Foetal Weight Calculator

During pregnancy, an ultrasound may include measurements of your baby’s head, abdomen, and femur. These measurements can be used to estimate fetal weight, also called estimated fetal weight (EFW). Because a fetus cannot be placed on a scale before birth, EFW is a calculation—not a direct weighing.

Our Foetal Weight Calculator, also known as a Fetal Weight Calculator, estimates EFW using ultrasound measurements and one of three calculation options. You can enter measurements in millimeters or centimeters and see the estimate in grams, kilograms, and pounds.

The calculator offers two Hadlock methods and a simplified abdominal circumference and femur length method. It also accepts gestational age and displays it alongside the result.

This tool is for general information only. It does not determine whether a baby is growing normally, calculate a growth percentile, or diagnose a pregnancy condition. Your maternity care professional should interpret ultrasound findings in the context of your pregnancy and clinical history.

What Is a Foetal Weight Calculator?

A Foetal Weight Calculator uses measurements taken during an ultrasound to estimate a fetus’s weight. The measurements describe parts of the baby’s body, such as the head, abdomen, and thigh bone. A formula combines these values to produce an estimated weight.

The calculator includes three methods:

• Hadlock (HC, AC, FL): Uses head circumference, abdominal circumference, and femur length.
• Hadlock (BPD, HC, AC, FL): Adds biparietal diameter to those three measurements.
• Simplified AC/FL estimate: Uses abdominal circumference and femur length.

The tool converts measurements to centimeters before applying its equations. The estimated result is shown in grams, with equivalent values in kilograms and pounds.

Each formula is an approximation. The result can differ from a baby’s actual weight at birth, and a single estimate does not describe growth over time.

How to Use the Foetal Weight Calculator

Step 1: Select a Calculation Method

Choose the method that matches the measurements you have from the ultrasound report.

If your report includes HC, AC, and FL, you can use the three-measurement Hadlock option. Choose the four-measurement Hadlock option when BPD, HC, AC, and FL are available. The simplified AC/FL option requires only abdominal circumference and femur length.

The methods need different inputs, so select the method before entering measurements. Use figures from the report rather than trying to estimate measurements yourself.

Step 2: Enter Gestational Age

Enter completed gestational weeks, then select the additional days. For example, enter 32 weeks and 4 days for a gestational age of 32 weeks and 4 days.

The calculator accepts completed weeks from 14 through 42 and an additional 0–6 days. At 42 weeks, only 0 additional days are accepted.

Gestational age is displayed with the result, but it is not used in the weight equations. It does not change the estimated weight or calculate a percentile for that week.

Step 3: Enter the Ultrasound Measurements

Enter the measurements needed for your selected method:

• BPD: Biparietal diameter
• HC: Head circumference
• AC: Abdominal circumference
• FL: Femur length

Each measurement has its own unit selector. Choose millimeters (mm) or centimeters (cm) to match the value on your ultrasound report. The calculator converts millimeters into centimeters before calculating.

For example, if an ultrasound report lists an abdominal circumference of 280 mm, enter 280 and select mm. The calculator converts this to 28 cm.

Step 4: Check Your Entries

Check that each number and unit match the report. A unit mismatch can significantly alter the calculation. For example, entering a value reported in millimeters while selecting centimeters makes it ten times larger than intended.

Step 5: Click Calculate

Select Calculate to display the estimated fetal weight, its equivalents in other units, the gestational age entered, the selected method, and an approximate ±10% comparison range.

The calculator requires different measurements for each method. If a required measurement is missing or outside the tool’s accepted input range, it displays an error for you to check the entry.

Fetal Ultrasound Measurements Explained

Understanding the abbreviations can help you match your ultrasound report to the calculator.

Biparietal Diameter (BPD): The width of the fetal head measured between the two parietal bones. It is one of several measurements used to assess fetal size.

Head Circumference (HC): A measurement around the fetal head. It is used in both Hadlock options in this calculator, but not in the simplified AC/FL option.

Abdominal Circumference (AC): A measurement around the fetal abdomen. It is included in all three calculation methods offered.

