The Due Date Calculator estimates when a pregnancy is expected to reach full term, using the first day of the last menstrual period, a known conception date, an ultrasound dating point, or an IVF transfer date. Enter whichever starting point is known with the most confidence, and the tool returns an estimated due date along with current gestational age when today's date is used as the reference.
A due date is a planning estimate, not a guarantee of when birth will occur. Most births do not happen exactly on the estimated date. This page shows the standard obstetric dating methods and the arithmetic behind each one.
Estimate your due date from your last menstrual period
Naegele's rule is the classic method for estimating a due date from the last menstrual period (LMP): add 280 days, or 40 weeks, to the first day of that period.
Due date ≈ LMP + 280 daysWorked example: an LMP of 1 January.
Adding 280 days to 1 January lands on approximately 8 October of the same year, the estimated due date under this method.
This method assumes a roughly 28-day cycle with ovulation near day 14. Shorter or longer cycles shift the true ovulation timing, which is one reason an early ultrasound often revises the initial LMP-based estimate.
Estimate your due date from a known conception date
When a conception date is known with reasonable confidence, for instance from ovulation tracking or a single recorded encounter, gestational length from conception is about 266 days, or 38 weeks.
Due date ≈ conception date + 266 daysThis is 14 days shorter than the LMP-based estimate, because conception itself typically occurs about 14 days after LMP in an average cycle, so both methods describe the same underlying 40-week pregnancy length measured from two different starting points.
Estimate your due date from an ultrasound
A first-trimester ultrasound reports gestational age directly from measurements such as crown-rump length, which is often more reliable than LMP dating when cycles are irregular or the LMP date is uncertain. The due date then projects forward from that scan-reported gestational age to a total of 280 days.
Worked example: an ultrasound at 8 weeks 3 days gestation, performed on 1 March.
Remaining time to full term: 280 − (8×7 + 3) = 280 − 59 = 221 days from the scan date, giving an estimated due date roughly 221 days after 1 March.
Understand why due dates are estimates, not guarantees
Only a small percentage of births occur exactly on the calculated due date. Full-term birth is generally considered to span a range of weeks around the estimate, not a single fixed day, since the actual length of any individual pregnancy varies for reasons that a calendar formula cannot capture.
A due date is best treated as the center of a range rather than a precise deadline.
Compare the different dating methods
| Method | Reference point | Days to due date |
|---|---|---|
| LMP (Naegele's rule) | First day of last period | 280 days |
| Conception date | Known or estimated conception | 266 days |
| Ultrasound | Scan-reported gestational age | 280 minus days already elapsed |
Each method should converge on roughly the same due date when the underlying dates are accurate; a meaningful disagreement between methods is one reason a clinician might prefer ultrasound dating over LMP dating for a specific pregnancy.
This is general information, not medical advice
This calculator provides an estimate for general planning and information. It does not replace prenatal care, clinical ultrasound dating, or guidance from a midwife or obstetric provider. Pregnancies vary, and any concerns about dating, timing or pregnancy health should be directed to a qualified healthcare provider rather than resolved by a calendar calculation alone.
Work through a second LMP example
An LMP of 15 June follows the same 280-day addition. Adding 280 days to 15 June lands on approximately 21 March of the following year, illustrating how the estimate can cross into the next calendar year depending on when the last period started.
Frequently asked questions
How is a due date calculated from the last menstrual period?
Add 280 days (40 weeks) to the first day of the last menstrual period, following Naegele's rule, the standard obstetric convention.
How accurate is a due date estimate?
Due dates are estimates, and only a small percentage of births occur exactly on the calculated date. Full-term birth spans a range of weeks around the estimate rather than a single fixed day.
Which dating method is most accurate?
A first-trimester ultrasound is often considered more reliable than LMP dating, particularly for people with irregular cycles or an uncertain LMP date, since it measures the fetus directly rather than assuming a standard cycle length.
What if my cycle is not exactly 28 days?
LMP-based dating assumes an average 28-day cycle with ovulation around day 14. A consistently shorter or longer cycle shifts the true ovulation date, which can make LMP dating less accurate; an ultrasound or known conception date may give a better estimate in that case.
How is a due date calculated from a conception date?
Add 266 days (38 weeks) to a known conception date, which accounts for conception typically occurring about 14 days after the last menstrual period in an average cycle.
Is this calculator a substitute for prenatal care?
No. This tool provides a general estimate for information purposes. Prenatal care, clinical ultrasound dating and guidance from a qualified healthcare provider remain the appropriate source for pregnancy-specific decisions.
Can the due date change during pregnancy?
Yes. If a later ultrasound suggests a different gestational age than initial LMP-based dating, a clinician may revise the estimated due date, particularly when the ultrasound was performed early in the pregnancy.
Summary
A due date estimate typically adds 280 days to the last menstrual period, 266 days to a known conception date, or projects forward from an ultrasound-reported gestational age, with all three methods designed to converge on the same approximate 40-week full-term point.
Because actual pregnancy length varies, a due date estimate is best read as the center of a range of likely birth dates rather than a fixed deadline, and any specific concerns belong with a prenatal care provider.