Estimated fetal weight calculator
Enter the measurements from an ultrasound report, in millimeters, to estimate the baby's weight with the Hadlock formula: with all four measurements, with head circumference, abdomen and femur, or with abdomen and femur only. It is an estimate, not a medical assessment.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Choose the Hadlock formula: all four measurements, HC with AC and FL, or AC with FL.
- Enter each measurement from the report in millimeters.
- Read the estimated weight in grams, kilograms and pounds.
How it works
Hadlock’s 1985 formulas give log10 of the weight in grams from measurements in centimeters:
- BPD, HC, AC, FL: 1.3596 − 0.00386 AC·FL + 0.0064 HC + 0.00061 BPD·AC + 0.0424 AC + 0.174 FL
- HC, AC, FL: 1.326 − 0.00326 AC·FL + 0.0107 HC + 0.0438 AC + 0.158 FL
- AC, FL: 1.304 + 0.05281 AC + 0.1938 FL − 0.004 AC·FL
The weight is 10 raised to that number.
Worked example
At a scan around 32 weeks: BPD 81 mm, HC 295 mm, AC 280 mm, FL 61 mm. In centimeters: 8.1, 29.5, 28 and 6.1.
log10 EFW = 1.3596 − 0.659288 + 0.1888 + 0.138348 + 1.1872 + 1.0614 = 3.27606, so EFW = 10^3.27606 ≈ 1,888 g (1.89 kg, 4.16 lb).
With only AC and FL the same scan gives about 1,913 g.
Tips
- Enter the measurements exactly as printed; mixing up AC and HC is the most common error.
- One estimate matters less than the growth between scans.
- Questions about the size of your baby are best discussed with your midwife or doctor.
Frequently asked questions
How accurate is the estimated fetal weight?
It is an estimate. Ultrasound measurements are hard to take precisely, and small differences in the abdomen or femur change the result a lot, so the real birth weight can differ by several hundred grams, more so in large babies.
Which Hadlock formula should I use?
The one your ultrasound machine used, if you want to check a report. The four-measurement version (BPD, HC, AC, FL) is a common default; the others help when a head measurement is unreliable, for example because of the baby's position.
Why are the measurements in millimeters?
Ultrasound reports usually print them in millimeters. The formulas themselves use centimeters, so the calculator divides by 10.
Does the calculator give a percentile?
No. A percentile compares the weight with a growth chart for the exact gestational age, and charts differ between countries and hospitals. Your report's percentile comes from the chart your clinic uses.