How to Calculate BMI: Formula & Examples

BMI Health Checker Team 7 min read6 April 2026

Step by step illustration of how to calculate BMI using the formula

How to Calculate BMI: The Complete Formula Guide

**Body Mass Index (BMI)** is one of the most widely used tools for screening weight status. Whether you are tracking your own health or studying for an exam, knowing how to calculate BMI by hand is a useful skill. This guide walks through both the metric and imperial formulas, provides worked examples, explains what the results mean, and covers special cases like reverse calculation and BMI for children.

The BMI Formula

Metric Formula (kg and metres)

BMI = Weight (kg) ÷ Height (m)²

This is the standard formula used by the World Health Organisation and most clinical settings worldwide.

Imperial Formula (pounds and inches)

BMI = [Weight (lbs) ÷ Height (inches)²] × 703

The multiplication factor of **703** converts the imperial units to match the metric scale. This formula is widely used in the United States.

Step-by-Step Calculation: Metric Example

**Scenario:** A woman weighs 68 kg and is 1.65 m tall.

**Square the height:** 1.65 × 1.65 = 2.7225

**Divide weight by the result:** 68 ÷ 2.7225 = 24.98

**Result:** BMI = **25.0** (borderline overweight)

Step-by-Step Calculation: Imperial Example

**Scenario:** A man weighs 185 lbs and is 5'10" (70 inches) tall.

**Convert height to inches:** 5 feet × 12 = 60, plus 10 = **70 inches**

**Square the height:** 70 × 70 = 4,900

**Divide weight by the result:** 185 ÷ 4,900 = 0.03776

**Multiply by 703:** 0.03776 × 703 = **26.5**

**Result:** BMI = **26.5** (overweight)

What Your BMI Number Means

| BMI | Category | What It Suggests |

|---|---|---|

| Below 18.5 | Underweight | Possible nutritional deficiency or underlying health issue |

| 18.5 – 24.9 | Normal weight | Lowest risk for weight-related health problems |

| 25.0 – 29.9 | Overweight | Moderately increased risk for cardiovascular disease and diabetes |

| 30.0 – 34.9 | Obese (Class I) | Significantly increased health risk |

| 35.0 – 39.9 | Obese (Class II) | High risk; medical intervention often recommended |

| 40.0 + | Obese (Class III) | Severe risk; multi-disciplinary treatment typically required |

What Does a BMI of 28 Mean?

A **BMI of 28** falls in the **overweight** category. At this level, you are not yet classified as obese, but your risk for conditions such as high blood pressure, elevated cholesterol, and type 2 diabetes is meaningfully higher than someone in the normal range. For most people, modest lifestyle changes — reducing calorie intake by 300–500 calories per day and adding 150 minutes of weekly exercise — can bring BMI down to a healthier level within months.

For context, a BMI of 28 corresponds to:

A 5'4" woman weighing about **163 lbs (74 kg)**

A 5'10" man weighing about **195 lbs (88 kg)**

How to Calculate BMI in Pounds and Inches: Detailed Walkthrough

This is the most commonly searched version of the formula, so let's break it down one more time with full detail.

**Formula:** BMI = (Weight in pounds × 703) ÷ (Height in inches × Height in inches)

**Example:** A person weighs 160 lbs and is 5'5" tall.

**Convert height:** 5 feet = 60 inches + 5 = **65 inches**

**Multiply weight by 703:** 160 × 703 = **112,480**

**Square the height:** 65 × 65 = **4,225**

**Divide:** 112,480 ÷ 4,225 = **26.6**

**Result:** BMI = **26.6** (overweight)

Both versions of the imperial formula (dividing then multiplying by 703, or multiplying first then dividing) give the same result. Use whichever feels more intuitive.

Reverse Calculation: Determining Height from BMI and Weight

Sometimes you need to work backwards — for instance, calculating what height would correspond to a specific BMI at a given weight. The rearranged formula is:

Height (m) = √(Weight (kg) ÷ BMI)

**Example:** A person weighs 80 kg and has a BMI of 25.

**Divide weight by BMI:** 80 ÷ 25 = 3.2

**Take the square root:** √3.2 = **1.789 m** (approximately 5'10.5")

In imperial: **Height (inches) = √[(Weight (lbs) × 703) ÷ BMI]**

**Example:** A person weighs 176 lbs and has a BMI of 25.

**Multiply weight by 703:** 176 × 703 = 123,728

**Divide by BMI:** 123,728 ÷ 25 = 4,949.12

**Take the square root:** √4,949.12 = **70.4 inches** (approximately 5'10")

BMI Formula for Children and Teens

BMI is calculated the same way for children and adolescents, but the interpretation is different. Instead of fixed category cut-offs, children's BMI is plotted on **age- and sex-specific growth charts** to determine a percentile:

Below 5th percentile: — Underweight

5th to 84th percentile: — Healthy weight

85th to 94th percentile: — Overweight

95th percentile and above: — Obese

This percentile approach accounts for the fact that body composition changes substantially during growth.

Common Mistakes When Calculating BMI

1. Forgetting to Square the Height

The most frequent error. If you divide weight by height (not height squared), you will get a nonsensical result.

2. Mixing Units

Using kilograms for weight but feet for height, or using metres with the imperial formula. Make sure all inputs use the same system.

3. Forgetting the 703 Multiplier

When using pounds and inches, the factor of 703 is essential. Without it, your result will be far too low.

4. Using Centimetres Instead of Metres

The metric formula requires height in **metres**. If your height is 170 cm, you must convert to 1.70 m before squaring.

Quick Reference BMI Table

| Height | BMI 20 | BMI 25 | BMI 30 | BMI 35 | BMI 40 |

|---|---|---|---|---|---|

| 5'0" | 102 lbs | 128 lbs | 153 lbs | 179 lbs | 204 lbs |

| 5'3" | 113 lbs | 141 lbs | 169 lbs | 197 lbs | 225 lbs |

| 5'6" | 124 lbs | 155 lbs | 186 lbs | 217 lbs | 247 lbs |

| 5'9" | 136 lbs | 169 lbs | 203 lbs | 237 lbs | 271 lbs |

| 6'0" | 148 lbs | 184 lbs | 221 lbs | 258 lbs | 295 lbs |

Key Takeaways

The BMI formula in metric is **weight (kg) ÷ height (m)²**. In imperial, it is **[weight (lbs) ÷ height (in)²] × 703**.

A BMI of **28** means overweight — action is warranted but the situation is very manageable with lifestyle changes.

Always **square the height** and ensure your units are consistent — these are the two most common sources of error.

The reverse formula lets you determine height from weight and BMI: **height = √(weight ÷ BMI)**.

For children, use **age-specific percentile charts** rather than fixed adult cut-offs.

For a quick, error-free result, use our [BMI calculator](/).