What does an estimated fetal weight (EFW) calculator compute?

It converts four ultrasound biometry measurements — biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL) — into a single estimated weight in grams using the Hadlock regression formula, the equation used in routine obstetric ultrasound.

How accurate is the Hadlock EFW formula?

Published accuracy is approximately ±15–20% (about 2 standard deviations), and accuracy tends to be worse at the extremes of fetal weight, such as suspected macrosomia or growth restriction. It is a point estimate, not a diagnosis.

What units does this calculator expect?

BPD, HC, AC, and FL are entered in millimeters (mm), matching standard ultrasound report units. The calculator converts internally to centimeters before applying the Hadlock regression, and displays the result in grams as well as pounds and ounces.

Why does the calculator show a range instead of one number?

Because EFW is a statistical estimate with an inherent ±15–20% margin of error, showing only a single gram figure implies false precision. The Margin of Error tab converts the point estimate into a realistic weight range.

Does this calculator replace a clinician's assessment?

No. EFW supports but does not replace clinical judgment. Results must be interpreted by the treating obstetric provider in the context of the full clinical picture and serial growth trends, not used alone to make care decisions.

Estimated Fetal Weight Calculator — Hadlock EFW from BPD, HC, AC, FL

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Walk-through

How to Use This Calculator

4 steps

1

Get BPD, HC, AC, and FL from the ultrasound report

Every routine growth ultrasound report lists four biometry measurements: biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL), all in millimeters. These are the four inputs the Hadlock formula needs.

2

Enter the four measurements

Type each value into the EFW Calculation tab in millimeters, exactly as printed on the report. The estimate updates instantly as you type — no need to press a calculate button.

3

Read the estimate as a range, not a single number

Switch to the Margin of Error tab to see the estimate expressed as a low-to-high weight range (±15% or ±20%). Ultrasound EFW is a statistical estimate, not a scale reading — the range is the more honest way to interpret it.

4

Compare two scans to see growth trend

If you have a follow-up scan, enter its BPD/HC/AC/FL as Scan B and the number of days since Scan A. The interval growth rate (grams per week) is often more clinically useful than either single estimate.

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Reference

Formula & Methodology

3 formulas

Hadlock 4-parameter EFW regression

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

BPD, HC, AC, and FL are entered in centimeters (the calculator converts your millimeter inputs automatically). EFW in grams is 10 raised to the computed value. This is the standard 4-parameter Hadlock equation used in routine obstetric ultrasound — see the Formula Reference tab for the same equation worked out with your current numbers.

Margin-of-error range

range = EFW × (1 − p) to EFW × (1 + p)

p is the chosen margin (0.15 or 0.20). Hadlock's own reported accuracy for the 4-parameter formula is approximately ±15–20% (about 2 standard deviations), and the range widens further at the extremes of fetal weight.

Interval growth rate

growth rate (g/week) = (EFW_B − EFW_A) / days_between × 7

Comparing two scans converts the raw weight difference into a weekly rate, which is what growth charts and clinicians typically reference rather than the raw gram difference.

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High YMYL

Trust, Methodology & Sources

Reviewed by Calculover Editorial Team · Updated 2026-07-23 · 2 sources

Editorial accountability

Author: Calculover Editorial Team - Clinical calculator content

Owner: Calculover Editorial Team - Health calculator editorial review

Reviewer: Calculover Editorial Team - Clinical content reviewer

Last reviewed: 2026-07-23

Last verified: 2026-07-23

Data effective: 2026-07-23

Methodology

Implements the standard 4-parameter Hadlock estimated fetal weight (EFW) regression — log10(EFW g) = 1.3596 − 0.00386×AC×FL + 0.0064×HC + 0.00061×BPD×AC + 0.0424×AC + 0.174×FL, with BPD/HC/AC/FL converted from millimeter ultrasound-report inputs to centimeters before the formula is applied. The margin-of-error range applies a selectable ±15% or ±20% band to the point estimate, matching the formula's own published accuracy range.

Assumption: Assumes BPD, HC, AC, and FL are accurately measured and entered in millimeters exactly as reported on the ultrasound.

Assumption: Assumes a singleton pregnancy; the formula was not derived from and is not validated for multiple gestations.

Assumption: The ±15–20% margin of error is a population-level accuracy figure — actual error for any individual estimate can fall outside this range, especially at the extremes of fetal weight.

Limitations & guidance

This is a point-in-time estimate from a single scan, not a growth-percentile assessment or a diagnosis of macrosomia or fetal growth restriction.

Measurement technique, fetal position, and maternal body habitus can introduce additional error beyond the formula's own published margin.

Not validated for use outside the biometry ranges typical of a routine second/third-trimester growth scan.

Professional guidance: For clinical/educational reference only. Ultrasound EFW carries an inherent margin of error of approximately ±15–20%, and results must be interpreted by the treating clinician in the context of serial growth trends — not as a substitute for clinical judgment or diagnostic imaging review.

Primary sources

Hadlock FP, Harrist RB, Sharman RS, Deter RL, Park SK. "Estimation of fetal weight with the use of head, body, and femur measurements — a prospective study." Am J Obstet Gynecol. 1985;151(3):333-337.

American College of Obstetricians and Gynecologists (ACOG) — Fetal Growth Restriction, Practice Bulletin

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Glossary

Key Terms Explained

7 terms

BPD (Biparietal Diameter) The distance between the two sides of the fetal skull, measured at the widest point, in millimeters. One of the four standard biometry measurements used in fetal weight and dating formulas.

