Use the Fetal (Ultrasound) tab to enter gestational age and fetal abdominal circumference (AC) to estimate the percentile from reference growth charts. Use the Adult Waist tab to calculate waist-to-height ratio (WHtR) and a waist-circumference risk category.

Abdominal Circumference Percentile Calculator

Fetal (Ultrasound)
Adult Waist

Enter gestational age and abdominal circumference to get the fetal percentile.

Data sources: WHO fetal growth references (abdominal circumference percentiles by gestational age), plus examples of fetal biometry calculators (Perinatology, FMF). Adult waist cut points shown (men 94/102 cm, women 80/88 cm) are commonly used screening thresholds in some guidelines. See: WHO fetal growth references, Perinatology fetal biometry, FMF biometry, WHO waist guidance summary.

Related Calculators

Abdominal Circumference Percentile Formula

Abdominal circumference (AC) percentiles are not calculated from a single universal algebraic equation. Instead, percentiles are obtained by comparing a measurement to a reference chart (such as the WHO fetal growth references) at the relevant gestational age, often using interpolation between chart values. For adults, this calculator reports risk screening metrics (WHtR and waist cut points), not a percentile.

GA = w + \frac{d}{7}
p \approx p_i + (p_{i+1}-p_i)\frac{AC-AC_{p_i}(GA)}{AC_{p_{i+1}}(GA)-AC_{p_i}(GA)}
WHtR = \frac{\text{waist}}{\text{height}}

Variables:

  • AC is abdominal circumference (fetal ultrasound biometry) or waist circumference (adult), in any unit (the calculator converts units internally).
  • GA is gestational age in weeks (w) and days (d) for the fetal percentile calculation.
  • p is the estimated percentile from the fetal reference chart; WHtR is the adult waist-to-height ratio (use the same units for waist and height).

To use the calculator, enter the requested measurements for the selected tab. The fetal tool looks up and interpolates reference values by gestational age to estimate a percentile, and the adult tool calculates WHtR and waist-based risk categories.

What is Abdominal Circumference?

Abdominal circumference can refer to different measurements depending on context:

Fetal (ultrasound): fetal abdominal circumference (AC) is a standard fetal biometry measurement taken on a specific transverse abdominal view (used in fetal growth assessment).

Adult: abdominal/waist circumference is the distance around the abdomen measured with a tape. Common protocols include measuring at the midpoint between the lowest rib and the top of the iliac crest (often used by WHO) or at the top of the iliac crest (often used in U.S. guidance). Because technique affects the number, it’s best to measure consistently using the same protocol each time.

How to Calculate Abdominal Circumference Percentile?

The following steps outline how to estimate the fetal abdominal circumference percentile (and how the adult tab works).

  1. Select the appropriate tab: Fetal (Ultrasound) or Adult Waist.
  2. Fetal: choose gestational age (weeks and days) and enter fetal abdominal circumference (AC) with the correct unit.
  3. Click Calculate to get an estimated fetal percentile and reference values (10th/50th/90th) at that gestational age.
  4. Adult: enter waist circumference and height (with units) and select sex to view WHtR and a waist-based risk category.
  5. Interpret adult waist cut points with caution: guideline thresholds can differ by ethnicity and clinical context.
  6. Use Reset to clear all fields.

Example Problem : 

Use the following example to test your understanding (fetal tab).

Gestational Age (GA) = 24w 0d

Fetal Abdominal Circumference (AC) = 197 mm

Estimated Percentile (p) ≈ 50% (because the WHO reference 50th percentile at 24 weeks is 197 mm)