Pregnancy · Ultrasound
Estimated Fetal Weight (Hadlock)
Enter the four ultrasound biometric measurements from your latest scan to estimate fetal weight using the Hadlock IV formula — the standard equation used in modern obstetric ultrasound.
Last reviewed 22 May 2026
Estimate fetal weight from ultrasound measurements
How to use this calculator
Find the four standard biometric measurements in your latest ultrasound report and enter each one in millimetres:
- BPD — biparietal diameter (the side-to-side distance across the fetal head).
- HC — head circumference.
- AC — abdominal circumference.
- FL — femur length.
The estimate appears in grams, pounds and ounces, and updates as you type.
Background — why Hadlock?
Frank Hadlock and colleagues published a series of regression equations in the 1980s that estimate fetal weight from ultrasound biometry. The four-parameter equation — usually called Hadlock IV or simply the Hadlock 1985 formula — has remained the obstetric standard since its publication. It is the calculation built into almost every modern ultrasound machine’s “EFW” readout.
The formula uses a logarithmic regression: each biometric input contributes a weighted term, plus interaction terms between AC×FL and BPD×AC, with the sum giving log₁₀ of the estimated weight in grams.
Interpreting your result
Hadlock IV has a mean prediction error of roughly ±10 to 15 % against actual birth weight when the scan is performed within a week or two of delivery. Accuracy is best in appropriately grown singletons and worse at the extremes of fetal size. Trust the growth curve more than any single number: an EFW that is on the 30th percentile and has been on the 30th percentile for the last three scans is reassuring; the same number after a previous scan at the 80th percentile is not.
Limitations
- The formula was derived in a population of singleton, predominantly white pregnancies; performance in twins, multiples and some ethnic groups may differ.
- Estimates near term are less reliable than mid-trimester ones because head measurements become harder to obtain reproducibly as the fetus engages.
- EFW alone does not diagnose macrosomia or growth restriction — those are clinical diagnoses that combine biometry with growth velocity, amniotic-fluid index, Doppler indices and maternal context.
- This tool is for informational use only. Do not change clinical care plans based on a calculated EFW without discussing the scan with the obstetric team that performed it.
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;151:333-7.
- Dudley NJ. A systematic review of the ultrasound estimation of fetal weight. Ultrasound Obstet Gynecol 2005;25:80-9.
- Royal College of Obstetricians and Gynaecologists (RCOG). The investigation and management of the small-for-gestational-age fetus (Green-top Guideline No. 31).