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Estimated Fetal Weight Calculator

Enter the four ultrasound measurements in centimeters to estimate fetal weight.

How it is calculated

The tool uses a four-measurement Hadlock regression. All inputs are ultrasound measurements in centimetres: BPD (biparietal diameter), HC (head circumference), AC (abdominal circumference) and FL (femur length). It first computes log₁₀(EFW) = 1.3596 − 0.00386·AC·FL + 0.0064·HC + 0.00061·BPD·AC + 0.0424·AC + 0.174·FL and then raises 10 to that power to get the weight in grams.

The result is shown in grams (whole number) and in pounds (grams ÷ 453.592, two decimals), together with the four values you entered. All four measurements must be above zero, otherwise nothing is returned. The tool does not use gestational age, so it cannot say whether the weight is normal for the week of pregnancy. It is an estimate, not medical advice.

Worked example

Enter BPD 9, HC 32, AC 32 and FL 7 cm. The terms of the log are: −0.00386 × 32 × 7 = −0.8646; 0.0064 × 32 = 0.2048; 0.00061 × 9 × 32 = 0.1757; 0.0424 × 32 = 1.3568; 0.174 × 7 = 1.218.

Adding them to 1.3596 gives log₁₀(EFW) ≈ 3.4502. Then 103.4502 ≈ 2,820 g. In pounds this is 2,820 ÷ 453.592 ≈ 6.22 lb.

Try this example →

Common mistakes

  • Entering millimetres, as many ultrasound reports print them. The tool expects centimetres, so 90 mm must be typed as 9. Values too large to be centimetres give no result.
  • Mixing up the circumferences: HC is the head and AC is the abdomen. Swapping them changes the weight noticeably because AC carries the largest weight in the equation.
  • Entering a rough or rounded AC. Because AC is multiplied with both FL and BPD, a small error in it spreads through the whole formula.
  • Reading the grams figure as a precise birth weight. It is an estimate from one scan and carries a margin of error.
  • Expecting a percentile. The tool gives a weight only and does not compare it with the gestational age.

Sources

About this calculator

This calculator implements the Hadlock formula (Hadlock et al., 1985), the most widely used method in obstetric ultrasound for estimating fetal weight from four biometric measurements: biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL), all measured in centimeters. The four-parameter version combines all of these into a single regression equation for log10(EFW), which is generally the most accurate of Hadlock's several published formulas.

Sonographers and obstetricians use this estimated fetal weight (EFW) to track fetal growth trends, screen for growth restriction or macrosomia, and support delivery planning, typically plotting the result on a gestational-age-specific growth percentile chart. Because ultrasound measurement error compounds through the formula, the estimate carries a margin of error of roughly ±10–15% and is always interpreted alongside clinical context by a qualified provider — not as a substitute for medical advice.

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Comments (1)

  • Editorial team

    Small measurement differences move the weight more than they seem to. Keeping BPD 9.2, HC 33 and FL 7.1 cm, an AC of 34 gives 3,212 g and an AC of 35 gives 3,368 g, so one extra centimetre adds 156 g. That is why a repeat scan, read by the clinician, tells you more than any single figure. If you have scan reports from different weeks, what changed between them?

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