BMI sounds technical, but the formula is actually straightforward — and you can work it out by hand with nothing more than basic arithmetic. Whether you prefer metric or imperial measurements, here's the exact method, step by step, with a worked example for each.
The BMI Formula — Metric Version
The metric formula is the "original" BMI calculation and is the simplest to work with:
BMI = Weight (kg) ÷ Height (m)²
Step-by-Step: Metric Example
Let's say you weigh 70 kg and are 1.70 m tall.
- Step 1: Square your height in meters: 1.70 × 1.70 = 2.89
- Step 2: Divide weight by that result: 70 ÷ 2.89 = 24.2
- Result: BMI = 24.2 — healthy weight range (18.5–24.9)
The BMI Formula — Imperial Version
If you work in pounds and inches, use this formula instead:
BMI = (Weight in lbs × 703) ÷ Height in inches²
The 703 factor converts from lb/in² to kg/m² — it's just a unit conversion constant built into the formula.
Step-by-Step: Imperial Example
Let's say you weigh 155 lbs and are 5 feet 8 inches (68 inches) tall.
- Step 1: Convert height to total inches: 5 × 12 + 8 = 68 inches
- Step 2: Square your height in inches: 68 × 68 = 4,624
- Step 3: Multiply weight by 703: 155 × 703 = 108,965
- Step 4: Divide: 108,965 ÷ 4,624 = 23.6
- Result: BMI = 23.6 — healthy weight range
Quick Reference: Pre-Calculated BMIs for Common Heights
Rather than calculating from scratch, use this table to find your approximate BMI. Find your height, then look across to your weight range:
| Height | BMI 18.5 (lbs) | BMI 22 (lbs) | BMI 25 (lbs) | BMI 30 (lbs) |
|---|---|---|---|---|
| 5′2″ (62 in) | 101 | 120 | 136 | 164 |
| 5′4″ (64 in) | 108 | 128 | 145 | 175 |
| 5′6″ (66 in) | 115 | 136 | 155 | 186 |
| 5′8″ (68 in) | 122 | 144 | 164 | 197 |
| 5′10″ (70 in) | 129 | 153 | 174 | 209 |
| 6′0″ (72 in) | 136 | 162 | 184 | 221 |
| 6′2″ (74 in) | 144 | 172 | 194 | 233 |
A Rough Mental Shortcut
If you just want a ballpark without any multiplication:
- Find your height in inches and square it (multiply by itself)
- Divide your weight in pounds by that number
- Multiply by 700 (close enough to 703 for a quick estimate)
For many people, this mental math produces a result within 0.2 BMI points of the exact answer — sufficient to know which category you're in.
Interpreting Your Result
| BMI Result | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0+ | Obese |
Want a precise result without the arithmetic? Use the free CheckBMI calculator — just enter your height and weight, and it does the math instantly.