What is Underweight BMI?

Underweight BMI means an adult has a body mass index below 18.5. BMI, or body mass index, compares your weight with your height and places the result into a standard adult weight category. A low BMI may suggest that your weight is below the usual range for your height.

BMI can be helpful for screening, but it does not explain why your BMI is low. It also does not directly measure body fat, muscle mass, bone density, nutrition status, medical history, or overall health.

Some people naturally have a lower body weight, while others may have a low BMI because of illness, appetite changes, stress, eating difficulties, or other health factors.

An underweight BMI is below 18.5 for adults. It may suggest that your weight is lower than the standard range for your height. BMI is a screening tool, not a diagnosis. A low BMI should be considered with symptoms, eating habits, weight changes, medical history, and guidance from a qualified healthcare professional.

What Is Underweight BMI?

Underweight BMI means your BMI score is below the healthy weight range for adults.

The CDC lists adult BMI categories as underweight, healthy weight, overweight, and obesity. For adults aged 20 and older, a BMI less than 18.5 falls into the underweight category.

The NHS also uses the same general adult BMI range and says that, for most adults, a BMI below 18.5 is in the underweight range.

You can check your BMI here, then compare your result with the BMI categories chart.

What Does a BMI Below 18.5 Mean?

A BMI below 18.5 means your weight is lower than the standard healthy weight range for your height.

For example, a person who is 5’7″ and weighs around 115 pounds has a BMI close to 18.0, which falls below the adult healthy weight range.

But BMI alone cannot tell why someone has a low BMI. It does not show whether a person has low body fat, low muscle mass, a smaller natural body frame, poor nutrition, a medical issue, or recent weight loss.

A low BMI may be more concerning if it comes with:

• Unplanned weight loss
• Low energy
• Feeling weak
• Loss of appetite
• Getting ill often
• Slow recovery from illness
• Irregular periods
• Digestive symptoms
• Hair thinning
• Dizziness
• Feeling cold often

The NHS lists unintentional weight loss, low body weight, lack of interest in eating and drinking, tiredness, weakness, and getting ill often as possible signs linked with malnutrition risk. It also notes that people with a BMI under 18.5 may be at risk of malnutrition.

How Is Underweight BMI Calculated?

BMI is calculated using weight and height.

Metric BMI Formula

Use this formula if weight is in kilograms and height is in meters:

BMI = weight in kg ÷ height in meters²

Example:

A person weighs 50 kg and is 1.70 m tall.

1.70 × 1.70 = 2.89
50 ÷ 2.89 = 17.3

BMI = 17.3

This falls in the underweight BMI range.

Imperial BMI Formula

Use this formula if weight is in pounds and height is in inches:

BMI = weight in pounds ÷ height in inches² × 703

Example:

A person weighs 110 lb and is 5 feet 7 inches tall.

5’7″ = 67 inches
67 × 67 = 4,489
110 ÷ 4,489 × 703 = 17.2

BMI = 17.2

This also falls in the underweight BMI range.

For a full formula breakdown, read How to Calculate BMI or BMI Formula.

Underweight BMI Chart for Adults

Here is a simple adult BMI category chart:

A BMI of 18.4 is underweight.
A BMI of 18.5 is the lower end of the healthy weight range.

This line can look very exact, but real health is not that simple. A BMI of 18.4 and 18.6 may be very close, so symptoms, weight history, nutrition, body composition, and medical context matter.

Possible Causes of Underweight BMI

A low BMI can happen for many reasons. It is not always caused by the same thing.

Possible reasons include:

Natural Body Size

Some adults naturally have a smaller body frame or lower body weight. If weight has been stable for years, appetite is normal, energy is good, and there are no symptoms, the meaning may be different from someone who recently lost weight.

Not Eating Enough Calories

Some people may not eat enough food to meet their daily energy needs. This can happen because of a busy lifestyle, low appetite, food access issues, restrictive eating patterns, or difficulty preparing meals.

Digestive Problems

Some digestive conditions can affect appetite, nutrient absorption, or body weight. Ongoing diarrhea, nausea, pain after eating, or unexplained weight loss should be discussed with a qualified healthcare professional.

Stress or Mental Health Factors

Stress, anxiety, depression, grief, and other mental health concerns can affect appetite and eating patterns. Eating disorders can also lead to low body weight and need professional care.

High Activity Levels

Very active people may burn more energy than they eat. This can happen in athletes, physically demanding jobs, or people with high daily movement.

Medical Conditions or Medicines

Some health conditions and medications can affect weight, appetite, digestion, metabolism, or fluid balance. If your BMI is low and you are not sure why, professional guidance is important.

Mayo Clinic notes that when someone is underweight, it is important to understand the reason, not only focus on gaining weight.

Is Underweight BMI a Health Risk?

An underweight BMI may be linked with health concerns for some adults, but BMI alone does not diagnose a problem.

Being underweight may be associated with:

• Low energy
• Weaker immune function
• Nutrient deficiencies
• Lower muscle mass
• Bone health concerns
• Fertility or menstrual changes
• Slower recovery from illness
• Higher risk of malnutrition in some cases

Cleveland Clinic notes that a BMI below 18.5 should be discussed with a healthcare provider because being underweight can have health risks or may be a sign of an underlying condition.

The important point is this: a low BMI should not be ignored, especially if it is new, unexplained, or connected with symptoms.

What BMI Does Not Show

BMI is limited because it only uses height and weight.

BMI does not directly measure:

• Body fat
• Muscle mass
• Bone density
• Waist size
• Fat distribution
• Nutrition quality
• Fitness level
• Blood pressure
• Blood sugar
• Cholesterol
• Medical history
• Overall health

This is why two adults with the same low BMI can have different health situations.

