Free Pregnancy Calculator - Estimate Your Due Date
Estimate your due date from the first day of your last menstrual period and cycle length, and see your current progress and trimester.
Formula & Methodology
This calculator uses Naegele's Rule, adding 280 days (40 weeks) to the first day of your last menstrual period (LMP) to estimate your due date. It then adjusts for your menstrual cycle length: cycles longer or shorter than the standard 28 days add or subtract the difference (Cycle Length − 28 days). The result is an estimated due date, not a guaranteed delivery date — clinical assessment such as an early ultrasound may revise it.
- Due Date = Last Menstrual Period (LMP) + 280 days (40 weeks)
- Cycle Adjustment = Cycle Length − 28 days
- Adjusted Due Date = LMP + 280 days + (Cycle Length − 28)
Worked Example
Using the Pregnancy Calculator: apply the formula above to your input values. For instance, Due Date = Last Menstrual Period (LMP) + 280 days (40 weeks), then Cycle Adjustment = Cycle Length − 28 days. The tool performs each step instantly and shows the result.
Frequently Asked Questions
How is my due date calculated?
It typically adds 280 days (40 weeks) to the first day of your last menstrual period to estimate the due date.
How accurate is a due date?
It is an estimate; only about 1 in 20 babies arrive exactly on the due date, with most within a couple of weeks either side.
What are pregnancy trimesters?
Pregnancy is divided into three trimesters of roughly 13 weeks each, tracking key stages of development.