Formula and calculation method
Estimated due date = last menstrual period + 280 days + (cycle length − 28). Gestational age is elapsed time since last menstrual period.
Calendar estimate assumes regular cycles; ultrasound and your maternity clinician may revise dating.
How to use due date calculator
- Set First day of last menstrual period. The example below uses 2026-01-01.
- Set Typical cycle length (days). The example below uses 28.
- Set Reference date. The example below uses 2026-06-01.
- Select Calculate to update the result. Reset restores the illustrated inputs. Copy, print or download your own result if you need a record.
Worked example
Example inputs
- First day of last menstrual period
- 2026-01-01
- Typical cycle length (days)
- 28
- Reference date
- 2026-06-01
- Estimated due date
- 2026-10-08
- Gestational age
- 21 weeks 4 days
Inputs and results explained
Due Date Calculator takes 3 inputs. Fill each one using the same units and conventions, then select Calculate.
- First day of last menstrual period: enter a date value in the format the field shows.
- Typical cycle length (days): a number from 21 to 45. Whole numbers only. The worked example uses 28.
- Reference date: enter a date value in the format the field shows.
The result panel reports Estimated due date and Gestational age, so you can see the whole outcome rather than a single figure.
Factors affecting your result
Calendar estimate assumes regular cycles; ultrasound and your maternity clinician may revise dating.
Changing first day of last menstrual period changes the scenario being evaluated. Adjust one input at a time when comparing results, keep a copy of the assumptions, and compare values using the same unit and period. A precise arithmetic result does not make an uncertain assumption precise.
Frequently asked questions
How does the due date calculator work?
Estimated due date = last menstrual period + 280 days + (cycle length − 28). Gestational age is elapsed time since last menstrual period.
What inputs does Due Date Calculator need?
Enter first day of last menstrual period, typical cycle length (days), reference date. Choose values that correspond to the same situation and measurement convention.
How should I interpret the due date calculator result?
Calendar estimate assumes regular cycles; ultrasound and your maternity clinician may revise dating.
Are my due date calculator inputs uploaded?
No. The calculation runs in this browser. Favorites, recent tool names, theme and formatting preferences may be saved locally, but calculation inputs are not sent to a calculation server or stored by this website. Shared links do not include entered values.
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