Estimate your baby's weight from ultrasound measurements (BPD, HC, AC, FL) using the standard Hadlock formula — instantly, right in your browser.
Find your baby's BPD, HC, AC, and FL measurements on your ultrasound report, enter them with the matching unit, then press Calculate. This tool provides a general estimate for informational purposes and does not replace advice from your doctor or sonographer.
This tool estimates a baby's weight before birth using the Hadlock four-parameter formula, one of the most widely used methods for calculating estimated fetal weight (EFW) from routine ultrasound measurements. It combines four values usually found on a growth or anatomy scan report — biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL) — into a single logarithmic equation developed by Dr. Frank Hadlock and colleagues in 1985. Enter all four measurements in either centimeters or millimeters, matching whatever unit your ultrasound report uses, and the calculator converts and computes the estimate locally in your browser. Like any formula based on indirect measurements rather than an actual scale, the result carries a typical margin of error of around ±15%, and accuracy naturally decreases somewhat at the extremes of fetal size. This tool performs only the standard calculation on the numbers you provide — it doesn't interpret whether a result is a cause for concern, which is a judgment your doctor or sonographer is best placed to make with the full context of your pregnancy.
Unlike a newborn who can simply be placed on a scale, a fetus still in the womb can only be weighed indirectly — by measuring its size on ultrasound and converting those measurements into an estimated weight using a mathematical formula. Several such formulas have been developed since ultrasound became a routine part of prenatal care in the 1970s and 1980s, but the version developed by Dr. Frank Hadlock and colleagues in 1985 remains one of the most widely cited and used in clinical practice worldwide, decades after its original publication.
The four-parameter Hadlock formula used in this calculator combines four separate measurements: biparietal diameter (BPD, the width of the head from side to side), head circumference (HC), abdominal circumference (AC), and femur length (FL, the length of the thigh bone). Each of these captures a different dimension of fetal growth — head size, trunk size, and limb length — and combining all four into one logarithmic equation tends to produce a more reliable estimate than relying on any single measurement alone, since it helps average out the natural variation in how individual babies are proportioned relative to their overall size.
No formula based on indirect measurements is perfectly precise, and the Hadlock method is no exception: published accuracy studies commonly cite a margin of error of roughly ±15%, with an average error around 7.5%, meaning a calculated estimate of 3,000 grams could realistically correspond to an actual birth weight anywhere from about 2,550 to 3,450 grams. This margin tends to widen further at the extremes of fetal size, which is worth keeping in mind particularly for pregnancies where a baby is estimated to be notably smaller or larger than average, since these are exactly the cases where precise weight matters most for clinical decision-making.
Despite these limitations, estimated fetal weight remains a genuinely useful tool in prenatal care, helping track growth trends across multiple ultrasounds over the course of a pregnancy, flag potential growth concerns for further evaluation, and inform delivery planning discussions between parents and their care team. The key is treating any single EFW reading as one data point within a broader clinical picture — interpreted by a qualified healthcare provider who has access to your full pregnancy history — rather than as a precise, standalone prediction of your baby's exact birth weight.
It's an approximation of a baby's weight before birth, calculated from ultrasound measurements of the fetus rather than a direct scale reading, since the baby obviously can't be weighed while still in the womb.
The Hadlock formula used here has a typical margin of error of about ±15%, with an average error of roughly 7.5%. That means a 3,000 g (6 lb 10 oz) estimate could realistically correspond to an actual birth weight anywhere from about 2,550 g to 3,450 g. Accuracy tends to be lower at the extremes of fetal size (very small or very large babies).
They come from a growth or anatomy ultrasound report, usually listed as BPD (biparietal diameter), HC (head circumference), AC (abdominal circumference), and FL (femur length). Ask your sonographer or check your printed ultrasound report.
The four-parameter Hadlock formula (using BPD, HC, AC, and FL together) is generally considered more accurate than simpler versions that use only two or three measurements, since combining head, abdomen, and limb measurements helps average out natural variation in how individual babies are proportioned.
No. This tool only performs the standard Hadlock calculation on the numbers you enter. It doesn't interpret your results, flag growth concerns, or replace your doctor's or sonographer's assessment — always discuss your ultrasound results with your healthcare provider.