Pregnancy Weeks Calculator

Estimate pregnancy weeks and days from the pregnancy reference date. Enter your own values below to see estimated gestational age and check the method behind it.

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Your inputs

Change the example values to match your situation.

Your result

Let’s work it out.

Fill in your inputs and select Calculate.
Your result and breakdown will appear here.

How this calculator works

Gestational age is counted from the first day of the last menstrual period. Due date assumes 280 days.

Before you use the result

  • An estimate; clinical dating and ultrasound can differ.

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What is Pregnancy Weeks Calculator?

Pregnancy Weeks Calculator helps you estimate pregnancy weeks and days from the pregnancy reference date.

The main result is estimated gestational age, with a breakdown of estimated due date. The formula, example and assumptions below explain how to interpret it.

How to use Pregnancy Weeks Calculator

Use the example to get familiar with the form, then enter your own figures. Any rates in the example are illustrative. Check the field labels and units before calculating.

  1. Gather first day of last menstrual period and calculate as of. Use values from the same situation or reporting period.
  2. Replace the example first day of last menstrual period with your own value, then complete the other fields. Check the displayed units.
  3. Select Calculate and read estimated gestational age with its result breakdown. Change one input and calculate again to compare a second scenario. Reset restores the original example.
  • Select the first day of last menstrual period as a calendar date. The example uses 2026-06-22.
  • Select the calculate as of as a calendar date. The example uses 2026-09-14.

Pregnancy Weeks Calculator: a worked example

With the inputs below, the result is 12 weeks 0 days (estimated gestational age). Follow the example, then replace these illustrative values with your own.

Example inputs and output
Input or outputExample value
First day of last menstrual period2026-06-22
Calculate as of2026-09-14
Estimated gestational age12 weeks 0 days
Estimated due date2027-03-29

Understanding the Pregnancy Weeks result

Read estimated gestational age alongside estimated due date. These describe different parts of the same calculation.

Displayed results use a readable number of decimal places. When checking a result by hand, keep extra precision until the last step. To compare scenarios, change one input at a time and keep the others fixed.

  • Example Estimated due date: 2027-03-29

Pregnancy Weeks Calculator: assumptions and common mistakes

A useful estimate starts with the right inputs. Check the following assumptions and limits before applying the result to your situation.

  • An estimate; clinical dating and ultrasound can differ.
  • Check whether endpoints are included and which weekend or holiday settings apply. An entered deadline is not a live notification from an authority.

For wider guidance, visit Government of India calendar. The formula and scope of this specific tool are stated on this page.

Pregnancy Weeks Calculator FAQs

What does Pregnancy Weeks Calculator calculate?

Pregnancy Weeks Calculator helps you estimate pregnancy weeks and days from the pregnancy reference date. Its main output is estimated gestational age.

What inputs do I need for Pregnancy Weeks Calculator?

You need first day of last menstrual period and calculate as of. Use the field labels and units to match your figures to the formula.

Which formula does Pregnancy Weeks Calculator use?

Gestational age is counted from the first day of the last menstrual period. Due date assumes 280 days.

What is a worked example for Pregnancy Weeks Calculator?

Example inputs: First day of last menstrual period: 2026-06-22; Calculate as of: 2026-09-14. The result is 12 weeks 0 days (estimated gestational age).

How should I enter first day of last menstrual period?

Select the first day of last menstrual period as a calendar date. The example uses 2026-06-22.

How do I choose calculate as of?

Select the calculate as of as a calendar date. The example uses 2026-09-14. The example uses 2026-09-14; replace it if your circumstances differ.

How should I read estimated gestational age?

Read estimated gestational age alongside estimated due date. These describe different parts of the same calculation.

Why might my Pregnancy Weeks Calculator result differ from my records?

An estimate; clinical dating and ultrasound can differ. Differences in entered values, units or rounding can also change the comparison.

Can I compare two scenarios in Pregnancy Weeks Calculator?

Yes. Note your first estimated gestational age, change first day of last menstrual period or another input, and select Calculate again. Editing a field clears the old result so it is not mistaken for a new calculation.

Is Pregnancy Weeks Calculator free, and where are my inputs calculated?

This calculator is free to use without signing in. Its formula runs in your browser; calculator inputs are not sent to a calculation API. The inputs reset to the example values when you reset the form or start a fresh page.