Underweight BMI Calculator (Weight Gain)
A specialized health tool for individuals with a BMI under 18.5 to calculate the caloric surplus needed to reach a healthy weight safely.
Recommended Intake:
This includes a +500 kcal surplus to gain ~0.5kg/week.
About
This tool addresses clinical underweight status by calculating the specific caloric surplus required to restore body mass to a healthy range. A Body Mass Index (BMI) below 18.5 is associated with compromised immune function, osteoporosis, and hormonal irregularities. The calculator determines the exact weight gain needed to cross the World Health Organization's threshold for "Normal Weight" and the "Ideal" midrange. It then generates a nutritional strategy focused on controlled anabolic gain rather than rapid fat accumulation. This promotes the restoration of lean tissue alongside essential adipose reserves.
Formulas
The calculation reverses the BMI formula to find the target weight and applies an energy surplus.
Target Weight:
Wtarget = 18.5 × H2
Daily Calories for Gain:
Ctotal = TDEE + Ssurplus
Where H is height in meters. The surplus S is typically 300 to 500 kcal.
Reference Data
| BMI Range | Classification | Health Risk | Action Required |
|---|---|---|---|
| < 16.0 | Severe Thinness | Critical | Medical Intervention |
| 16.0 - 16.9 | Moderate Thinness | High | Supervised Gain |
| 17.0 - 18.4 | Mild Thinness | Moderate | Caloric Surplus |
| 18.5 - 24.9 | Normal Range | Low | Maintenance |
| Target Gain | +0.5 kg/wk | Safe | +500 kcal/day |
| Rapid Gain | +1.0 kg/wk | Hard | +1000 kcal/day |