Pregnancy Due Date Calculator
Estimate your due date from the first day of your last period using Naegele’s rule.
Formula and sources updated · Data sources · 2View algorithm, constants and default inputs (JSON)
Results
About this calculator
Use the first day of the last menstrual period and an editable reference date. The estimate is LMP + 280 days + (cycle length − 28), assuming a regular cycle; conception is an approximate cycle-length-minus-14 offset, not a confirmed date.
Worked example
calculated with the default values below
First day of last period 2026-10-03 · Average cycle length 28 days · Reference date 2026-10-31 Estimated due date 2027-07-10 · Estimated conception 2026-10-17 · Second trimester begins 2027-01-09 · Third trimester begins 2027-04-17 · Gestational days at reference date 28 days · Gestational weeks at reference date 4.0 weeks · Days remaining 252 days · Weeks remaining 36.0 weeks
Method and assumptions
Use the first day of the last menstrual period and an editable reference date. The estimate is LMP + 280 days + (cycle length − 28), assuming a regular cycle; conception is an approximate cycle-length-minus-14 offset, not a confirmed date. Trimester landmarks use 14 weeks 0 days and 28 weeks 0 days from LMP in both entries. Dates are date-only, without UTC/local-day drift. A clinician and early ultrasound establish clinical dating; this cannot confirm pregnancy.
Data sources
Disclaimer
These calculators are for general information only and are not a medical device, a diagnosis or a treatment plan. Reference ranges describe populations, not individuals, and do not account for pregnancy, medication, disability or athletic training. Discuss your own situation with a qualified healthcare professional.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
D = LMP + 280 + (C − 28); Con ≈ LMP + (C − 14); w = (reference − LMP)/7; T2 = LMP + 98; T3 = LMP + 196
Dates are date-only, without UTC/local-day drift. A clinician and early ultrasound establish clinical dating; this cannot confirm pregnancy.
Frequently asked questions
How accurate is the estimated due date?
Use the first day of the last menstrual period and an editable reference date. The estimate is LMP + 280 days + (cycle length − 28), assuming a regular cycle; conception is an approximate cycle-length-minus-14 offset, not a confirmed date. Trimester landmarks use 14 weeks 0 days and 28 weeks 0 days from LMP in both entries. Dates are date-only, without UTC/local-day drift. A clinician and early ultrasound establish clinical dating; this cannot confirm pregnancy.
What if my cycle is not twenty-eight days?
Use the first day of the last menstrual period and an editable reference date. The estimate is LMP + 280 days + (cycle length − 28), assuming a regular cycle; conception is an approximate cycle-length-minus-14 offset, not a confirmed date. Trimester landmarks use 14 weeks 0 days and 28 weeks 0 days from LMP in both entries. Dates are date-only, without UTC/local-day drift. A clinician and early ultrasound establish clinical dating; this cannot confirm pregnancy.