Femur Length (FL): The length of the fetal thigh bone. It is used in all three methods.

Ultrasound measurements are obtained by trained professionals using imaging equipment. This calculator is intended for entering those reported measurements; it is not a guide to taking fetal measurements at home.

Foetal Weight Calculator Formulas Explained

The Hadlock equations calculate the base-10 logarithm of estimated fetal weight in grams. In the equations below, all measurements must be expressed in centimeters.

Hadlock Formula Using HC, AC, and FL

log₁₀(EFW in grams) = 1.326 − 0.00326 × (AC × FL) + 0.0107 × HC + 0.0438 × AC + 0.158 × FL

The calculator applies this method when you select Hadlock (HC, AC, FL).

Hadlock Formula Using BPD, HC, AC, and FL

log₁₀(EFW in grams) = 1.3596 − 0.00386 × (AC × FL) + 0.0064 × HC + 0.00061 × (BPD × AC) + 0.0424 × AC + 0.174 × FL

This option includes all four ultrasound measurements. The BPD term is included in the equation, along with HC, AC, and FL.

Simplified AC/FL Estimate

For the calculator option labelled “AC and FL (Shepard-style simplified estimate),” the calculation is:

log₁₀(EFW in grams) = 1.304 − 0.003 × (AC × FL) + 0.052 × AC + 0.193 × FL

This option uses only AC and FL. The calculator applies the simplified equation shown above; it should not be assumed to be interchangeable with every formula described as a Shepard method in other sources or clinical reports.

Converting the Formula Result to Grams

After calculating the logarithmic value, the calculator raises 10 to that result:

EFW (g) = 10 raised to the calculated log₁₀ value

It then converts grams into kilograms and pounds:

• Kilograms = grams ÷ 1,000
• Pounds = grams ÷ 453.59237

These conversions change the displayed unit, not the underlying estimate.

Example: Estimating Fetal Weight with Hadlock

Suppose an ultrasound report provides these measurements:

• Gestational age: 32 weeks and 4 days
• HC: 29.0 cm
• AC: 28.0 cm
• FL: 6.2 cm
• Calculation method: Hadlock (HC, AC, FL)

First, use the three measurements in the formula:

log₁₀(EFW) = 1.326 − 0.00326 × (28.0 × 6.2) + 0.0107 × 29.0 + 0.0438 × 28.0 + 0.158 × 6.2

The individual terms are approximately:

• AC × FL = 173.6
• 0.00326 × 173.6 = 0.566
• 0.0107 × 29.0 = 0.310
• 0.0438 × 28.0 = 1.226
• 0.158 × 6.2 = 0.980

Substituting these values gives a logarithmic result of about 3.276. Raising 10 to that value produces an estimated weight of approximately 1,890 grams.

The approximate unit conversions are:

• 1,890 g
• 1.89 kg
• 4.17 lb

The calculator also displays a ±10% arithmetic range:

• Lower value: approximately 1,701 g
• Upper value: approximately 2,079 g

This is an example of how the calculation works, not a prediction or interpretation of a particular pregnancy. A clinician should assess the actual ultrasound measurements and result.

What Does the ±10% Range Mean?

The calculator displays a lower and upper figure by subtracting and adding 10% to the calculated estimate:

Lower figure = EFW × 0.90

Upper figure = EFW × 1.10

For an estimate of 1,890 g, this gives approximately 1,701–2,079 g.

This is a simple comparison band generated by the calculator. It is not a personalized confidence interval, a guarantee that the actual weight falls within those limits, or a statement of the accuracy of the ultrasound. Actual differences can vary. Your healthcare professional can explain how the estimate should be interpreted in your circumstances.

Why Gestational Age Matters—but Does Not Change This Calculation

Gestational age is important when healthcare professionals assess fetal growth. An EFW is usually considered alongside the pregnancy’s dating, ultrasound findings, and other relevant clinical information.

In this calculator, gestational age is included in the result for reference, but it is not part of any of the three weight equations. Changing the entered week or day while keeping the ultrasound measurements and method the same will not change the estimated weight.

