How accurate is a due date calculator?

A due date calculator gives an estimate based on typical pregnancy length and the date of your last menstrual period. Real due dates vary, and your provider may adjust dates using early ultrasound, especially if your cycle length is irregular.

What if my cycle is irregular?

If your cycles are irregular, an LMP-based due date may be less reliable. Use this tool as a starting point, then confirm dating with a healthcare professional.

When should I see a doctor?

If you think you may be pregnant, schedule prenatal care early. Seek urgent care for severe pain, heavy bleeding, dizziness, or other concerning symptoms.

Free toolNo signupLMP-based estimate

Calculate Due Date By Cycle Length

Use this due date calculator to estimate your pregnancy due date by LMP. Enter the first day of your last menstrual period (LMP) and your average cycle length. You'll get an estimated due date (EDD) plus a typical 37 to 42 week range from LMP. This is an estimate for planning and is informational only, not medical advice. Confirm dates with a healthcare professional, especially if your cycle is irregular. See Methods & assumptions for the formula and range.

Methods & assumptions

This variant focuses on Calculate Due Date By Cycle Length.

This due date calculator by LMP estimates an estimated due date (EDD) from the first day of your last menstrual period (LMP) and your average cycle length. In simple terms: we start from your LMP, count forward 40 weeks, and then adjust by how long your cycle usually is (similar to Naegele's rule).

Estimated due date (EDD)

EDD = LMP + 280 days + (cycle length - 28)

If your cycle is longer than 28 days, the estimate shifts later. If it's shorter, it shifts earlier.

Typical delivery window (37-42 weeks)

range = LMP + 37 weeks to LMP + 42 weeks

This range shows common variation in pregnancy length, even with an accurate due date.

Inputs: first day of LMP (required) and cycle length in days (required, 21-35).
Outputs: estimated due date (EDD) and a typical 37 to 42 week range from LMP.
Rounding: dates are rounded to the day and shown in US format.
Next step: confirm pregnancy dating with your healthcare provider.

If you're unsure of your LMP date, have irregular cycles, or your dates don't match early ultrasound, your provider may adjust your due date. This tool is informational only and not medical advice. See editorial policy & methodology.