The extra term shifts the window earlier or later based on when ovulation actually occurs. An ultrasound adjustment (adjustmentDays) can further refine the estimate if early imaging suggests a different gestational age. The result is an estimated due date (EDD), not a guarantee — only about 5% of babies are born on their exact EDD, but ~80% arrive within two weeks of it.
How to use
Suppose your LMP was January 1, 2024, your average cycle is 32 days, and you have no ultrasound adjustment. The formula adds 280 + (32 − 28) = 284 days to January 1, 2024. Counting 284 days forward lands on October 11, 2024 — your estimated due date. If an early ultrasound suggests you are 3 days further along, add adjustmentDays = 3, giving 287 days total, shifting the EDD to October 14, 2024. Enter your own LMP, cycle length, and any ultrasound offset to get your personalized result.
Frequently asked questions
How accurate is the due date calculator based on last menstrual period?
LMP-based due dates are accurate to within about ±2 weeks for most women with regular cycles. The estimate assumes ovulation on day 14, which is not universal. Women with longer or shorter cycles will get a more precise result when the cycle length field is adjusted. First-trimester ultrasound remains the gold standard for dating and can narrow uncertainty to ±5–7 days.
What does the ultrasound adjustment days field do in the due date calculator?
When an early ultrasound measures crown-rump length and assigns a gestational age that differs from your LMP-based date, your provider may revise your EDD by a few days. Entering that difference as a positive or negative number in the Ultrasound Adjustment field applies the same shift to the formula. For example, if your sonographer says you are 4 days further along than the LMP suggests, enter +4. This keeps your calculated date aligned with clinical findings.
Why do different due date calculation methods give slightly different results?
Naegele's Rule (LMP-based) and conception-date methods start from different reference points. LMP dating counts from the last period, while conception dating counts ~266 days from fertilization. Because ovulation timing varies, the two approaches can differ by several days. Ultrasound biometry adds a third, often more accurate, reference. All three methods are estimates; your healthcare provider will use a combination to assign an official EDD.