One person may naturally have a low body weight and feel well. Another may have recent weight loss, poor appetite, or an underlying health issue.

To understand these limits better, read BMI limitations and why BMI can be misleading.

When Should You Be Concerned About a Low BMI?

A low BMI may need more attention if it is paired with symptoms or recent changes.

Consider speaking with a qualified healthcare professional if:

• Your BMI is below 18.5 and you are not sure why
• You have lost weight without trying
• You feel weak, tired, dizzy, or cold often
• Your appetite has changed
• You get sick often or recover slowly
• You have digestive symptoms
• You have missed or irregular periods
• Eating feels stressful or difficult
• You are worried about your relationship with food or body weight
• You have a medical condition or take medicine that affects appetite or weight

The NHS notes that unintentional weight loss of 5% to 10% or more over 3 to 6 months can be one of the main signs of malnutrition.

Safe Next Steps If Your BMI Is Underweight

If your BMI is below 18.5, avoid extreme advice or quick-fix plans. The safest next step is to understand why your BMI is low.

1. Recheck Your BMI

Make sure your height and weight were entered correctly.

Small errors can change your BMI result, especially near the 18.5 cutoff.

2. Look at Recent Weight Changes

Ask yourself:

• Has my weight always been low?
• Did I lose weight recently?
• Was the weight loss intentional or unintentional?
• Has my appetite changed?
• Do I have symptoms?

A stable low BMI may mean something different from sudden weight loss.

3. Review Eating Patterns

Think about whether you are eating enough throughout the day. Some people unintentionally undereat because of stress, routine, low appetite, or limited access to meals.

Mayo Clinic suggests choosing nutritious, calorie-containing foods when trying to gain weight and emphasizes understanding the reason someone is underweight.

4. Avoid Unsafe Weight Gain Advice

Do not rely on junk food, extreme overeating, or unverified supplements. The goal is not just to increase weight. The goal is to support your body with enough energy, protein, vitamins, minerals, and balanced meals.

5. Speak With a Healthcare Professional

A doctor, registered dietitian, or qualified healthcare professional can help check whether low BMI is related to nutrition, digestion, medication, stress, eating difficulties, or another health factor.

How to Support Healthy Weight Gain Safely

Some people with underweight BMI may be advised to gain weight. This should be done carefully.

General supportive habits may include:

• Eating regular meals
• Adding nutritious snacks
• Including protein with meals
• Using healthy calorie additions such as nuts, seeds, avocado, olive oil, yogurt, or nut butters
• Drinking calorie-containing smoothies if appetite is low
• Strength training when appropriate
• Getting help for appetite, digestive, or mental health concerns

This is general education, not a personal plan. If you have medical symptoms, eating disorder concerns, digestive problems, or ongoing weight loss, speak with a qualified healthcare professional before making major changes.

Underweight BMI vs Healthy Weight BMI

Here is how underweight BMI compares with the healthy weight range:

A healthy BMI range does not guarantee perfect health. An underweight BMI does not automatically diagnose poor health. Both categories still need context.

FAQs

What BMI is considered underweight?

For adults, a BMI below 18.5 is considered underweight. A BMI from 18.5 to 24.9 is usually considered healthy weight. BMI is a screening tool, so a low BMI should be understood with symptoms, weight history, nutrition, and medical context.

Is a BMI below 18.5 always unhealthy?

Not always. Some people naturally have a lower body weight. However, a BMI below 18.5 may suggest possible nutrition or health concerns, especially if it is new, unexplained, or linked with symptoms such as fatigue, weakness, appetite changes, or unplanned weight loss.

What causes underweight BMI?

Underweight BMI can be caused by many factors, including natural body size, not eating enough, high activity levels, digestive problems, stress, mental health concerns, eating disorders, medical conditions, or medications. The cause matters, so it is useful to speak with a qualified healthcare professional if you are unsure.

What are the risks of being underweight?

Being underweight may be linked with low energy, nutrient deficiencies, weaker immune function, lower muscle mass, bone health concerns, fertility or menstrual changes, and slower recovery from illness. BMI alone cannot diagnose these issues, but a low BMI may suggest that further assessment is useful.

What should I do if my BMI is underweight?

First, recheck your height and weight to make sure the result is accurate. Then consider whether your weight has changed recently or whether you have symptoms. If your BMI is below 18.5, especially with unplanned weight loss or low appetite, speak with a qualified healthcare professional.

Can BMI be wrong for underweight people?

BMI can be incomplete for underweight people because it does not show body fat, muscle mass, nutrition status, or medical history. A person with a low BMI may have different reasons for that result. BMI should be treated as a screening number, not a final health judgment.

How can I gain weight safely if I am underweight?

Safe weight gain usually means adding enough nutritious calories, protein, and balanced meals rather than relying on low-nutrient foods. A healthcare professional or registered dietitian can help identify the reason for low BMI and suggest a plan that fits your health needs.

Disclaimer
BMIBox content is for educational use only. It does not replace medical advice, diagnosis, or treatment. BMI is a screening tool and does not directly measure body fat, muscle mass, waist size, or overall health. Speak with a qualified healthcare professional if you have concerns about your weight, BMI, or health.

Sources

• CDC: Adult BMI Categories.
• NHS: Obesity and Adult BMI Ranges.
• NHS: Malnutrition Signs and Low BMI.
• Mayo Clinic: Underweight and Healthy Weight Gain.
• Cleveland Clinic: Underweight Symptoms, Causes, and Health Risks.