# Fetal Weight Percentile Calculator (Hadlock)

URL: https://speedycalculator.com/calculators/health/fetal-weight-percentile/
Last updated: 2026-10-30

> The fetal weight percentile calculator estimates fetal weight from ultrasound measurements – head circumference, abdominal circumference and femur length – with the Hadlock formula, and gives the percentile for gestational age.

## Inputs

- Gestational age (weeks)
- Head circumference (HC) (cm, in, m, ft, mm)
- Abdominal circumference (AC) (cm, in, m, ft, mm)
- Femur length (FL) (cm, in, m, ft, mm)

## Results

- Estimated fetal weight
- Average weight at this gestation
- Fetal weight percentile
- Category

## Formula

```
log₁₀ EFW (g) = 1.326 − 0.00326 × AC × FL + 0.0107 × HC + 0.0438 × AC + 0.158 × FL   (cm)
Mean weight for GA: ln W = 0.578 + 0.332 × GA − 0.00354 × GA²; SD ≈ 12.7% of the mean
Percentile = Φ((EFW − mean) ÷ SD)
```

## Explanation

Estimated fetal weight below the 10th percentile defines a small-for-gestational-age (SGA) baby and may signal fetal growth restriction; above the 90th percentile is large-for-gestational-age. Ultrasound estimates can be off by 10–15%.

### How to use the Fetal Weight Percentile Calculator

- **Gestational age**
- **Head circumference (HC)**
- **Abdominal circumference (AC)**
- **Femur length (FL)**

Results update as you type, and every field accepts the units shown in its menu.

### How it works

Enter the gestational age and the three ultrasound measurements.

### Tips

- Measure lying length for babies under 2 and standing height from age 2.
- Use the exact age in months for babies – a few weeks matters.
- Follow the trend across several visits; crossing two major percentile lines is more important than one value.
- Discuss concerns with your paediatrician.

### Common mistakes to avoid

- Comparing a baby with the wrong sex chart.
- Reading the 50th percentile as "normal" and everything else as abnormal.
- Using corrected age incorrectly for premature babies.

### Inputs explained

FieldWhat to enterGestational ageIn weeksHead circumference (HC)Units: cm, in, m, ft, mmAbdominal circumference (AC)Units: cm, in, m, ft, mmFemur length (FL)Units: cm, in, m, ft, mm

### What each result means

- **Estimated fetal weight** = 10^(1.326 - 0.00326×[Abdominal circumference (AC)]×[Femur length (FL)] + 0.0107×[Head circumference (HC)] + 0.0438×[Abdominal circumference (AC)] + 0.158×[Femur length (FL)])
- **Average weight at this gestation** = exp(0.578 + 0.332×[Gestational age] - 0.00354×[Gestational age]^2)
- **Fetal weight percentile** = ncdf(([Estimated fetal weight] - [Average weight at this gestation])÷(0.127×[Average weight at this gestation]))×100

*Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.*

## Worked example

Inputs — Gestational age: 30 weeks; Head circumference (HC): 27.5 cm; Abdominal circumference (AC): 25.5 cm; Femur length (FL): 5.7 cm.

Results — Estimated fetal weight = 1,459 g; Average weight at this gestation = 1,559 g; **Fetal weight percentile = 31 %**; Category = Appropriate for gestational age.

## Key terms

- **EFW**: Estimated fetal weight.
- **SGA/LGA**: Small/large for gestational age.

## FAQ

**What percentile is small for a baby?**
Below the 10th percentile is small for gestational age.

**How accurate is estimated fetal weight?**
Usually within 10–15% of the actual birth weight.

**Is a small baby always unhealthy?**
No – many are constitutionally small; Doppler and growth over time help tell the difference.


## Sources

- [Hadlock FP et al. Estimation of fetal weight with the use of head, body, and femur measurements. Am J Obstet Gynecol, 1985](https://pubmed.ncbi.nlm.nih.gov/3881966/)
- [Hadlock FP et al. In utero analysis of fetal growth: a sonographic weight standard. Radiology, 1991](https://pubmed.ncbi.nlm.nih.gov/1887034/)