THE NUMORIX GUIDE
How to use the Pregnancy Due Date Calculator
Last reviewed September 14, 2026
What this calculator does
In LMP mode, the engine adds 280 calendar days to the last menstrual period.
Formula and method
In LMP mode, the engine adds 280 calendar days to the last menstrual period. In weeks-from-due-date mode, it subtracts the as-of date from the due date, computes gestational days as 280 minus days remaining, and reports floor(gestational days / 7) weeks plus the remainder days. Trimester labels are first below 13 weeks, second below 27 weeks, and third at 27 weeks or later.
Variables and inputs
LMP mode takes a last-menstrual-period date. The alternate mode takes an estimated due date and an as-of date in YYYY-MM-DD form and returns weeks, leftover days, trimester, and days remaining. The LMP mode also calculates current week and remaining days from the browser's current date.
Worked example
An LMP of January 15, 2025 gives January 15 + 280 days = October 22, 2025. In the alternate mode, an October 22 due date viewed on July 30 has 84 days remaining; 280 - 84 = 196 gestational days, which is 28 weeks and 0 days, so this engine labels it trimester 3.
How to interpret the result
The due date is an estimate of a 40-week pregnancy, not a promise of the birth date. The weeks-from-due-date result is a reproducible snapshot only when the due date and as-of date are both retained. In LMP mode, the current-week and remaining-day values change with the day the calculation is run.
Common mistakes to avoid
Enter the LMP date rather than an estimated conception date; conception is usually later than the first day of the period. In the alternate mode, use valid full dates and keep the as-of date separate from the due date. Do not mistake a trimester label for a clinical assessment of fetal development.
Assumptions and limitations
The 280-day convention assumes dating from the LMP and can be inaccurate when ovulation was late, cycles are irregular, or the LMP is uncertain. Ultrasound dating and clinical history may revise the estimate. This tool does not assess viability, fetal growth, labor timing, or pregnancy complications; contact a prenatal clinician for medical decisions.