BMI Calculator
Calculate your Body Mass Index — a quick screening measure that uses your weight and height to show whether you’re in the healthy weight range. The categories are for adults 20 and older (CDC).
Your Results
Weight at Different BMI Levels
| BMI | Category | Weight | Difference |
|---|
Go Beyond BMI — Track What Actually Matters
BMI is a starting point. The app tracks the calories and macros you log and your weight trend — so you can work toward a healthy weight, not just calculate it.
What is BMI?
Body Mass Index (BMI) is a simple numerical value calculated from your weight and height. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet as a way to quickly assess body weight relative to height across populations.
Today the WHO and the CDC use BMI to sort adults into weight categories: underweight, normal (healthy) weight, overweight and obesity. BMI does not measure body fat directly, but it is moderately to strongly associated with other measures of body fat (CDC).
How BMI is Calculated
The BMI formula is straightforward:
For example, if you weigh 75 kg and are 1.75 m tall:
If you’re using imperial units (pounds and inches), the formula includes a conversion factor:
This calculator handles all unit conversions automatically — just enter your weight and height in whichever units you prefer.
BMI Categories & Chart
The CDC uses these categories for adults 20 and older, regardless of age, sex or race. The WHO uses the same limits for overweight (25) and obesity (30):
| BMI Range | Category |
|---|---|
| < 18.5 | Underweight |
| 18.5 – 24.9 | Normal Weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obese Class I |
| 35.0 – 39.9 | Obese Class II |
| ≥ 40.0 | Obese Class III |
A high BMI adds to the risk of type 2 diabetes, cardiovascular disease, liver and kidney disease and several cancers (WHO), and too little weight can be a health risk too (CDC). For children and teens aged 2 to 19, the CDC reads BMI as a percentile for age and sex.
BMI for Women vs Men
The BMI formula and categories are the same for women and men (CDC). What the number reflects differs, though — in a study of 13,601 US adults (Romero-Corral 2008):
- Women: BMI followed body fat more closely than lean mass.
- Men: BMI followed lean mass more closely than body fat — a muscular man can reach a high BMI without excess fat.
Two people with the same BMI can therefore carry different amounts of body fat. In the overweight range (25–29.9), BMI could not tell body fat from lean mass in either sex (Romero-Corral 2008).
That is why BMI is a screening measure: the CDC says it should be considered with other factors when assessing a person’s health.
BMI Limitations
BMI has several well-known limitations:
- It doesn’t tell muscle from fat. BMI does not distinguish between fat, muscle and bone mass (CDC), so a muscular athlete and a sedentary person of the same height and weight get the same BMI.
- It doesn’t show where your fat is. BMI does not indicate where in the body you carry fat (CDC).
- It misses many people with excess fat. In 13,601 US adults, a BMI of 30 or more found fewer than half of those whose body fat was above 25% (men) or 35% (women) (Romero-Corral 2008).
- Age matters. In the same study, BMI got less accurate as age increased (Romero-Corral 2008).
Bottom line: BMI is a useful first check of your weight, but it’s not the whole picture. The CDC says it should be considered with other factors, such as blood pressure, cholesterol levels and a physical examination.
Healthy Weight Range
Your healthy weight range is the healthy BMI range (18.5–24.9) applied to your height:
Max = 24.9 × Height (m)²
BMI vs Body Fat Percentage
BMI estimates weight status from your height and weight alone; body fat percentage measures how much of your body mass is fat. It is harder to measure: a DEXA scan measures body composition precisely, but it is expensive and not widely available (CDC).
The two can differ a lot. In the 13,601 US adults, body fat (estimated by bioelectrical impedance) was above the WHO reference of 25% in 43.9% of men and above 35% in 52.3% of women, while a BMI of 30 or more was found in only 19.1% of men and 24.7% of women (Romero-Corral 2008).
If you know your body fat percentage, our TDEE calculator can use the Katch-McArdle formula, which estimates your basal metabolic rate from your lean body mass (Cunningham 1991).
Beyond BMI — Actionable Steps
Knowing your BMI is the first step. Here’s what to do with that number:
- If your BMI is in the healthy range (18.5–24.9): Focus on maintaining your weight through balanced nutrition and regular activity.
- If your BMI is 25 or above: Calculate your TDEE and eat less than it. People who lose weight at a gradual, steady pace of about 0.5–1 kg (1–2 lb) a week are more likely to keep it off (CDC) — at the commonly used 7,700 kcal per kg, that is a deficit of about 550–1,100 kcal a day.
- If your BMI is below 18.5: Consult a healthcare professional and focus on nutrient-dense foods to gradually increase weight.
Whatever your starting point, the key is tracking what you eat consistently. AI Food Coach makes this easy — snap a photo of your food on a kitchen scale, and AI reads the weight and estimates the macros in seconds.
