What is the Pregnancy Due Date Calculator?
This is a cycle-adjusted form of Naegele’s rule.
The Calcwise Pregnancy Due Date tool is designed to give a fast answer while keeping the arithmetic visible. Enter the requested values, check the units, and the result updates in your browser. Your calculator entries are not sent to a Calcwise server.
How to use this calculator
- Enter values using the units shown beside each field. In the interactive calculator, each field also shows a plain-language example or meaning guide.
- Check that the values describe the same period or situation where relevant.
- Read the main result and the supporting figures shown beside it.
- Use the reset button to return to the example values.
Inputs used
- First day of last menstrual period
- Typical cycle length — Used to adjust the 28-day assumption
Pregnancy Due Date Calculator formula
The formula is shown so you can understand the relationship between the inputs instead of treating the result as a black box.
Worked example
With a 28-day cycle, the estimate is 280 days after the first day of the last menstrual period.
Your own result may differ because it uses the exact values you enter. For comparisons, keep the method and units consistent from one calculation to the next.
Important assumptions and limitations
An estimated due date is not exact. Ultrasound dating and clinical care can revise it.
Calculator outputs are estimates unless the underlying arithmetic is exact by definition. For decisions involving tax filing, lending, health, contracts or institutional grading rules, use the calculator as a first check and confirm the final decision with the relevant authoritative source.
Frequently asked questions
What does the Pregnancy Due Date Calculator calculate?
This is a cycle-adjusted form of Naegele’s rule.
What formula does the Pregnancy Due Date Calculator use?
Estimated due date = LMP + 280 days + (cycle length − 28 days)
Is the Pregnancy Due Date Calculator result exact?
An estimated due date is not exact. Ultrasound dating and clinical care can revise it.
How should I use the Pregnancy Due Date Calculator result?
Use the result as a practical estimate or planning check. Keep the inputs consistent, read the assumptions on this page, and verify important financial, academic or health decisions with the appropriate official or professional source when needed.
Pro mode: Dating cross-check
Optionally compare the LMP estimate with a known conception/ovulation date.
Enable Pro mode above the input form to reveal the advanced scenario fields. The normal result stays visible, and a separate Pro analysis appears underneath so you can compare the basic answer with the deeper model.
Advanced options
- Known conception / ovulation date