About the Estimated Fetal Weight Calculator
This estimated fetal weight (EFW) calculator converts the four standard ultrasound measurements — biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC) and femur length (FL) — into a weight estimate using the Hadlock formulas, the models built into most ultrasound machines. It then compares that weight with the Hadlock in-utero weight standard for the gestational age you enter and reports an approximate percentile.
It is meant for expectant parents who want to understand the numbers on a growth scan report, and for students learning how EFW is derived. Enter measurements in millimetres exactly as printed on the report, pick the formula your report used (the four-measurement Hadlock model is the most common), and add the gestational age on the day of the scan.
An ultrasound weight is an estimate: Hadlock reported that its best models have a standard deviation of about 7.5% of actual weight, so a baby estimated at 3,000 g could plausibly weigh 2,550–3,450 g. Percentiles depend on the reference chart, and your hospital may use a different one (such as INTERGROWTH-21st or a customised chart), so always go by your care team’s interpretation.
With the default inputs, the estimated fetal weight is 1,928 g. Change any value above to recalculate instantly.
How to use the estimated fetal weight calculator
- 1Find BPD, HC, AC and FL on your growth scan report (usually in millimetres).
- 2Choose the Hadlock formula that matches your report — the four-measurement model is the usual default.
- 3Enter each measurement in millimetres.
- 4Enter the gestational age on the day of the scan in weeks and days.
- 5Read the estimated weight, its likely range and the percentile, and discuss any concerns with your care team.
Formula and method
The weight estimate uses the regression models published by Hadlock and colleagues in 1985, with every measurement in centimetres (the calculator divides your millimetre values by 10) and the result in grams. Models that combine head, abdomen and femur measurements were the most accurate in Hadlock’s prospective study, with one standard deviation equal to about 7.5% of actual birth weight.
The percentile compares the estimate with Hadlock’s 1991 in-utero weight standard, whose mean weight W at gestational age GA (in weeks, including days as a fraction) follows the equation above, with a reported spread of ±12.7% (1 SD) at every week. The calculator converts the difference into a z-score and then a percentile with the normal distribution. Ultrasound software that reads percentiles from Hadlock’s printed table, rather than this equation, can show a percentile a point or two different.
- BPD
- Biparietal diameter (cm)
- HC
- Head circumference (cm)
- AC
- Abdominal circumference (cm)
- FL
- Femur length (cm)
- GA
- Gestational age in weeks (e.g. 32 weeks 3 days = 32.43)
- W
- Average (50th percentile) fetal weight at GA, grams
Worked examples
32-week growth scan, Hadlock 4
With BPD 8.2 cm, HC 29.6 cm, AC 28.0 cm and FL 6.2 cm, the Hadlock 4 equation gives log₁₀ EFW ≈ 3.285, or about 1,928 g (4 lb 4 oz). The Hadlock average at 32 weeks is about 1,953 g, so the baby sits near the 46th percentile — right in the middle of the normal range.
Small abdomen at 29 weeks (HC + AC + FL)
HC 26.0 cm, AC 21.5 cm and FL 5.0 cm give an estimate of about 967 g, while the average at 29 weeks is about 1,379 g. That is roughly 30% below average, under the 1st percentile on the Hadlock standard — the kind of result that usually prompts a repeat scan and Doppler assessment.
AC + FL only at 36 weeks 2 days
Using only abdominal circumference (34.0 cm) and femur length (7.2 cm), the estimate is about 3,279 g. At 36 weeks 2 days the Hadlock average is about 2,875 g, so this baby is around the 87th percentile — on the larger side but still below the 90th percentile.
Frequently asked questions
How accurate is estimated fetal weight on ultrasound?+
Hadlock’s best models (head, abdomen and femur) had a standard deviation of about 7.5% of actual weight, so roughly 95% of babies weigh within ±15% of the estimate. Accuracy is lower for very large or very small babies, with low amniotic fluid, or when the baby’s position makes measuring difficult.
Which Hadlock formula is the most accurate?+
Formulas that include a head measurement, the abdominal circumference and femur length perform best; the four-measurement model (BPD, HC, AC, FL) is the default on most ultrasound machines. AC + FL is used when the head cannot be measured well, for example when it is low in the pelvis.
What percentile is considered small or large for gestational age?+
An estimated fetal weight below the 10th percentile is usually described as small for gestational age (SGA) and above the 90th percentile as large for gestational age (LGA). These are screening cut-offs: many SGA babies are simply constitutionally small, which is why growth over time and Doppler results matter.
Why does my scan report show a different percentile?+
Hospitals use different reference charts — Hadlock, INTERGROWTH-21st, WHO fetal growth charts or customised charts adjusted for parental size and ethnicity — and these give different percentiles for the same weight. Even Hadlock-based software may differ slightly depending on whether it uses the published table or the equation.
What does AC mean on an ultrasound?+
AC is the abdominal circumference, measured around the baby’s belly at the level of the stomach and liver. It reflects liver size and fat stores, so it is the measurement most sensitive to growth restriction or excess growth, and it carries the most weight in every Hadlock formula.
Can I use this calculator to predict birth weight?+
Only roughly. The estimate describes the baby on the day of the scan; babies typically gain around 200 g a week in late pregnancy, and the ±15% error band means a 4,000 g estimate could be a 3,400 g or 4,600 g baby. Clinical decisions should not rest on a single EFW.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.