Estimate your due date from your last period or conception date, using the same 280 day rule your provider starts from.
Estimate only, not medical advice.
Both entry points here run through the same underlying arithmetic. Choosing "last menstrual period" adds 280 days to the first day of your last period, the calculation known as Naegele's rule. Choosing "conception date" instead adds 266 days, roughly 280 minus the two weeks between the start of a cycle and ovulation. Either path lands on the same due date if your cycle runs a textbook 28 days.
A due date is the midpoint of a normal range, not a deadline. Term length varies naturally from one pregnancy to the next even when dating is accurate, which is why only a small share of babies arrive on the exact calculated day. Most births happen within roughly two weeks on either side of the estimate, and a birth several days early or late is not on its own a sign that anything is wrong. An early ultrasound can tighten this estimate; your provider may adjust your due date if a first trimester scan disagrees with the last period date by more than a few days.
Naegele's rule assumes ovulation happens around day 14 of a 28 day cycle. If your average cycle runs longer or shorter, ovulation shifts with it, so this calculator adds or subtracts the difference from the standard 280 days. The table below shows how that adjustment scales.
| Average cycle length | Adjustment to the 280 day baseline |
|---|---|
| 24 days | 4 days earlier |
| 26 days | 2 days earlier |
| 28 days | No adjustment |
| 30 days | 2 days later |
| 32 days | 4 days later |
| 35 days | 7 days later |
Cycle length is the single biggest lever in this calculation. If yours has varied a lot recently, or you are not confident in the date, an early ultrasound is the more reliable dating method and will generally take precedence over date math like this.
The "Copy shareable link" button above encodes your inputs into the page address, so you can send the exact same result to a partner, family member, or your own notes without retyping the dates. "Print summary" produces a clean, one page version of the results panel with the header, navigation, and footer removed, useful for a folder of prenatal paperwork or a first appointment.
Estimate only, not medical advice. Generated by pregnancy-tools.com
It is the standard method of adding 280 days, 40 weeks, to the first day of your last menstrual period to estimate a due date. It assumes a 28 day cycle with ovulation around day 14.
Naegele's rule assumes a 28 day cycle. If yours runs longer or shorter, ovulation shifts, so we adjust the estimate by the difference between your cycle length and 28 days.
Rarely. Only a small share of babies are born on the exact estimated date, and most arrive within about two weeks before or after it.
It can. An early ultrasound, particularly in the first trimester, is considered a more precise dating method than LMP alone, and your provider may use it to set or adjust your due date.
Yes, but a later ultrasound is less precise for dating than an early one, so if your dates were never confirmed early, expect a wider margin of error.