This pregnancy due date calculator estimates an expected delivery date from the first day of the last menstrual period and average cycle length. It is a calendar estimate and does not replace dating by a maternity healthcare professional.
How to use this calculator
Enter the first day of the last menstrual period, not the final day. The standard estimate assumes a 28-day cycle. If your usual cycle differs, enter its average length to adjust the estimate by the difference from 28 days.
Formula & calculation method
Estimated due date = last menstrual period + 280 days + (cycle length − 28 days)The 280-day convention corresponds to 40 weeks from the last menstrual period. Cycle adjustment shifts ovulation timing in this simplified model. Gestational age on today's date is also displayed when the entered date is not in the future.
Worked example
With a 28-day cycle, adding 280 days gives the standard estimate. A consistently 32-day cycle adds four days in this model, while a shorter cycle subtracts the corresponding difference.
Why this is useful
The calculator makes the date convention transparent and provides both the estimated due date and current gestational age where applicable. It can help with early planning before individualized clinical dating is available.
Limitations to keep in mind
Only a minority of births occur exactly on the estimated date. Irregular cycles, uncertain dates, conception circumstances, and ultrasound findings can change clinical dating. Always rely on maternity professionals for personalized assessment and care.
Frequently asked questions
Is the due date a guaranteed delivery date?
No. It is an estimate around 40 weeks, and actual birth can occur before or after that date.
Should I enter the first or last day of bleeding?
Enter the first day of the last menstrual period, which is the convention used for this gestational-age estimate.
Can an ultrasound change the due date?
Yes. A maternity professional may use ultrasound and clinical information to establish or revise dating.