BMI Calculator for Weight Gain
Wondering if you're underweight? Enter your height and weight (in metric or imperial) to get your BMI and your WHO weight category. If you're underweight or at the low end of healthy, the tool also shows exactly how much weight you'd need to add to reach a healthy BMI of 18.5 and 20.0.
Your BMI
WHO BMI categories
| BMI range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
What BMI tells you — and what it doesn't
Body Mass Index (BMI) is your weight in kilograms divided by your height in metres squared. The World Health Organization and national health bodies like the NHS and CDC use it as a quick screening tool to flag people who may be underweight, overweight, or obese. For weight gain, the number that matters most is the 18.5 cutoff: a BMI below 18.5 is classified as underweight, which can signal that you are not eating enough, are losing muscle, or have an underlying health issue worth investigating.
How this calculator works
The tool converts your height to metres and your weight to kilograms, then computes BMI = kg ÷ m² and rounds it to one decimal place. It compares the result to the WHO cutoffs (underweight below 18.5, healthy 18.5–24.9, overweight 25–29.9, obese 30 and above). If you fall in the underweight or low-healthy range, it back-calculates the body weight you would need to reach a BMI of 18.5 and a more comfortable 20.0, using target weight = BMI × height².
A worked example
Take someone 178 cm tall weighing 55 kg. In metres that is 1.78 m, so height² = 1.78 × 1.78 = 3.1684 m². BMI = 55 ÷ 3.1684 = 17.36, which rounds to 17.4 — underweight. To reach a BMI of 18.5, target weight = 18.5 × 3.1684 = 58.6 kg (about 129 lb), so this person would need to add roughly 3.6 kg (~8 lb). To reach a sturdier BMI of 20.0, target weight = 20.0 × 3.1684 = 63.4 kg (about 140 lb).
If you're underweight
Being underweight is associated with lower bone density, weaker immunity, fatigue, and in some cases fertility and hormonal issues. The fix is usually a sustained calorie surplus built from nutrient-dense foods, paired with resistance training so the new weight is muscle rather than just fat. But because a low BMI can also be a symptom of a medical condition, the right first step is a check-up. Use a calorie surplus calculator to set your daily target once you have the all-clear.
Sources: World Health Organization — BMI classification · NHS — Underweight adults · CDC — About Adult BMI · NIH / NIDDK Weight Management.
Frequently asked questions
- What BMI is considered underweight?
- The WHO, NHS, and CDC all classify a BMI below 18.5 as underweight. A BMI of 18.5 to 24.9 is the healthy range, 25 to 29.9 is overweight, and 30 or above is obese.
- How much weight do I need to gain to be a healthy BMI?
- Enough to bring your BMI to at least 18.5. Target weight = 18.5 × your height in metres squared. The calculator above does this for you and also shows the weight for a BMI of 20.0.
- Is BMI accurate for muscular people?
- Not always. BMI does not distinguish muscle from fat, so very muscular athletes can read as "overweight" while carrying little fat. It is a population screening tool, best paired with measures like waist circumference or body-fat percentage.
- Can I be skinny but still have a healthy BMI?
- Yes. Many naturally lean people sit comfortably within 18.5–24.9. If your BMI is healthy but you want to look more muscular, the goal is a small surplus plus resistance training rather than chasing a number on the scale.
- Should I see a doctor if I'm underweight?
- Yes. A BMI under 18.5 can be a sign of an underlying medical issue, so it is worth a check-up before you start a bulking plan. A doctor or registered dietitian can rule out causes and help you gain weight safely.