What BMI is considered underweight for adults?

For adults, BMI below 18.5 is commonly categorized as underweight. This is a screening category, not a diagnosis.

Does this calculator prescribe a weight-gain plan?

No. It estimates BMI, a healthy-weight range, and calorie context. Individual weight-gain planning depends on appetite, symptoms, medical history, activity, and clinician guidance when needed.

Can teens use this calculator?

No. Children and teens use BMI-for-age percentiles rather than fixed adult BMI thresholds.

Underweight Calculator

Check adult underweight status from BMI, healthy-weight range, gain needed to reach BMI 18.5, and cautious calorie context.

Adult Underweight Screening

This calculator uses adult BMI categories to show whether weight is below the common healthy-weight range. It also estimates the weight gain needed to reach BMI 18.5 and a mid-range reference point.

BMI = \frac{weight _{k g}}{height _{m}^{2}}

Adult body mass index formula.

weight_{18.5} = 18.5 \times height_{m}^{2}

Weight at the lower adult healthy-weight boundary.

How To Use The Result

BMI is a screening measure. It does not explain why someone is underweight and does not replace care for unintended weight loss, eating disorder symptoms, pregnancy, illness, or malnutrition risk.

Screening only

BMI below 16, rapid unintentional weight loss, weakness, digestive symptoms, or concern about eating behavior should be discussed with a qualified clinician.

Frequently Asked Questions

Sources and References

Calculations are based on the listed reference sources. Links open in a new tab.

Updated: June 2, 2026

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