How to Calculate BMI Formula Manually: Step by Step Guide with Examples

You don't always have access to an online BMI calculator. Maybe you're in a doctor's office, teaching a health class, or just want to understand how the numbers work.

The good news? Calculating BMI manually is simple math. You only need your height, weight, and basic multiplication and division skills.

In this guide, I'll walk you through the exact formulas, step-by-step examples for both metric and imperial units, and common mistakes to avoid.

The BMI Formula (Two Versions)

There are two formulas depending on which measurement system you use:

Metric Formula (kilograms and meters):

BMI = weight (kg) ÷ height² (m²)

Imperial Formula (pounds and inches):

BMI = [weight (lbs) × 703] ÷ height² (in²)

The number 703 is a conversion factor that makes the imperial formula produce the same result as the metric formula.

Step-by-Step: Metric Calculation

Let's break this down into simple steps.

Step 1: Measure Your Height in Meters

If you know your height in centimeters, convert it to meters by dividing by 100.

Example: 170 cm ÷ 100 = 1.70 meters

Step 2: Square Your Height

Multiply your height by itself (height × height).

Example: 1.70 × 1.70 = 2.89

Step 3: Measure Your Weight in Kilograms

Use a reliable scale. For accuracy, weigh yourself in the morning, after using the bathroom, before eating.

Example: 70 kg

Step 4: Divide Weight by Height Squared

Take your weight and divide it by the number from Step 2.

Example: 70 ÷ 2.89 = 24.2

Step 5: Round to One Decimal Place

Standard BMI reporting uses one decimal place.

Example: 24.2

This person's BMI is 24.2, which falls in the Normal range (18.5-24.9).

More Metric Examples

Height (cm)	Height (m)	Height²	Weight (kg)	BMI	Category
160	1.60	2.56	55	55 ÷ 2.56 = 21.5	Normal
165	1.65	2.72	65	65 ÷ 2.72 = 23.9	Normal
175	1.75	3.06	80	80 ÷ 3.06 = 26.1	Overweight
180	1.80	3.24	95	95 ÷ 3.24 = 29.3	Overweight
162	1.62	2.62	45	45 ÷ 2.62 = 17.2	Underweight

Step-by-Step: Imperial Calculation (Pounds and Inches)

If you're in the US or using imperial measurements, follow these steps.

Step 1: Convert Height to Total Inches

If you know your height in feet and inches, convert to total inches:

Total inches = (feet × 12) + inches

Example: 5 feet 7 inches = (5 × 12) + 7 = 60 + 7 = 67 inches

Step 2: Square Your Height in Inches

Multiply your height in inches by itself.

Example: 67 × 67 = 4,489

Step 3: Measure Your Weight in Pounds

Example: 154 lbs

Step 4: Multiply Weight by 703

The conversion factor 703 makes the imperial formula match the metric result.

Example: 154 × 703 = 108,262

Step 5: Divide by Height Squared

Take the result from Step 4 and divide by the number from Step 2.

Example: 108,262 ÷ 4,489 = 24.1

Step 6: Round to One Decimal Place

Example: 24.1

This person's BMI is 24.1, which falls in the Normal range (18.5-24.9).

More Imperial Examples

Height (ft/in)	Height (in)	Height²	Weight (lbs)	Weight × 703	BMI	Category
5'0"	60	3,600	110	77,330	21.5	Normal
5'4"	64	4,096	140	98,420	24.0	Normal
5'8"	68	4,624	180	126,540	27.4	Overweight
5'10"	70	4,900	210	147,630	30.1	Obese Class I
6'0"	72	5,184	250	175,750	33.9	Obese Class I

Common Mistakes to Avoid

Mistake #1: Forgetting to Convert cm to meters

Wrong: Using centimeters directly in the metric formula (170 instead of 1.70)

170 ÷ (170 × 170) = 170 ÷ 28,900 = 0.006 (completely wrong!)

Correct: 170 cm = 1.70 m

Mistake #2: Using Pounds Without the 703 Conversion Factor

Wrong: weight (lbs) ÷ height² (in²)

154 ÷ 4,489 = 0.03 (completely wrong!)

Correct: [weight × 703] ÷ height²

Mistake #3: Squaring Incorrectly

Wrong: 1.70 × 2 = 3.40 (that's doubling, not squaring)

Correct: 1.70 × 1.70 = 2.89 (squaring)

Mistake #4: Measuring Height Incorrectly

Measure without shoes. Stand with back straight, heels together, looking straight ahead.

How to Interpret Your BMI Result

Once you have your BMI number, here's what it means:

• Below 18.5: Underweight — may indicate nutritional deficiency, consult healthcare provider
• 18.5 - 24.9: Normal weight — healthy range, maintain with balanced lifestyle
• 25 - 29.9: Overweight — increased health risk, lifestyle modification recommended
• 30 - 34.9: Obese Class I — high risk, weight management program recommended
• 35 - 39.9: Obese Class II — very high risk, medical evaluation advised
• 40 and above: Obese Class III — extremely high risk, bariatric evaluation may be appropriate

Quick Reference: BMI Categories at a Glance

Category	BMI Range	Health Risk
Underweight	Below 18.5	Increased
Normal weight	18.5 - 24.9	Lowest
Overweight	25 - 29.9	Increased
Obese Class I	30 - 34.9	High
Obese Class II	35 - 39.9	Very High
Obese Class III	40+	Extremely High

Why BMI Has Limitations

While BMI is a useful screening tool, it has important limitations:

• Does not distinguish between muscle and fat (athletes may have high BMI but low body fat)
• Does not account for fat distribution (waist circumference matters)
• Less accurate for older adults (muscle loss skews results)
• Different ethnic groups have different risk thresholds
• Does not apply to pregnant women

For best results, combine BMI with waist circumference measurement.

Frequently Asked Questions

What is a good BMI for my age?

For adults 20-65, the healthy BMI range is 18.5-24.9 regardless of age. For seniors 65+, optimal BMI is slightly higher (23-27).

How often should I calculate my BMI?

Every 3-6 months is sufficient for healthy adults tracking weight. More frequently if actively trying to lose or gain weight.

Is BMI accurate for athletes?

No. BMI may overestimate body fat in muscular individuals. Athletes should consider body fat percentage and waist measurement instead.

Can I calculate BMI for children using the same formula?

The calculation is the same, but interpretation is different. Children's BMIs are compared to percentiles based on age and gender, not adult categories.

Scientific References

• World Health Organization. (2021). BMI classification.
• Centers for Disease Control and Prevention. (2022). Calculating BMI.
• National Heart, Lung, and Blood Institute. BMI tools and resources.

← Back to Blog