Due Date Calculator
Estimate a pregnancy due date from the first day of the last menstrual period or a conception date.
Wednesday, June 16, 2027
Currently about week 0
About this calculator
How This Due Date Calculator Works
Enter the first day of your last menstrual period (or a known conception date), and the calculator estimates a due date using standard obstetric conventions.
The Formula
From the last menstrual period (LMP): Due date = LMP + 280 days (40 weeks) — this is Naegele's rule, the standard method used by most healthcare providers, based on a typical 28-day cycle with ovulation around day 14. From a known conception date: Due date = Conception date + 266 days (38 weeks), since conception itself is roughly 2 weeks after the LMP in a typical cycle.
Why This Is an Estimate, Not a Guarantee
Only a small percentage of babies are actually born on their calculated due date — a full-term pregnancy is considered normal anywhere from 3 weeks before to 2 weeks after it. Cycle length varies between individuals (the 280-day formula assumes a 28-day cycle), and an early ultrasound is generally considered more accurate than LMP-based dating alone. Use this calculator as a planning estimate, and defer to your healthcare provider's dating for anything medical.
Worked example
From the first day of the last menstrual period, the estimated due date is 280 days (40 weeks) later. From a known conception date, it's 266 days (38 weeks) later.
Frequently asked questions
How accurate is an estimated due date?
Only about 1 in 20 babies arrive exactly on the estimated date — it's a statistical average, not a prediction. Full term is generally considered anywhere from 37 to 42 weeks.
Why do LMP and conception dates give different day counts?
The LMP method assumes conception happens about 2 weeks after the last period started (a typical cycle), so 280 days from LMP and 266 days from conception land on close to the same date for an average cycle.
What if I have an irregular cycle?
This calculator uses standard averages and doesn't adjust for cycle length — an ultrasound-based estimate from a healthcare provider is more accurate for irregular cycles.