What BMI Is Underweight?
For most adults, a BMI below 18.5 is considered underweight.
Underweight
Underweight BMI Calculator
This underweight BMI calculator helps you see whether your BMI falls below the commonly accepted healthy range.
Your BMI Result
Fill in your height and weight, then click calculate to view your BMI result and healthy weight range.
Underweight BMI Formula
Metric: BMI = weight (kg) ÷ height² (m²)
Imperial: BMI = (weight (lbs) ÷ height² (in²)) × 703
Underweight BMI Categories
| Category | BMI Range | Health Risk |
|---|---|---|
| Underweight | < 18.5 | Possible nutritional deficiency |
| Normal Weight | 18.5 – 24.9 | Low risk |
| Overweight | 25 – 29.9 | Moderate risk |
| Obese | ≥ 30 | High risk |
Frequently Asked Questions
The Body Mass Index (BMI) is a screening tool used to identify whether an adult is underweight, at a healthy weight, overweight, or obese. It is calculated by dividing a person’s weight in kilograms by the square of their height in meters.
A BMI below 18.5 is generally classified as underweight for adults.
Not always, but it can indicate nutritional or medical concerns and should be considered in context.
If you are concerned about being underweight, it is a good idea to speak with a healthcare professional.
Explore Similar Calculators
Browse related calculator variations and find the version that matches your preferred units or use case.
Overweight BMI Calculator
Check whether your BMI falls in the overweight range.
Open calculator
Body Mass Index Calculator
Estimate body mass index instantly using height and weight.
Open calculator
BMI Calculator in lbs and inches
Use imperial units to calculate BMI with pounds and inches.
Open calculator
Imperial BMI Calculator
Calculate BMI using pounds and inches.
Open calculator
Check Out Other Calculators
Age Calculator
Calculate your exact age in years, months, and days from your date of birth.
Open calculator
Percentage Calculator
Solve percentage-based problems quickly, including increases, decreases, and proportions.
Open calculator
Discount Calculator
Figure out final prices and total savings when discounts are applied.
Open calculator
Date Calculator
Calculate the duration between two dates for planning and timeline management.
Open calculator