Fetal Weight Calculator
Four Ultrasound Measurements, One Weight Estimate
An ultrasound can't put a baby on a scale, so estimated fetal weight is built instead from four linear measurements — head, abdomen, and thigh bone dimensions — fed through a formula derived from measuring thousands of babies at birth against their last prenatal scan. The Hadlock formula, published in 1985, is still the reference standard used across obstetric ultrasound today, and it's the formula behind this calculator.
The Formula
EFW (grams) = 10log&sub10;(EFW)
BPD is biparietal diameter (the head's side-to-side width), HC is head circumference, AC is abdominal circumference, and FL is femur length — all measured in centimeters. This is the 4-parameter Hadlock formula, generally considered the most accurate of the several Hadlock variants because it draws on both head and body measurements.
How Sensitive the Estimate Is to Each Measurement
Holding BPD at 8.5 cm, HC at 30.0 cm, and FL at 6.5 cm fixed, and changing only AC:
| AC | Estimated weight |
|---|---|
| 24 cm | 1,669.1 g (3 lb 10.9 oz) |
| 27 cm | 1,949.7 g (4 lb 4.8 oz) |
| 30 cm | 2,277.5 g (5 lb 0.3 oz) |
| 33 cm | 2,660.3 g (5 lb 13.8 oz) |
| 36 cm | 3,107.6 g (6 lb 13.6 oz) |
Computed directly with the formula above. Notice how much weight tracks abdominal circumference specifically — AC carries the heaviest coefficients in the formula (both directly and through the BPD×AC and AC×FL cross-terms), which is why sonographers take particular care measuring it.
Why the Estimate Has a Margin of Error
Published accuracy for the 4-parameter Hadlock formula runs around ±15% of actual birth weight for most pregnancies — tighter for babies in the normal weight range, wider at the extremes. A single scan's estimate is a useful data point, not a guarantee, which is exactly why it's tracked across multiple visits rather than read as a final answer.
Where This Estimate Is Used
- Growth trend monitoring — a single EFW matters less than how it compares to the prior scan and to gestational-age norms.
- Delivery planning — unusually high or low estimates factor into decisions about delivery timing and method.
- Cross-checking a report — if you have the four raw measurements from a scan report, this lets you verify the EFW your sonographer calculated.
How to Use This Calculator
- Enter BPD — Biparietal Diameter, in centimeters.
- Enter HC — Head Circumference, in centimeters.
- Enter AC — Abdominal Circumference, in centimeters.
- Enter FL — Femur Length, in centimeters.
- Select Calculate for the estimated fetal weight in grams and pounds/ounces.
Related Calculations
To see how far along the pregnancy is at the time of the scan, use the Pregnancy Week Calculator. For tracking your own weight gain across pregnancy, see the Pregnancy Weight Gain Calculator.