Estimated Fetal Weight Calculator: Hadlock Formula
Estimated fetal weight (EFW) is a key measurement obtained during obstetric ultrasound examinations to monitor fetal growth and wellbeing during pregnancy. The Hadlock formula, developed by Dr Frank Hadlock and colleagues and published in the American Journal of Roentgenology in 1985, is the most widely used algorithm for this purpose in clinical practice. It combines three ultrasound biometry measurements: head circumference (HC), abdominal circumference (AC), and femur length (FL), all entered here in millimetres and converted internally to centimetres for the formula. The logarithmic output is raised to the power of 10 to give estimated weight in grams, which this calculator also converts to pounds and ounces. Gestational-age percentiles use Hadlock's 1991 normative tables, allowing EFW to be classified as small for gestational age (below the 10th percentile), appropriate for gestational age (10th to 90th percentile), or large for gestational age (above the 90th percentile). These classifications carry clinical implications that your obstetrician will interpret in the context of your full antenatal history. The measurements entered here must come from a formal ultrasound examination performed by a trained sonographer using standardised technique. No calculation can substitute for direct clinical assessment by a qualified obstetric provider.
Estimated fetal weight: -- g (--), --.
The Hadlock formula (1985)
All measurements are entered in mm and divided by 10 to give centimetres before applying the formula. The Hadlock equation operates in centimetres.
HC_cm = HC_mm / 10
AC_cm = AC_mm / 10
FL_cm = FL_mm / 10
log10(EFW_g) = 1.3596 + 0.0064*(HC_cm) + 0.0424*(AC_cm) + 0.174*(FL_cm) - 0.00386*(AC_cm * FL_cm)
EFW_g = 10 ^ log10(EFW_g)
lb = floor(EFW_g / 453.592)
oz = (EFW_g - lb * 453.592) / 28.35
Worked example
Gestational age 32 weeks, HC = 300 mm (30.0 cm), AC = 275 mm (27.5 cm), FL = 62 mm (6.2 cm):
- 0.0064 x 30.0 = 0.192
- 0.0424 x 27.5 = 1.166
- 0.174 x 6.2 = 1.079
- 0.00386 x (27.5 x 6.2) = 0.00386 x 170.5 = 0.658
- log10(EFW) = 1.3596 + 0.192 + 1.166 + 1.079 - 0.658 = 3.138
- EFW = 10^3.138 = approximately 1,373 g
- 1,373 g / 453.592 = 3 lb, remainder (1,373 - 1,360.8) / 28.35 = 0.43 oz = 3 lb 0.4 oz
- At 32 weeks: p10 = 1,379 g, p50 = 1,779 g, p90 = 2,237 g. EFW of 1,373 g is below p10 (small for gestational age)
Hadlock (1991) normative EFW percentiles by gestational age
| Gestational age (weeks) | 10th percentile (g) | 50th percentile (g) | 90th percentile (g) |
|---|---|---|---|
| 20 | 249 | 331 | 434 |
| 24 | 492 | 659 | 871 |
| 28 | 859 | 1,135 | 1,466 |
| 32 | 1,379 | 1,779 | 2,237 |
| 36 | 2,018 | 2,622 | 3,277 |
| 40 | 2,761 | 3,600 | 4,490 |
Values at gestational ages between the rows above are interpolated linearly. For ages below 20 weeks or above 40 weeks, this calculator does not assign a percentile category.
EFW classifications
| Percentile | Classification | Clinical significance |
|---|---|---|
| Below 10th | Small for gestational age (SGA) | May warrant increased monitoring; discuss with obstetrician |
| 10th to 90th | Appropriate for gestational age (AGA) | Within expected normal range |
| Above 90th | Large for gestational age (LGA) | May be associated with gestational diabetes or other factors; discuss with obstetrician |
Limitations of EFW estimation
All ultrasound-based EFW formulas are estimates. The Hadlock formula has a typical 1 standard deviation error of approximately 7 to 10 percent of actual birth weight in validation studies. This means that for a fetus with a true weight of 2,000 g, the EFW estimate may vary from roughly 1,800 g to 2,200 g. The abdominal circumference is particularly sensitive to measurement technique and fetal positioning.
EFW accuracy is also reduced in: late third trimester (especially beyond 40 weeks), oligohydramnios or polyhydramnios, multiple pregnancies, and cases where fetal position limits biometry quality. The American College of Obstetricians and Gynecologists (ACOG) recommends that EFW values be interpreted alongside clinical history, growth trajectory over serial scans, and Doppler studies when indicated.
Fetal weight calculator: frequently asked questions
What is the Hadlock formula for estimated fetal weight?
The Hadlock formula, published by Dr Frank Hadlock and colleagues in the American Journal of Roentgenology in 1985, uses ultrasound biometry measurements to estimate fetal weight. The version used here employs head circumference (HC), abdominal circumference (AC), and femur length (FL), all measured in centimetres. The formula is: log10(EFW_g) = 1.3596 + 0.0064*HC + 0.0424*AC + 0.174*FL - 0.00386*(AC*FL). The EFW in grams is then 10 raised to the power of that result.
How accurate is estimated fetal weight from ultrasound?
EFW from ultrasound biometry is an estimate with inherent variability. The Hadlock formula has a typical error of plus or minus 7 to 10 percent of actual birth weight in prospective studies. Accuracy decreases at extreme gestational ages and for very large or very small fetuses. The measurements themselves must be taken by a trained sonographer using correct technique. No formula can substitute for clinical assessment by a qualified obstetric provider.
What biometry measurements does this calculator need?
This calculator requires three measurements obtained from a standard fetal ultrasound: head circumference (HC) in millimetres, abdominal circumference (AC) in millimetres, and femur length (FL) in millimetres. These are divided by 10 internally to convert to centimetres before applying the Hadlock formula. These measurements must be performed by a trained sonographer or obstetrician using standardised planes and technique.
What do the EFW percentiles mean?
EFW percentiles are derived from Hadlock's 1991 reference tables and classify estimated fetal weight relative to the expected range for the given gestational age. A fetus below the 10th percentile is considered small for gestational age (SGA), which may warrant closer monitoring. A fetus above the 90th percentile is considered large for gestational age (LGA). Fetuses between the 10th and 90th percentiles are considered appropriate for gestational age (AGA). These classifications have clinical implications that your obstetrician will discuss with you.
Can I use this calculator without an ultrasound?
No. The HC, AC, and FL values must come from a formal ultrasound examination performed by a qualified sonographer or obstetrician. These measurements cannot be obtained by any other means and cannot be estimated from physical examination alone. This calculator is provided to help parents and healthcare providers understand what ultrasound-derived values mean, not to replace the ultrasound itself or the clinical interpretation of results.
Official sources
- Hadlock FP, Harrist RB, Sharman RS, Deter RL, Park SK. Estimation of fetal weight with the use of head, body, and femur measurements. American Journal of Obstetrics and Gynecology. 1985;151(3):333-337.
- Hadlock FP, Harrist RB, Martinez-Poyer J. In utero analysis of fetal growth: a sonographic weight standard. Radiology. 1991;181(1):129-133.
- American College of Obstetricians and Gynecologists: Fetal Growth Restriction (Practice Bulletin).
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology. General information only, not medical advice.