Due Date Calculator
Result
Estimated due date
- Days until the due date
- 5 days
- Early term starts
- September 17, 2026
- Full term starts
- October 1, 2026
- Late term starts
- October 15, 2026
- Postterm starts
- October 22, 2026
A due date calculator turns the first day of your last menstrual period into an estimated due date, and then into the dates that follow it. It adds 280 days to that first day, the convention behind an average 40-week pregnancy, and shifts the whole line by your cycle length so a 35-day cycle is not treated as a 28-day one. Because a due date is a midpoint rather than an appointment, this page also reports the days when early term, full term, late term and postterm begin, so you see a window instead of a single day.
The four stages around your due date
| Stage | Weeks of pregnancy | What it means |
|---|---|---|
| Preterm | Before 37 weeks 0 days | Everything earlier than early term. ACOG's glossary defines preterm as less than 37 weeks of pregnancy. It has no start date of its own, which is why the calculator reports the other four stages as dates and leaves this one as a boundary. |
| Early term | 37 weeks 0 days to 38 weeks 6 days | Three weeks to one week before the due date. ACOG split this window out of the old single definition of term because outcomes still improve with each week gained between 37 and 39 weeks. |
| Full term | 39 weeks 0 days to 40 weeks 6 days | One week before the due date to one week after it. ACOG reports that the frequency of adverse outcomes is lowest among uncomplicated pregnancies delivered in this window. |
| Late term | 41 weeks 0 days to 41 weeks 6 days | One to two weeks after the due date. It is the last stage before postterm, and ACOG counts it from the due date rather than from the last period. |
| Postterm | 42 weeks 0 days and beyond | Two weeks after the due date and later. Reaching this stage is uncommon, and ACOG's four names exist so that a delivery at 41 weeks is not described with the same word as one at 38. |
The weeks are counted from the first day of your last period, which is the same count the calculator uses. ACOG names these four stages rather than calling everything from 37 weeks onwards term, because outcomes differ across the range.
Formula
Due date = first day of your last period + 280 days + (cycle length - 28 days)
- LMP
- The first day of your last menstrual period. Every dating convention on this page counts from it, which is why the two weeks before conception are already part of the count.
- Cycle length
- Days from the first day of one period to the first day of the next. It moves the whole line: a longer cycle means later ovulation, and a later due date with it. (days)
- 280 days
- The convention ACOG describes: the estimated due date is 280 days after the first day of the last menstrual period, assuming a 28-day cycle. (days)
- 28 days
- The cycle length that convention assumes. Subtracting it is what turns the formula into a shift for other cycles rather than a second rule. (days)
- Due date
- The estimated date of delivery, and the anchor for the four stage dates below it.
- Stage dates
- Early term, full term, late term and postterm are counted from the due date: three weeks before it to two weeks after it.
Use it when you know the first day of your last period and how long your cycles usually run. If you know the due date already and want to work backwards to conception, the conception calculator does that instead; if you want the same input counted in weeks rather than turned into a date, use the pregnancy calculator.
Worked examples
A 28-day cycle beginning 1 January 2026
- Add 280 days to 2026-01-01: the estimated due date is 2026-10-08.
- The cycle length is the assumed 28 days, so nothing shifts and the sum stays 280.
- Early term starts 3 weeks before the due date: 2026-09-17.
- Full term starts 1 week before (2026-10-01), late term 1 week after (2026-10-15), and postterm 2 weeks after (2026-10-22).
The count of days is measured from 15 January 2026, which is 266 days before this due date. Change the cycle length and the four stage dates move with the due date, never on their own.
A 30-day cycle beginning 20 November 2025
- The cycle is 30 days, which is 2 days longer than the assumed 28.
- Add the difference to the convention: 280 + 2 = 282 days after 2025-11-20.
- 2025-11-20 + 282 days = 2026-08-29.
- The stage dates follow the new due date: 2026-08-08, 2026-08-22, 2026-09-05 and 2026-09-12.