The calculator also does not compare the estimate with a gestational-age growth chart. It does not calculate a percentile or identify a fetus as small or large for gestational age.

Benefits and Limitations of Using the Calculator

The Foetal Weight Calculator can help you:

• See how ultrasound biometry is used to estimate fetal weight.
• Convert measurements from millimeters to centimeters automatically.
• View the estimated weight in grams, kilograms, and pounds.
• Compare the three available calculation options.
• See the entered gestational age and selected method alongside the result.
• Understand the arithmetic behind the estimate.

It also has important limitations. It does not evaluate a series of scans, fetal growth rate, pregnancy dating, fetal anatomy, or medical history. It cannot diagnose growth restriction, macrosomia, or another pregnancy condition. The simplified AC/FL method is a separate approximation, not a substitute for the method documented by a clinician.

For meaningful interpretation, use the measurements from the ultrasound report and discuss the findings with your obstetrician, midwife, or other maternity care professional. Do not make decisions about care based solely on this calculator.

Frequently Asked Questions

1. What is a foetal weight calculator?

A foetal, or fetal, weight calculator estimates a baby’s weight using ultrasound measurements such as head circumference, abdominal circumference, femur length, and sometimes biparietal diameter. It produces a mathematical estimate rather than directly weighing the baby.

2. Which ultrasound measurements do I need?

The required measurements depend on the selected method. Hadlock (HC, AC, FL) requires head circumference, abdominal circumference, and femur length. Hadlock (BPD, HC, AC, FL) requires all four measurements. The simplified AC/FL option requires abdominal circumference and femur length.

3. Can I enter measurements in millimeters?

Yes. Each ultrasound measurement has its own unit selection. Choose millimeters or centimeters to match the report. The calculator converts millimeters to centimeters before applying the selected equation.

4. Does gestational age affect the estimated weight?

Not in this calculator. Gestational age is displayed with the result but is not included in the weight equations. The tool does not compare the result with a growth chart or calculate an age-based percentile.

5. What does EFW mean?

EFW means estimated fetal weight. It is calculated from ultrasound measurements because fetal weight cannot be measured directly before birth. It is an estimate, not a confirmed weight.

6. What is the Hadlock formula?

Hadlock refers to ultrasound-based equations that combine fetal biometric measurements to estimate weight. This calculator offers a three-measurement version using HC, AC, and FL, and a four-measurement version that also uses BPD.

7. Is the simplified AC/FL option the same as a standard Shepard formula?

The calculator labels this option “Shepard-style simplified estimate,” but applies the AC/FL equation listed in the article. Formula names and versions may vary between sources and clinical systems, so do not assume this result matches the equation used in your ultrasound report.

8. What does the ±10% range mean?

It is the calculator’s estimated weight plus and minus 10%. It is an arithmetic comparison range, not a guarantee of accuracy or a personalized clinical confidence interval. The actual weight may fall outside it.

9. Does this calculator show whether my baby is growing normally?

No. It calculates an estimated weight from one set of measurements. It does not calculate a growth percentile, compare serial scans, or diagnose a growth concern. A healthcare professional should interpret the measurements and pregnancy history.

10. Should I make pregnancy decisions based on this result?

No. Use this calculator for general information only. Your maternity care professional should interpret ultrasound findings and advise you about care. Do not change treatment or make pregnancy decisions based solely on an online estimate.

Conclusion

The Foetal Weight Calculator provides a convenient way to estimate fetal weight using ultrasound measurements. It offers two Hadlock options and a simplified AC/FL method, accepts measurements in millimeters or centimeters, and displays the estimate in grams, kilograms, and pounds.

It can also show an arithmetic ±10% comparison range and record gestational age alongside the result. However, gestational age does not affect the calculated weight, and the tool does not determine growth percentiles or diagnose pregnancy conditions.

Use the calculator to understand how an estimated fetal weight is derived, then rely on your maternity care professional to interpret the ultrasound and guide your care.