Calculate abdominal circumference, head circumference, or ratio from any two inputs in cm or inches with this body measurement calculator.
Medical note: Educational use only; not a diagnostic tool. Use measurements from a clinician (or values listed on an ultrasound/medical report). Interpretation depends on age/gestational age, measurement technique, and clinical reference ranges—please discuss results with an OB-GYN, pediatric clinician, or other qualified healthcare professional.
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Abdominal Circumference To Head Circumference Ratio Formula
The abdominal circumference to head circumference ratio compares abdominal circumference with head circumference using the same length unit for both measurements.
Ratio = AC / HC
AC = HC * Ratio
HC = AC / Ratio
- Ratio = abdominal circumference to head circumference ratio, unitless
- AC = abdominal circumference
- HC = head circumference
The calculator lets you enter any two values and solves for the missing one:
- Find ratio: enter abdominal circumference and head circumference.
- Find abdominal circumference: enter head circumference and the ratio.
- Find head circumference: enter abdominal circumference and the ratio.
If you use different units, such as inches for one measurement and centimeters for the other, the calculator converts the length values to centimeters before applying the formula. The final circumference result is converted back to the unit selected for that field.
Common AC/HC Ratio Reference Points
AC/HC ratio values are usually interpreted with gestational age specific reference charts, not by one fixed cutoff. The table below gives a simple way to read the number before comparing it with the appropriate clinical reference range.
| Ratio result | What it means mathematically | Basic interpretation |
|---|---|---|
| Less than 1.00 | AC is smaller than HC | Common in many fetal measurements, depending on gestational age. |
| 1.00 | AC equals HC | The two circumferences are the same size. |
| Greater than 1.00 | AC is larger than HC | May be normal or abnormal depending on gestational age and the full ultrasound findings. |
Length Unit Conversion Used
| Unit | Centimeter equivalent | Example |
|---|---|---|
| Centimeters | 1 cm = 1 cm | 30 cm stays 30 cm |
| Inches | 1 in = 2.54 cm | 12 in = 30.48 cm |
Example Calculations
Example 1: Calculate the AC/HC ratio
Suppose abdominal circumference is 28.5 cm and head circumference is 31.0 cm.
Ratio = AC / HC
Ratio = 28.5 / 31.0 = 0.9194
The AC/HC ratio is 0.9194.
Example 2: Calculate abdominal circumference
Suppose head circumference is 32.0 cm and the ratio is 0.95.
AC = HC * Ratio
AC = 32.0 * 0.95 = 30.4 cm
The abdominal circumference is 30.4 cm.
FAQ
Is the AC/HC ratio unitless?
Yes. The ratio is unitless because it divides one length by another length. The units cancel out, as long as both measurements are converted to the same unit before division.
Can I enter one measurement in inches and the other in centimeters?
Yes. The calculator converts both circumference values to centimeters for the calculation. For example, if head circumference is entered in inches, it is multiplied by 2.54 before the ratio is calculated.
Does the AC/HC ratio diagnose a medical condition?
No. The ratio is only one measurement comparison. In prenatal ultrasound, it should be interpreted with gestational age, measurement quality, growth charts, estimated fetal weight, and other clinical findings. Ask a qualified clinician to interpret medical results.