A longer cycle pushes the date later, not earlier: ovulation comes later in the cycle, so the same last period corresponds to a younger pregnancy.
A short 15-day cycle beginning 1 January 2026
- The cycle is 13 days shorter than the assumed 28.
- 280 - 13 = 267 days after 2026-01-01, which is 2026-09-25.
- The same 13-day shift moves every stage date earlier.
Short cycles are the reason the cycle length field runs down to 15 days. Without it the page would silently report a date a fortnight later than the arithmetic in this formula suggests.
Limitations
Your due date is a midpoint, not an appointment. ACOG describes the 280-day rule as one that assumes a regular 28-day cycle and does not account for inaccurate recall of the last period, irregular cycles, or variability in the timing of ovulation. The shift applied here for other cycle lengths follows from that convention rather than replacing it, and it cannot stand in for an early ultrasound, which stays the most accurate way to date a pregnancy and can revise the date this page gives you. A negative count of days simply means the estimated date has already passed.
Frequently asked questions
- How accurate is a due date calculated from my last period?
- It is a midpoint, not a prediction of the day you will give birth. ACOG describes the 280-day rule as one that assumes a regular 28-day cycle, and notes that only about half of women recall their last menstrual period accurately. An ultrasound before 14 weeks is the most accurate way to date a pregnancy, and it can replace the date this page gives you.
- Why does the calculator ask for my cycle length?
- Because the 280-day convention assumes a 28-day cycle, in which ovulation falls 14 days before the next period. If your cycle runs 35 days, ovulation comes about 7 days later, so the due date moves about 7 days later too. The shift is arithmetic that follows from that convention rather than a rule of its own, and it matters most when your cycles are consistently longer or shorter than 28 days.
- What do early term, full term, late term and postterm mean?
- They are the four names ACOG gives to the weeks around your due date. Early term is 37 weeks 0 days through 38 weeks 6 days, full term is 39 weeks 0 days through 40 weeks 6 days, late term is 41 weeks 0 days through 41 weeks 6 days, and postterm is 42 weeks 0 days and beyond. ACOG frames the whole span as the three weeks before your due date through the two weeks after it.
- My due date has already passed and the count of days is negative.
- That is expected. The count is simply the distance to the estimated date, and it keeps counting once the date is behind you. A due date is a midpoint: ACOG's own definition of term stretches from three weeks before it to two weeks after it, so the window around the date is wider than the date itself.
- Should I use my last period or an ultrasound scan?
- An early ultrasound is the more accurate of the two. ACOG advises changing the estimated date when a scan before 14 weeks differs from the last period by more than 7 days, and describes a pregnancy without an ultrasound that confirms the date before 22 weeks as suboptimally dated. Use this page to see the shape of the timeline, and let your clinician set the date that goes in your notes.
- Does a longer cycle always mean a later due date?
- It means a later ovulation, and with it a later due date under this convention. The number of days between ovulation and the next period is the most consistent part of a cycle, which is why ovulation is placed 14 days before the next period rather than on day 14 of the cycle. It is also why a 60-day cycle moves the date 32 days later rather than staying near 40 weeks.
References
- Methods for Estimating the Due Date (Committee Opinion 700) — American College of Obstetricians and Gynecologists, United States, with the American Institute of Ultrasound in Medicine and the Society for Maternal-Fetal Medicine (May 2017). Source of the 280-day convention, its 28-day assumption, and the accuracy of ultrasound dating by trimester.
- Definition of Term Pregnancy (Committee Opinion 579) — American College of Obstetricians and Gynecologists, United States, with the Society for Maternal-Fetal Medicine (November 2013). Source of the four stage names, the weeks they cover, and the finding that outcomes are best between 39 0/7 and 40 6/7 weeks.
- Having a Baby (patient FAQ) — American College of Obstetricians and Gynecologists, United States. The plain-language statement that pregnancy is measured in weeks from the first day of the last period and lasts about 40 weeks.