Due Date Calculator
Estimates how many days remain until your due date and your current pregnancy week from how many days ago your last period (or conception/ultrasound reference) was, plus cycle length and any ultrasound adjustment.
Last updated: May 2026
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 last period was 42 days ago, your average cycle is 32 days, and the LMP method is selected. Gestation = 280 + (32 − 28) = 284 days, so 284 − 42 = 242 days remain, and you are in week floor(42 / 7) = 6. Result: "242 days until your estimated due date (currently week 6)". Choosing the Conception method bases it on 266 days; the Ultrasound method adds your ultrasound adjustment instead of the cycle correction.
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.