Educational estimate for adults, not medical advice, a diagnosis, or a treatment target. Review the method and limitations and discuss individual health questions with a qualified clinician.
Your result
How this calculation works
Estimated due date = first day of last menstrual period + 280 days + (cycle length − 28) days. Displayed weeks and days are elapsed time since the entered period date.
Estimate an expected due date using the first day of the last menstrual period. The estimate adjusts for a regular cycle of 21–35 days. Ultrasound and clinical assessment may establish a different date.
How to use this calculator
- Enter or choose first day of last menstrual period.
- Enter or choose usual cycle length (days).
- Enter or choose calculate weeks on.
- Select Calculate and review the result and assumptions.
Worked example
A last menstrual period beginning January 1, 2025 with a 28-day cycle gives an estimated due date of October 8, 2025.
Assumptions and limitations
Not a diagnosis, confirmation of pregnancy, or prediction of the delivery day. Irregular cycles, assisted reproduction, and clinical findings may require a different method. LMP-based gestational age shown here is not cycle-adjusted.
Formula references
Frequently asked questions
Will birth happen on this exact date?
Not necessarily. A due date is an estimate, not a prediction of the actual delivery date.
Should this override my clinician’s date?
No. Discuss dating with your clinician, particularly for irregular cycles, ultrasound differences, or fertility treatment.