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.
Check whether an adult BMI is below 18.5.
See how far a 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
BMI about 18.4
BMI about 18.5
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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 a 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 the body, activity, date, or lab values exactly in the units shown on the page. Height, weight, age, sex, time, and activity level can change health estimates a lot, so treat each label like a rule instead of a suggestion. If you are unsure which option fits, choose the closest honest match and read the result as a rough estimate.
In plain language: BMI is weight in kilograms divided by height in meters squared. The calculator compares the result with the adult underweight threshold of BMI less than 18.5. Read the result together with the notes on the page, because health and fitness numbers often need personal context.
Use the result as a learning number, not a final answer about your body or health. The supporting lines can show categories, ranges, calories, dates, or targets, but those numbers still need context like age, medical history, pregnancy status, training level, and advice from a qualified professional.
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 the units, date, and personal details before reading the answer. For example, pounds and kilograms, inches and centimeters, or a wrong activity level can change the result quickly. If the number feels surprising, rerun it slowly and compare it with the examples.
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.