Use this free underweight BMI calculator to compare adult BMI with the BMI 18.5 screening threshold and healthy BMI reference range.
50 kg at 170 cm
BMI cannot diagnose anorexia or any eating disorder. If food, weight, exercise, or body image feels hard to control, talk with a qualified health professional.
Recent underweight BMI checks will appear here.
BMI is a screening estimate. It cannot diagnose anorexia, malnutrition, or any eating disorder.
Inputs and recent answers stay in this browser tab and are not sent to a server.
Check whether an adult BMI is below 18.5.
See how far the entered weight is from the BMI 18.5 reference threshold.
Read why BMI alone cannot diagnose an eating disorder.
Use a safer alternative to harmful anorexic-BMI style pages.
BMI about 17.3; about 3.5 kg to BMI 18.5
BMI about 18.4; about 0.4 kg to BMI 18.5
BMI about 18.5; already at the 18.5 boundary
BMI about 18.7; above the underweight threshold
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Plain-language answers about when to use the estimate, what the formula means, what it cannot decide for you, and how privacy works.
Use it for simple educational checks, trend tracking, or planning tasks like these: Check whether an adult BMI is below 18.5. See how far the entered weight is from the BMI 18.5 reference threshold. It can help you understand a number, but it cannot explain your whole health situation.
Enter adult height in centimeters and weight in kilograms. The calculator uses those two numbers only, so a pounds-versus-kilograms or inches-versus-centimeters mix-up can change the BMI category and the distance to the 18.5 threshold.
In plain language: BMI is weight in kilograms divided by height in meters squared. The calculator compares the result with the adult underweight threshold below 18.5 and shows the BMI 18.5 to 24.9 reference range for the same height. Read the result together with the notes on the page, because health and fitness numbers often need personal context.
Read the answer as an adult BMI screening check, not a diagnosis or a personal weight goal. The category line compares BMI with the underweight threshold below 18.5, and the "To BMI 18.5" line shows how far the entered weight is from that reference boundary. Real health context still depends on symptoms, history, age, pregnancy status, eating patterns, medications, and professional care.
No. BMI below 18.5 is an adult screening category, not an anorexia diagnosis. Eating disorders are diagnosed from a bigger clinical picture, including behavior, distress, medical signs, and professional assessment.
It shows the difference between the entered weight and the weight that would put the same height at BMI 18.5. Treat it as a reference boundary, not a personal target or treatment plan.
No. Children and teens use BMI-for-age percentiles, pregnancy changes weight for a different reason, and athletes may have body composition that BMI cannot explain. Use qualified guidance for those situations.
A calculator is not enough if eating, exercise, body image, weight change, dizziness, fainting, missed periods, chest pain, or weakness feels hard to manage. Use qualified medical or mental-health support instead of relying on a BMI label.
BMI alone cannot diagnose anorexia or any eating disorder, and turning a low BMI into a target can be harmful. This page uses underweight-screening language and points back to professional support.
Yes. Eating disorders and nutrition problems can exist at many body sizes. If eating, restriction, purging, over-exercise, or body image is causing distress, the BMI number should not be used as reassurance by itself.
BMI cannot diagnose anorexia, malnutrition, or any eating disorder. If eating, weight, exercise, or body image feels hard to control, talk with a qualified health professional. Use the calculator as a learning tool, then ask a qualified professional about decisions that affect care, pregnancy, medication, nutrition, or safety.
Check that height is in centimeters, weight is in kilograms, and the page is being used for an adult BMI screen. Do not use the "To BMI 18.5" line as a self-directed weight target, and do not use BMI to diagnose anorexia, malnutrition, or recovery status.
No. The calculator runs in your browser tab. Recent answers stay only on the page while you use it, and they are not sent to a server.