Due Date Calculator
Estimate your due date from the first day of your last menstrual period and average cycle length, using the modified Naegele's rule, with a calendar-only milestone timeline.
Calculator Inputs
The 4-digit year of the first day of your last menstrual period.
The month of the first day of your last menstrual period (1-12).
The day of the first day of your last menstrual period (1-31).
Your typical cycle length in days (21-35).
- Estimated Due Date — Day
- 8
- Estimated Due Date — Year
- 2026
- Estimated Due Date — Month
- 10
- 12-Week Milestone — Year
- 2026
- 12-Week Milestone — Month
- 3
- 12-Week Milestone — Day
- 26
- 20-Week Milestone — Year
- 2026
- 20-Week Milestone — Month
- 5
- 20-Week Milestone — Day
- 21
- 28-Week Milestone — Year
- 2026
- 28-Week Milestone — Month
- 7
- 28-Week Milestone — Day
- 16
- 40-Week (Due Date) — Year
- 2026
- 40-Week (Due Date) — Month
- 10
- 40-Week (Due Date) — Day
- 8
About these results
CalcSpectrum calculators produce automated estimates using established formulas and the specific values you enter. Because real-world outcomes depend on input accuracy, mathematical assumptions, and rounding conventions, please verify critical figures with an appropriately qualified financial, tax, medical, legal, or other professional, as relevant.
Health & Fitness Notice: Wellness and body metric calculations provide informational estimates based on generalized population formulas. They do not provide medical diagnosis, clinical advice, or treatment plans. Consult an appropriately qualified medical or healthcare professional for personal guidance.
How It's Calculated
Formula
This calculator uses the modified Naegele's rule: the standard obstetric estimate of 280 days (40 weeks) from the first day of your last menstrual period, adjusted for your average cycle length when it differs from the 28 days the base rule assumes. A longer cycle shifts the estimated due date later; a shorter cycle shifts it earlier. The timeline shown (12, 20, 28, and 40 weeks) is a purely calendar-based set of milestone dates counted back from your estimated due date — it does not describe fetal development stages or any week-specific clinical events, only calendar dates. This is an ESTIMATE based on a published formula, not a guaranteed birth date: most births occur within a range of days around the estimated due date, and this calculator does not infer or report current pregnancy status — it only computes dates from the inputs you provide.
Worked Examples
28-day cycle, LMP January 1, 2026
- Base estimate: Jan 1, 2026 + 280 days = Oct 8, 2026
- Cycle adjustment: 28-day cycle, so no adjustment needed
- Estimated due date: Oct 8, 2026
30-day cycle, LMP January 1, 2026
- Base estimate: Jan 1, 2026 + 280 days = Oct 8, 2026
- Cycle adjustment: +2 days (30 - 28)
- Estimated due date: Oct 10, 2026
Frequently Asked Questions
Why does the cycle length change the due date?
The base Naegele's rule assumes ovulation occurs around day 14 of a 28-day cycle. A longer or shorter average cycle shifts when ovulation likely occurred, so the estimate adds or subtracts the difference from the base 28-day assumption.
Is the estimated due date the exact day the baby will be born?
No. This is a formula-based estimate, not a guarantee. Most births happen within a range of days around the estimated due date rather than on the exact date.
What do the 12/20/28/40-week milestones show?
They are purely calendar dates counted back from your estimated due date at those gestational-age points — this calculator does not describe fetal development or clinical events at each stage, only the calendar date itself.
Can this tell me how many weeks pregnant I am today?
No. This calculator only estimates dates from your last menstrual period and cycle length — it does not take a current date as input and does not report or infer current pregnancy status.