Health / Browser calculator

Due Date Calculator

Due Date Calculator works from the inputs you provide and returns estimated due date, gestational age. Use the calculation panel to explore a scenario, then check the formula and limitations before applying the result.

Your inputs

Minimum: 21 · Maximum: 45 · Whole numbers

Your results

Enter your values and calculate to see the result.

This calculator is for informational purposes only and is not a substitute for professional medical advice.

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

  1. Set First day of last menstrual period. The example below uses 2026-01-01.
  2. Set Typical cycle length (days). The example below uses 28.
  3. Set Reference date. The example below uses 2026-06-01.
  4. 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

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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