How to Use MUAC to Calculate BMI and Assess Nutritional Health

October 20, 2025 •

2 min read

According to the National Institutes of Health, Mid-Upper Arm Circumference (MUAC) measurement is a viable alternative to Body Mass Index (BMI) for nutritional assessment, especially in situations where a person cannot be weighed or measured for height. This guide explains how to use MUAC to calculate BMI indirectly, detailing the process, interpretation, and its role as a rapid screening tool.

Quick Summary

This guide outlines the practical steps for measuring Mid-Upper Arm Circumference (MUAC) and interpreting the results to estimate Body Mass Index (BMI) category. It covers the clinical scenarios where MUAC is a necessary alternative, explores the accuracy and limitations of this correlation, and provides a clear breakdown of established MUAC cut-off points.

Key Points

MUAC is a screening tool: It provides an estimate of a BMI category, not an exact BMI score.
Measurement is simple: Find the midpoint between the shoulder bone and elbow tip on the non-dominant arm and measure the circumference.
Specific cut-offs indicate nutritional status: MUAC < 23.5 cm suggests a BMI < 20 kg/m² (likely underweight), while > 32.0 cm suggests a BMI > 30 kg/m² (likely obese).
Ideal for challenging situations: Use MUAC when measuring height and weight is difficult, such as with bed-ridden patients, those with oedema, or in low-resource areas.
Accuracy is population-dependent: Optimal MUAC cut-offs can vary by ethnicity and gender, and may require local validation for precise use.
Doesn't replace BMI entirely: It is less reliable for muscular individuals and should be followed up with more detailed assessments where possible.

What is MUAC and Why Is It Used for BMI?

Mid-Upper Arm Circumference (MUAC) is a simple measurement used for nutritional screening, especially when standard BMI measurements (weight and height) are difficult. MUAC correlates strongly with BMI.

MUAC is particularly useful in situations such as:

Critically ill or bed-ridden patients.
Individuals with physical disabilities or conditions affecting height measurement.
Patients with fluid retention.
Community nutritional surveys.

Step-by-Step Guide to Measuring MUAC

Measuring MUAC is straightforward.

Preparation: Use a non-stretchable tape on the patient's left arm, if possible. Remove loose clothing.
Locate Mid-Point: With the arm bent at 90 degrees, find the midpoint between the shoulder bone and elbow tip.
Mark the Spot: Mark this midpoint.
Take the Measurement: With the arm relaxed at the side, wrap the tape snugly around the marked midpoint, without pinching the skin.
Read and Record: Read the measurement in centimeters to the nearest 0.1 cm.

Interpreting MUAC Measurements with Established BMI Cut-offs

MUAC cut-off points can reliably indicate nutritional status categories like underweight and obesity.

MUAC-to-BMI Interpretation Table for Adults (General Guidelines)


MUAC Measurement	Corresponding BMI Category (Likely)	Nutritional Status Interpretation
< 23.5 cm	< 20 kg/m²	Likely Underweight/Nutritionally at Risk
23.5 cm - 32.0 cm	20 kg/m² - 30 kg/m²	Likely Normal Weight to Overweight
> 32.0 cm	> 30 kg/m²	Likely Obese

Note that specific populations may have different cut-offs; for instance, a study in Sudan used ≤ 25.5 cm for underweight detection in adults.

Limitations of Using MUAC to Calculate BMI

MUAC has limitations compared to direct BMI calculation.

Approximation: It provides an estimate of a BMI category, not a precise number.
Doesn't Differentiate Fat and Muscle: Like BMI, it doesn't distinguish between fat and muscle mass, making it less reliable for very muscular individuals.
Population Specificity: Cut-off accuracy varies between populations due to differences in body composition, gender, and age. Age- and sex-specific cut-offs can improve accuracy.

Why and When MUAC is the Superior Choice

MUAC is often superior in many settings due to its simplicity and low cost, making it ideal for mass screenings in developing countries or emergencies. It is less affected by acute illness, fluid retention, and physical disabilities than weight-based measurements. MUAC is also useful for monitoring nutritional changes over time.

Conclusion: MUAC's Role in Nutritional Assessment

MUAC is a practical and effective alternative for nutritional health screening when direct BMI calculation isn't feasible. Its correlation with BMI, ease of measurement, and cost-effectiveness make it a valuable tool for healthcare providers. Using standard MUAC cut-offs helps identify individuals who may be underweight or obese, enabling timely intervention. Understanding its strengths and limitations ensures appropriate use for public health. For more detailed information on nutrition screening, resources like the BAPEN MUST explanatory booklet can be consulted.

Frequently Asked Questions

No, MUAC is an excellent proxy and screening tool but does not provide an exact BMI score. It is an effective alternative when BMI measurements are impractical, offering a reliable estimate of nutritional status.

A MUAC measurement below 23.5 cm is often used as a cut-off point to indicate that an adult is likely to have a BMI of less than 20 kg/m², suggesting a risk of undernutrition.

Research shows MUAC has good accuracy for predicting obesity, often with a cut-off of > 32.0 cm corresponding to a BMI > 30 kg/m². However, precise cut-offs can vary between different studies and populations.

MUAC does not differentiate between fat and muscle mass, a limitation it shares with BMI. Therefore, it may not be an accurate nutritional indicator for very muscular athletes.

Measuring MUAC requires only a flexible, non-stretchable measuring tape. Specialized color-coded MUAC tapes are also available for simplified field screening.

Yes, MUAC is particularly useful for patients with oedema (fluid retention) or ascites, as their weight measurements would be unreliable. MUAC provides a measurement of the arm's circumference that is less affected by these conditions.

Yes, MUAC is widely used for nutritional screening in children, especially in younger age groups. Specific age- and sex-based cut-offs are applied for interpreting results in pediatric populations.