Pregnancy Due Date Calculator Guide: Formula & Worked Example
Short answer
Estimated due date from the last menstrual period. In the fixed example below, estimated due date is Mon, March 8, 2027. Your answer depends on the inputs and assumptions you choose.
Formula
Naegele's rule: LMP + 280 days
Use the relationship above with consistent units. The inputs are first day of last period, cycle length. Use the input scale shown beside each field and keep time periods and units consistent. Only round the final result for display.
Worked example
- First day of last period
- 2026-06-01
- Cycle length
- 28
- Estimated due date: Mon, March 8, 2027
- Currently pregnant: 23 weeks 1 days
- End of 1st trimester: Mon, August 31, 2026
- End of 2nd trimester: Mon, December 7, 2026
Understanding the result
Estimated due date from the last menstrual period. Start with the base example, then replace each value with the figure that matches your situation.
First day of last period
The example uses 2026-06-01 for first day of last period. This is an illustration, not a recommended value. Keep the unit and time period shown by the field.
Cycle length
The example uses 28 days for cycle length. This is an illustration, not a recommended value. Keep the unit and time period shown by the field.
What the comparison means
The comparison changes only cycle length, keeping every other example input fixed. This isolates that input's effect on the primary result. It does not predict future outcomes.
This is a screening or planning estimate, not a diagnosis. Age, pregnancy, medication and individual medical history can change how the result should be interpreted.
Comparison
| Scenario | Input | Result |
|---|---|---|
| Lower input | Cycle length: 22.4 days | Estimated due date: Tue, March 2, 2027 |
| Base example | Cycle length: 28 days | Estimated due date: Mon, March 8, 2027 |
| Higher input | Cycle length: 33.6 days | Estimated due date: Sat, March 13, 2027 |
Common mistakes
- Using the wrong units for cycle length.
- Comparing results while changing more than one input at a time.
- This is a screening or planning estimate, not a diagnosis. Age, pregnancy, medication and individual medical history can change how the result should be interpreted.
Checklist
- Confirm first day of last period.
- Confirm cycle length is expressed in days.
- Check the result against your source or provider.
Calculators Percentage Editorial Team
Calculator guides from the Calculators Percentage team. These introductory examples are educational, not professional advice.
Last reviewed:
This is a screening or planning estimate, not a diagnosis. Age, pregnancy, medication and individual medical history can change how the result should be interpreted.