Pregnancy Weeks Calculator

Estimate gestational age and time to due date from your last menstrual period. Informational only — confirm with your provider.

How the Math Works

The Pregnancy Weeks Calculator uses a straightforward mathematical conversion based on the standard obstetric measure of pregnancy duration. Pregnancy is traditionally counted from the first day of the last menstrual period (LMP), which is approximately 40 weeks or 280 days total. To determine current gestational age, the calculator divides the number of days since the LMP by 7 (the number of days in a week), yielding the equivalent number of weeks. For example, 70 days since LMP equals 10 weeks (70 ÷ 7 = 10), and 210 days equals 30 weeks. This linear conversion provides an accurate weekly breakdown of pregnancy progress throughout the 40-week gestation period.

Practical Applications

To use this calculator, simply count the number of days from the first day of your last menstrual period to the current date. Enter this day count into the calculator, and it will instantly display your pregnancy progress in weeks. The calculator also helps determine your estimated due date by adding 280 days to your LMP, which corresponds to 40 weeks gestation. You can use this calculation during every prenatal appointment, when tracking fetal movement, or when planning important pregnancy milestones to ensure you're within appropriate timeframes for various prenatal tests and procedures.

Day-to-Day Use

This calculator helps you track meaningful milestones throughout your pregnancy journey, from confirming you're on track in the first trimester to preparing for your due date in the third trimester. Knowing your exact gestational age helps you understand which prenatal screenings and tests are appropriate, such as when to schedule an anatomy scan around 18-20 weeks or when to expect gestational diabetes screening at 24-28 weeks. It also provides peace of mind by helping you monitor your baby's development timeline and understand when to contact your healthcare provider if you notice any concerns about your pregnancy progression or if you experience your due date approaching without signs of labor.

Worked example

100 days → 14 weeks 2 days (2nd trimester).

FAQ