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Gestational Age Calculator

Calculate gestational age based on ultrasound measurements (CRL).

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Gestational Age Calculator

Age based on ultrasound

Complete Guide

Overview

Calculate gestational age based on ultrasound measurements (CRL). Use this calculator to get instant, accurate results based on standard health formulas and guidelines.

⚠️This tool is for educational and screening purposes only and is not a substitute for professional medical advice.

How to Use

1

Enter your details in the input fields above.

2

Select your preferred units (metric or imperial) if available.

3

Click the Calculate button to view your instant results and interpretation.

Formula & Calculation

Refer to the in-depth guide below for complete calculation formulas.

Benefits of Using This Calculator

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Accurate Results

Provides instant calculations for gestational age calculator.

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Clear Guidance

Compares your score against standard medical reference ranges.

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Fast & Free

No sign-up required — fast, private calculation on any device.

Pro Tips

  • Enter accurate measurements for the most reliable results.
  • Track your results over time to identify long-term trends.
  • Consult a qualified healthcare professional for medical interpretation.
In-Depth Guide

Gestational Age Calculator: Determine Age from Ultrasound Measurements

During early pregnancy ultrasounds, the sonographer measures Crown-Rump Length (CRL) — the distance from the top of the fetal head to the bottom of the buttocks. This measurement directly translates to gestational age with remarkable accuracy in the first trimester.

What Is Crown-Rump Length?

CRL is the most accurate ultrasound measurement for dating a pregnancy in the first trimester (approximately weeks 6–13). After the first trimester, the fetus curls into a C-shape, making CRL less practical — other measurements (biparietal diameter, femur length) take over.

The Formula

Gestational Age (days) ≈ CRL (mm) + 42
Valid for CRL 10–84 mm (approximately weeks 6–13)
Weeks = Days ÷ 7

Worked Example

CRL from ultrasound: 55 mm.
Days = 55 + 42 = 97 days → Weeks = 97 ÷ 7 = 13 weeks, 6 days

Calculate gestational age from your CRL with our Gestational Age Calculator.

CRL Reference Values

CRL (mm)Approximate Gestational Age
5 mm6 weeks
20 mm8 weeks
40 mm10 weeks
60 mm11.5 weeks
84 mm13 weeks

Why Ultrasound Dating Matters

  • More accurate than LMP-based dates when cycles are irregular
  • If ultrasound date differs from LMP-based EDD by more than 7 days (first trimester), the ultrasound date takes precedence
  • Confirms viable pregnancy and location (rules out ectopic pregnancy)
  • Detects multiples early

Final Thoughts

CRL-based gestational age calculation is the gold standard for early pregnancy dating. If your ultrasound date differs meaningfully from your LMP date, discuss with your midwife or OB which date to use for your care schedule.

Estimate gestational age from CRL with our free Gestational Age Calculator.