CalculatorX

OBGYN; Reference

LMP Calculator

LMP Calculator counts days from the first day of the last menstrual period and divides by 7. Eighty-four days is 12.0 weeks. It does not add 280 days, so it is not a due date, and a scan can disagree if the cycle was irregular.

LMP Calculator counts days from the first day of the last menstrual period and divides by 7. Eighty-four days is 12.0 weeks. It does not add 280 days, so it is not a due date, and a scan can disagree if the cycle was irregular.

Reviewed 2026-11-08. The result is an educational estimate, not professional advice.

Enter your values

This calculation stays in your browser. Nothing is sent when you press Calculate.

Result

Enter values to calculate.

Formula

Pregnancy weeks ≈ days since last menstrual period ÷ 7

Example

If LMP was 12 weeks ago, the estimate is around 12 weeks pregnant.

Illustration for the Pregnancy Weeks Calculator used by CalculatorX.
Illustration only. Use the formula and the result box for the number.

How to use LMP Calculator

Enter the first day of that period and the date you want to measure to. A later day in the period makes the estimate shorter than the usual method.

The formula that actually runs is: Pregnancy weeks ≈ days since last menstrual period ÷ 7

If LMP was 12 weeks ago, the estimate is around 12 weeks pregnant.

Where this estimate stops

This is approximate arithmetic. Bleeding, pain, or doubt about the date needs a clinician, not a website.

The calculation stays in your browser. CalculatorX does not save the numbers as an account.

If you need a different figure, look at Pregnancy Weeks Calculator, Pregnancy Calculator, Ovulation Calculator, EDD Calculator.

How to use this calculator

  1. Start with the numbers you already have. Enter the first day of that period and the date you want to measure to. A later day in the period makes the estimate shorter than the usual method.
  2. Press Calculate. The result uses only the fields on this form and the formula printed under it.
  3. Read the limit. This is approximate arithmetic. Bleeding, pain, or doubt about the date needs a clinician, not a website.

Questions about LMP Calculator

How does LMP Calculator get its result?

Enter the first day of that period and the date you want to measure to. A later day in the period makes the estimate shorter than the usual method.

What formula is on the lmp calculator page?

Pregnancy weeks ≈ days since last menstrual period ÷ 7

Can I trust “lmp calculator” as a final decision?

This is approximate arithmetic. Bleeding, pain, or doubt about the date needs a clinician, not a website.

Is this stored or sent anywhere?

No. The page calculates in your browser and does not create an account.

What this helps you do

  • Estimate LMP from the inputs you already have.
  • Check the result against the formula and the worked example.
  • Change one value at a time and compare the new estimate.

Limits to keep in mind

  • The figure is only as complete as the inputs on this form.
  • Rounding, fees, and local rules can change the real-world number.
  • Use it to learn and compare. Confirm important finance or health decisions with a qualified professional.