Toolkit Shelf

Health Calculators

Pregnancy Week Calculator

Use this pregnancy week calculator to estimate gestational age from the first day of the last menstrual period.

Reviewed May 25, 2026EstimateFormula shown

Quick answer

Pregnancy Week Calculator: what it calculates

Pregnancy Week Calculator calculates estimated gestational age from last menstrual period, cycle length and as-of date. The core method is Gestational age = days since first day of last menstrual period / 7.

ResultEstimated gestational age
InputsLast menstrual period, Cycle length, As-of date
FormulaPregnancy week formula

Live calculator

Pregnancy week

Estimated gestational age3w 4d

Approximate first trimester based on last menstrual period.

Estimated due dateFeb 5, 2027

255 days remaining in a 40-week estimate.

Care note

Pregnancy dating can change after clinical review or ultrasound. Use this for rough planning and confirm dates with a qualified clinician.

Formula

Pregnancy week formula

Gestational age = days since first day of last menstrual period / 7

A 40-week estimate is commonly counted from the first day of the last menstrual period.

How to use

Steps

  1. Enter the first day of the last menstrual period.
  2. Enter average cycle length if it differs from 28 days.
  3. Choose the date you want to calculate as of.
  4. Review estimated weeks, days, trimester, and due date.

Example

Sample calculation

Last periodMay 1, 2026
As ofMay 26, 2026
Pregnancy week3w 4d

Calculator use

Best for

  • Quick estimated gestational age from last menstrual period, cycle length and as-of date.
  • Wellness, nutrition, pregnancy, or fitness planning when a rough estimate is enough.
  • Scenario comparisons before changing calorie, macro, or body-measurement assumptions.
  • Planning alongside professional medical or nutrition advice, not as a diagnosis.

Before relying on it

Check first

  • Entering last menstrual period, cycle length and as-of date from different time periods or scenarios.
  • Treating a calculator result as medical advice or a diagnosis.
  • Using stale body, activity, pregnancy, or nutrition inputs when your situation has changed.
  • Ignoring context such as age, medication, training status, medical history, or professional guidance.

Details

What to know before using the result

These notes make the assumptions explicit, especially where the same search query can mean slightly different things.

Dating methodLMP-based estimate

Gestational age is commonly counted from the first day of the last menstrual period, not from the estimated conception date.

Clinical datingMay change

A clinician may revise dates using ultrasound, cycle history, assisted reproduction dates, or other clinical information.

Cycle adjustmentRough personalization

Cycle length can shift estimated ovulation and due date timing, but irregular cycles can make estimates less reliable.

Source notes

References used for this calculator

ACOGStates that estimated due date is calculated from the first day of the last menstrual period and average pregnancy length is 280 days or 40 weeks.ACOGProvides clinical guidance on methods for estimating the due date and documenting gestational age.

Benchmarks

How to read the result

The calculator is a decision aid, not a fixed rule. Use the output to compare scenarios and document your assumptions.

0 - 13 weeksFirst trimester

A broad trimester reference used by the calculator.

14 - 27 weeksSecond trimester

Dating should still be confirmed clinically.

28+ weeksThird trimester

Late-pregnancy dating by ultrasound can be less reliable than early dating.

Calculator accuracy

Methodology and assumptions

The formula, inputs, example, and limitations are shown so the result is checkable, not just a number in a box.

Formula

Gestational age = days since first day of last menstrual period / 7

Inputs used

Last menstrual period, Cycle length, As-of date

Limitations

Results are estimates for quick planning and should be checked before important financial, legal, tax, health, or business decisions.

Last reviewed

May 25, 2026

Cite this page

Toolkit Shelf. Pregnancy Week Calculator. Retrieved May 25, 2026, from https://toolkitshelf.com/tools/pregnancy-week-calculator

FAQ

Common questions

How do I calculate how many weeks pregnant I am?

Count the days from the first day of the last menstrual period to the as-of date, then divide by seven.

Is pregnancy counted from conception?

Gestational age is commonly counted from the first day of the last menstrual period, which is usually about two weeks before conception.

Can my due date change?

Yes. A clinician may update the estimated due date after reviewing ultrasound and clinical information.