HC (Head Circumference) The circumference of the fetal head at the same level as the BPD measurement, in millimeters. Less affected than BPD by head shape variation, so it is included alongside BPD in the 4-parameter Hadlock formula.

AC (Abdominal Circumference) The circumference of the fetal abdomen at the level of the stomach and umbilical vein, in millimeters. AC is the single measurement most predictive of fetal weight and the one most affected by growth abnormalities like macrosomia or growth restriction.

FL (Femur Length) The length of the fetal thigh bone (femur diaphysis), in millimeters, used both for weight estimation and as an independent gestational-age dating measurement.

EFW (Estimated Fetal Weight) The predicted fetal weight in grams, calculated from ultrasound biometry using a regression formula such as Hadlock's. It is a statistical estimate with an inherent margin of error, not a direct measurement of weight.

Margin of error The expected range of variation between the ultrasound-estimated weight and the actual birth weight, commonly cited as roughly ±15–20% for the Hadlock formula. Accuracy is generally worse at the extremes of fetal weight.

Macrosomia A fetus estimated to be significantly larger than average for gestational age (commonly defined as EFW above the 90th percentile, or above a fixed weight threshold near term). EFW is one input into this assessment, alongside growth-chart percentiles.

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Scenarios

Real-World Examples

2 worked examples

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Routine third-trimester scan

~32-week growth ultrasound

BPD 82 mm HC 295 mm AC 275 mm FL 61 mm

These measurements produce an EFW of about 1,840 g (roughly 4 lb 1 oz), typical for around 32 weeks of gestation. At the standard ±15% margin, the realistic range is about 1,565–2,115 g — a useful reminder that the single-gram figure on a report is a midpoint, not an exact weight.

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Follow-up growth check

Two scans, 14 days apart

Scan A EFW ≈1,840 g Scan B EFW ≈2,360 g

A roughly 520 g difference over 14 days works out to about 260 g/week — within the normal third-trimester range (typically 200–230 g/week on average, with considerable individual variation). Comparing the trend across scans, rather than judging a single EFW in isolation, is what most growth-monitoring protocols actually rely on.

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Deep Dive

Understanding Estimated Fetal Weight (EFW) and the Hadlock Formula

3 chapters · 4 min read

Estimated fetal weight (EFW) converts four routine ultrasound measurements into a predicted weight in grams. It's one of the most common numbers reported during a prenatal growth scan, and one of the most commonly misread — a single EFW figure is a statistical estimate with a real margin of error, not a scale reading.

How the Hadlock formula works

The calculator implements the 4-parameter Hadlock regression (Hadlock et al., Am J Obstet Gynecol, 1985), which combines biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL) into a single log-linear equation. AC carries the most weight in the formula because abdominal size is the measurement most sensitive to how well a fetus is growing — it's also the measurement most affected in conditions like macrosomia or growth restriction, which is part of why the formula leans on it so heavily.

Multiple Hadlock formula variants exist depending on which subset of measurements is available (some use only AC and FL, for example). This calculator uses the standard 4-parameter version, since all four measurements are captured on a routine growth ultrasound and it's the version in most common clinical use.

Why the estimate always needs a range, not just a number

Published validation studies put the Hadlock formula's accuracy at roughly ±15–20% (about 2 standard deviations) around the true birth weight, and that error tends to widen further at the extremes — very small or very large fetuses are harder to estimate precisely. A reported EFW of 3,000 g, for example, realistically corresponds to an actual birth weight range spanning several hundred grams in either direction.

This is why the calculator's Margin of Error tab converts the point estimate into a low-high range instead of treating the raw gram figure as exact. Clinically, EFW is used as one input among several — growth-chart percentile trends over multiple scans generally carry more diagnostic weight than any single measurement.

Limits and when to look further

EFW does not replace a formal fetal weight percentile assessment (which requires comparing EFW against a gestational-age-specific growth standard), a biophysical profile, or clinical judgment from the treating provider. It also assumes reasonably accurate, well-obtained measurements — operator technique, fetal position, and maternal body habitus can all introduce additional measurement error beyond the formula's own margin. If a single scan's EFW is unexpectedly high or low, the typical next step is a repeat scan or a formal growth-trend assessment rather than acting on one number in isolation.

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Questions

Frequently Asked Questions

5 questions

What does the Estimated Fetal Weight Calculator actually compute?+

It applies the standard 4-parameter Hadlock regression to four ultrasound biometry measurements — BPD, HC, AC, and FL — to produce a predicted fetal weight in grams (and pounds/ounces), plus a realistic ±15–20% margin-of-error range.

How accurate is this calculator?+

It implements the published Hadlock et al. (1985) formula exactly, so its accuracy matches that formula's own validated performance: roughly ±15–20% of actual birth weight, with wider error at the extremes of fetal size. It is a screening estimate, not a diagnostic measurement — always interpret it together with your treating clinician.

What units does the calculator expect?+

Enter BPD, HC, AC, and FL in millimeters (mm), matching how they're printed on a standard ultrasound report. The calculator converts to centimeters internally before applying the regression, and displays the result in grams plus pounds and ounces.

Can I share my results?+

Yes — the calculator encodes your inputs into the page URL, so copying the address bar (or using the Share button) is enough to send the exact same calculation to someone else.

Where does the formula come from?+

Hadlock FP, Harrist RB, Sharman RS, Deter RL, Park SK. "Estimation of fetal weight with the use of head, body, and femur measurements — a prospective study." American Journal of Obstetrics and Gynecology. 1985;151(3):333-337. This is the standard 4-parameter EFW formula used in routine obstetric ultrasound practice.

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Estimated Fetal Weight Calculator

Ultrasound Biometry