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Key detailsAbout this tool
Due Date Calculator
Estimate an expected delivery date using an LMP-based or conception-based pregnancy-dating model, with cycle-length adjustment for LMP dating.
Learn how an expected delivery date can be estimated from the last menstrual period or conception date and why the result is only an estimate.
Last menstrual period method
A standard calculation adds approximately 280 days to the first day of the last menstrual period, with a cycle-length adjustment when a person’s average cycle differs from 28 days.
Conception method
When a conception date is available, a simplified model adds about 266 days.
Why due dates vary
Only a small proportion of births occur exactly on the estimated date. Normal pregnancies can span a range of gestational lengths.
Ultrasound and clinical dating
Clinical teams may use ultrasound and other information when determining the most appropriate estimated due date.
How to use the calculator
Choose the dating method that best matches the information you have and review the result as an estimated date.
Health note
Do not use an online estimate to make decisions about induction, medication, prenatal care or other medical treatment.
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Questions & Answers
Frequently Asked Questions
What does the Due Date Calculator calculate?
It provides an estimate from the inputs you enter. Review the detailed guide on this page for formulas, interpretation, assumptions and limitations.
How is an estimated due date calculated?
For an LMP-based estimate, a simplified model uses about 280 days from the first day of the last menstrual period and adjusts for cycle length. A conception-based model uses about 266 days.
Does cycle length change the due date estimate?
Yes. If the LMP is known, the entered average cycle length can shift the LMP-based estimate away from the default 28-day cycle assumption.
Is the calculated due date guaranteed?
No. A due date is an estimate. Actual delivery can occur before or after the calculated date, and clinical dating may revise the estimate.