What is the cutoff for low BMI?

December 26, 2025 •

3 min read

According to health data, a body mass index (BMI) below 18.5 is officially the cutoff for low BMI, placing an adult in the underweight category. This metric, widely used by health professionals, is a simple screening tool to identify potential health risks related to being underweight.

Quick Summary

The standard cutoff for an adult to be considered underweight is a BMI below 18.5. This classification is used to screen for potential health issues and does not offer a complete picture of an individual's health.

Key Points

Standard Cutoff: The World Health Organization (WHO) and Centers for Disease Control and Prevention (CDC) set the underweight cutoff for adults at a Body Mass Index (BMI) below 18.5.
BMI Calculation: BMI is determined by dividing a person's weight in kilograms by the square of their height in meters ($$kg/m^2$$).
Health Risks: Low BMI can lead to health problems, including a weakened immune system, osteoporosis, anemia, and fertility issues.
Causes: Factors such as genetics, a high metabolism, underlying medical conditions (like thyroid issues or eating disorders), and certain medications can cause a low BMI.
Beyond the Number: BMI is a screening tool, not a perfect measure. Factors like muscle mass, age, and ethnicity are not fully accounted for, so it's essential to consider a person's overall health picture.
Professional Guidance: If concerned about low BMI, consulting a healthcare provider is recommended to identify the cause and create a safe, effective plan for weight management.
Weight Gain Strategies: Healthy strategies for gaining weight include increasing the intake of nutrient-dense, calorie-dense foods and incorporating strength training exercises.
Child BMI: The underweight cutoff for children and teens is a BMI below the 5th percentile, which is based on age- and sex-specific growth charts.

Understanding the Standard Cutoff for Low BMI

The World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) utilize a Body Mass Index (BMI) of less than 18.5 to define underweight in adults. This benchmark is a critical starting point for healthcare providers to evaluate an individual's nutritional status and potential health risks associated with a low body weight. It's important to recognize that this standard cutoff is a general measure and does not fully account for variations in body composition, such as muscle mass and bone density. For example, a highly muscular individual might register a low BMI while being in excellent health, while a less active person with the same BMI could be undernourished.

How Is BMI Calculated?

The calculation of BMI is straightforward, requiring only your weight and height to produce a single numerical value. The standard metric formula is weight in kilograms divided by the square of height in meters ($$BMI = weight(kg) / height(m)^2$$). For those using imperial units, the calculation is weight in pounds divided by height in inches squared, with the result multiplied by 703 ($$BMI = (weight(lb) / height(in)^2) * 703$$). While online tools are readily available for this calculation, understanding the process can be helpful. As an illustration, an individual weighing 70 kg with a height of 1.75 m would have a BMI of approximately 22.9, falling within the healthy weight range.

Health Implications of a Low BMI

Having a low BMI is linked to various health complications, primarily stemming from insufficient nutrient intake. Learn more about the potential health issues and factors that influence a low BMI on the {Link: American Heart Association website https://www.heart.org/en/healthy-living/healthy-eating/losing-weight/bmi-in-adults}.

Low BMI vs. Healthy Weight: A Comparison

Understanding the contrast between a low BMI and a healthy weight provides further context for the standard cutoff. Below is a comparison of key aspects:


Feature	Low BMI (< 18.5)	Healthy Weight (18.5–24.9)
Nutritional Status	Often indicates insufficient nutrient intake.	Generally reflects adequate nutrient intake.
Immune System	Can result in a weakened immune response.	Associated with normal immune function.
Bone Health	Increased risk of osteoporosis and fractures.	Lower risk of bone density issues.
Energy Levels	Often linked to fatigue and low energy.	Generally associated with stable energy.
Fertility	Can negatively impact fertility in women.	Typically associated with normal fertility.
Metabolism	High metabolic rate may be a factor.	A more balanced metabolic rate is common.

Conclusion

The standard cutoff for low BMI in adults is less than 18.5 kg/m2, which serves as a general indicator for being underweight. This figure is derived from population-based data and functions as a screening tool rather than a definitive health assessment. A low BMI can signal an increased risk for various health concerns, including malnutrition, a compromised immune system, and potential fertility issues. However, it's crucial to consider that factors like significant muscle mass, genetic predisposition, and ethnicity can influence BMI. Therefore, a comprehensive health evaluation by a medical professional is always recommended if you are concerned about your BMI. Consulting with a doctor or dietitian can help determine the appropriate course of action for your individual health requirements. For those aiming to gain weight, incorporating nutrient-dense foods and engaging in strength training can be beneficial strategies.

What is the cutoff for low BMI? Keypoints

Learn more about the keypoints regarding the cutoff for low BMI on the {Link: Inspira Health Network website https://www.inspirahealthnetwork.org/news/healthy-living/skinny-being-underweight-it-concern}.

FAQs

Learn more about frequently asked questions regarding the cutoff for low BMI on the {Link: American Heart Association website https://www.heart.org/en/healthy-living/healthy-eating/losing-weight/bmi-in-adults}.

Frequently Asked Questions

Not necessarily. While a low BMI can indicate health risks like malnutrition, it can also be a result of genetics or high muscle mass in athletes. A doctor should evaluate the full health context.

Key risks include a weakened immune system, weakened bones (osteoporosis), anemia, and hormonal imbalances that can affect fertility.

For younger individuals, BMI is measured using age- and sex-specific percentile growth charts, rather than a fixed cutoff point like for adults. A BMI below the 5th percentile is considered underweight for children and teens.

Yes, because BMI is based on total weight, people with a high amount of muscle mass (which is denser than fat) may have a high BMI even if they have little body fat. This is a key limitation of BMI as a standalone health measure.

The first step is to consult a healthcare provider. They can help identify any underlying causes for the low weight and develop a healthy plan for weight gain, if necessary.

Yes, a significantly low BMI can be a sign of underlying conditions such as an overactive thyroid, gastrointestinal disorders, or an eating disorder like anorexia nervosa.

Some health organizations recognize that different populations, particularly those of Asian descent, may be at higher health risk at lower BMI levels and may use slightly adjusted cutoffs.

Healthy weight gain focuses on increasing calorie intake from nutrient-dense foods, eating more frequently, and incorporating strength training exercises to build muscle mass.

According to the World Health Organization, a BMI below 17.0 indicates moderate and severe thinness, while a BMI below 16.0 is considered severe thinness.

A low BMI often suggests undernutrition, meaning the body isn't getting enough calories or essential nutrients, which can lead to various health problems.

Yes, stress and mental health conditions like depression can suppress appetite and lead to unintentional weight loss, contributing to a lower BMI.