How to Calculate BMI — Imperial and Metric Formula With Examples

BMI — Body Mass Index — is calculated from just two measurements: your weight and your height. The math is straightforward, but the imperial formula involves an extra step that trips people up. This guide walks through both the metric and imperial formulas with worked examples so you can verify the math yourself.

If you would rather skip the arithmetic: the free BMI calculator handles both imperial and metric, shows your result in real time as you type, and gives your category and healthy weight range alongside the number.

Metric BMI Formula — Kilograms and Centimeters

The metric formula is the most straightforward version:

BMI = weight (kg) ÷ height (m)²

Note that height must be in meters, not centimeters. Divide your height in centimeters by 100 to convert.

Worked example:

• Weight: 70 kg
• Height: 170 cm = 1.70 m
• Height squared: 1.70 × 1.70 = 2.89
• BMI: 70 ÷ 2.89 = 24.2
• Category: Normal weight (18.5–24.9)

Another example:

• Weight: 90 kg
• Height: 175 cm = 1.75 m
• Height squared: 1.75 × 1.75 = 3.0625
• BMI: 90 ÷ 3.0625 = 29.4
• Category: Overweight (25–29.9)

Imperial BMI Formula — Pounds and Inches

The imperial formula uses a conversion factor of 703 to account for the difference between the metric and imperial measurement systems:

BMI = [weight (lbs) ÷ height (inches)²] × 703

Convert height entirely to inches first (5 feet 9 inches = 69 inches).

Worked example:

• Weight: 170 lbs
• Height: 5'10" = 70 inches
• Height squared: 70 × 70 = 4,900
• Weight ÷ height²: 170 ÷ 4,900 = 0.03469
• Multiply by 703: 0.03469 × 703 = 24.4
• Category: Normal weight

Another example:

• Weight: 200 lbs
• Height: 5'8" = 68 inches
• Height squared: 68 × 68 = 4,624
• 200 ÷ 4,624 × 703 = 30.4
• Category: Obese Class I

The 703 constant (precisely 703.07) converts from lbs/in² to the equivalent of kg/m². The free BMI calculator handles this automatically in the imperial mode — just enter lbs, feet, and inches.

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Common BMI Calculation Errors

• Forgetting to square the height: The most common error. BMI divides by height-squared, not height. Using height instead of height-squared gives a number roughly 5-7 times larger than the actual BMI.
• Using centimeters instead of meters in the metric formula: Entering 170 instead of 1.70 gives a BMI of 0.24 instead of 24 — off by 100x. Always divide centimeters by 100 before squaring.
• Mixing units: The imperial formula requires pounds and inches — not pounds and feet. 5'9" is 69 inches, not 5.75 feet. Convert before calculating.
• Forgetting the 703 multiplier in imperial: Without the multiplier, the result is in lb/in² units, which looks like 0.034 — not a recognizable BMI number.

These errors are why using the free BMI calculator directly is often faster and more reliable than manual calculation — it handles units correctly regardless of whether you enter imperial or metric values.

Interpreting Your BMI Result

BMI Range	Category	Typical Action
Below 18.5	Underweight	Consider nutritional evaluation; rule out underlying cause
18.5–24.9	Normal weight	Maintain current habits; no BMI-driven intervention needed
25.0–29.9	Overweight	Review lifestyle factors; check waist circumference and blood markers
30.0–34.9	Obese Class I	Clinical weight management discussion with a doctor; lifestyle intervention
35.0–39.9	Obese Class II	Medical weight management; possible medication eligibility
40.0+	Obese Class III	Bariatric surgery may be an option; comprehensive medical care

BMI is a starting point for health conversations, not a clinical diagnosis. A single BMI number does not tell your doctor (or you) about your fitness level, body composition, metabolic health, or overall wellbeing.

Frequently Asked Questions

What is the BMI formula in pounds and inches?

BMI (imperial) = [weight in pounds ÷ (height in inches)²] × 703. Convert height entirely to inches first (e.g., 5'9" = 69 inches). Square the inch value (69² = 4,761). Divide your weight by that number. Multiply the result by 703. Example: 160 lbs at 5'8" (68 in) = [160 ÷ 4,624] × 703 = 24.3.

What is the BMI formula in kg and cm?

BMI (metric) = weight (kg) ÷ height (m)². Convert height from centimeters to meters by dividing by 100 (e.g., 175 cm = 1.75 m). Square the meter value (1.75² = 3.0625). Divide weight in kg by that number. Example: 70 kg at 175 cm = 70 ÷ 3.0625 = 22.9.

Why does the imperial BMI formula use 703?

The standard BMI formula uses metric units (kg and meters). To get the equivalent result using pounds and inches, you need a conversion factor of 703.07 (sometimes rounded to 703). This number comes from the unit conversion between kg/m² and lb/in². Multiplying by 703 makes the imperial result equivalent to the metric result so they produce the same BMI category.