# Free BMI Calculator

Last reviewed: 2026-07-29

Enter your height and weight to get your body mass index, the WHO category it falls in, and the weight range that would put you inside the healthy band. Metric or imperial, instantly, with nothing stored.

## Quick Answer: How do you calculate BMI?

BMI is your weight in kilograms divided by the square of your height in metres (kg/m²). For adults, the WHO classifies a BMI under 18.5 as underweight, 18.5–24.9 as a healthy weight, 25.0–29.9 as overweight, and 30.0 or above as obesity. BMI is a screening ratio, not a body-composition measurement: it cannot separate fat from muscle.

- Formula: BMI = weight (kg) ÷ height (m)²
- Imperial: BMI = 703 × weight (lb) ÷ height (in)²
- Healthy adult range: 18.5 to 24.9
- BMI does not measure body fat, and reads high for muscular people

BMI is the fastest screening number in health and fitness, which is exactly why it gets over-read. It puts a population-level ratio on one person, and a ratio built from two numbers cannot know whether the weight it counted is muscle, fat, bone, or water.

That makes it a useful starting point and a poor finishing point. Use the calculator for the screen, then move to the numbers that actually drive a plan — maintenance calories and body composition.

Use the calculator: https://www.slimai.ai/calculators/bmi-calculator

## What Your BMI Actually Tells You About Your Goal

BMI on its own is a category, not a plan. What matters is what it points you toward next — and that differs completely depending on whether you are trying to lose fat, gain weight, or hold where you are.

### Weight loss — A BMI above 25 tells you a deficit is warranted — not how big it should be

The category flags that losing weight is reasonable. It says nothing about pace. For that you need maintenance calories, then a deficit you can actually hold — around 0.5 kg a week for most people. Work out the number with the TDEE calculator, and use the healthy weight range above as the destination rather than a round number you picked.

### Weight gain — A BMI under 18.5 means gaining, but gaining the right tissue

Eating more will move the number. Eating more with enough protein and resistance training moves it in the direction you want. Aim for a surplus of roughly 250–350 calories a day and 1.6–2.2 g of protein per kg of body weight, and expect a slower, steadier climb than a bulk-at-all-costs approach.

### Maintenance — A healthy BMI shifts the question to composition

Once you are inside 18.5–24.9, the scale stops being the interesting number. Two people at an identical BMI can carry very different amounts of fat and lean mass. From here, body-fat percentage, waist measurement, and how you train tell you more than another kilogram lost.

## What BMI Measures — and What It Misses

Body mass index was devised in the 1830s by Adolphe Quetelet as a way to describe populations, not to diagnose individuals. It survived because it is cheap: two measurements anyone can take, no equipment, no lab.

The cost of that simplicity is that BMI counts total mass. A 95 kg rugby player at 12% body fat and a 95 kg sedentary adult at 34% body fat, both 1.80 m tall, share a BMI of 29.3 and almost nothing else. The ratio cannot tell them apart, because nothing in the formula describes what the weight is made of.

It also ignores fat distribution, which is the part most closely tied to metabolic risk. Two people at the same BMI carrying different amounts of visceral fat around the abdomen are not in the same position, and BMI has no way to say so.

- **Muscle reads as risk:** Trained and muscular people are routinely classified as overweight by BMI. The number is right and the interpretation is wrong.
- **Older adults can read falsely reassuring:** Muscle mass declines with age. Someone can lose lean mass, gain fat, and keep the same BMI throughout.
- **It is not adjusted for frame or ethnicity:** Risk thresholds differ across populations; several health bodies apply lower cut-offs for people of South Asian descent, for example.
- **It says nothing about where fat sits:** Waist circumference and waist-to-height ratio track abdominal fat, which BMI cannot see at all.

## BMI, Body Fat, and the Number That Replaces It

The reason BMI persists is that the better measurements are harder to get. DEXA scans and hydrostatic weighing are accurate and impractical for weekly use. Skinfold calipers need a trained hand. Bioimpedance scales swing with hydration.

That gap is what SlimAI Body Assessment is built for. It estimates body fat percentage, fat mass, lean mass, and regional fat distribution from two smartphone photos: AI reads six tape-style measurements, and validated peer-reviewed formulas — the US Navy circumference method, Relative Fat Mass, and CUN-BAE — turn those into a result shown as a likely range with a confidence rating. A tape-measure mode gives the same estimate with no photos at all.

It is an estimate rather than a clinical measurement, and it is not a medical diagnosis. But it answers the question BMI structurally cannot: how much of your weight is fat, and where your body is storing it.

## How to Use BMI Without Being Misled by It

Treat BMI as one line in a wider picture. On its own it is a screening flag; alongside a waist measurement, a body-fat estimate, and a weight trend over weeks, it becomes genuinely useful.

Weigh yourself under consistent conditions — same time of day, same clothing, ideally first thing in the morning after using the bathroom. Day-to-day swings of one to two kilograms are normal and driven by water, food volume, sodium, and glycogen. A single reading is noise. A four-week trend is signal.

And be clear about who BMI is not for. It is not validated for children and teenagers, who need age-and-sex-specific percentile charts, and it is not appropriate during pregnancy. Athletes with high muscle mass should read it with heavy caveats.

- **Pair it with a waist measurement:** Keeping your waist under half your height is a simple, widely used check that captures what BMI misses.
- **Watch the trend, not the reading:** Weigh regularly under the same conditions and read the weekly average rather than the daily figure.
- **Cross-check with body fat:** A body-fat estimate tells you whether a change on the scale came from fat or from lean mass.
- **Change one thing at a time:** If calories, training, and sleep all change at once, no number can tell you which one moved the needle.

## From a Number on a Website to a Number That Follows You

This page gives you today’s BMI. It does not know what you weighed last month, and it will not know what you weigh next week — nothing here is stored.

That is the difference an account makes. When you set up SlimAI Calorie Tracker, your height, weight, age, activity level, and goal become a profile the app keeps. BMI, healthy weight range, and body-fat trend are recalculated as you log, and shown on the Statistics screen with Progress, Weight, and Insights tabs you can switch between weekly and monthly.

From that same profile SlimAI sets a daily calorie goal and gram-level protein, carbohydrate, and fat targets, then counts every meal you log against them. The number stops being something you look up and starts being something that updates itself.

## BMI Categories for Adults (WHO)

These are the World Health Organization cut-offs for adults aged 18 and over. They apply to the general adult population and not to children, pregnancy, or athletes with unusually high muscle mass.

| Category | BMI range | What it usually points toward |
|---|---|---|
| Underweight | Below 18.5 | Gaining weight, ideally as lean mass, with enough protein and resistance training |
| Healthy weight | 18.5 – 24.9 | Maintenance and body composition rather than further loss |
| Overweight | 25.0 – 29.9 | A moderate calorie deficit and a steady rate of loss |
| Obesity class I | 30.0 – 34.9 | Sustained weight loss with professional guidance involved |
| Obesity class II | 35.0 – 39.9 | Medical guidance before starting a weight-loss programme |
| Obesity class III | 40.0 and above | Medical care first; tracking as a support tool alongside it |

Several health bodies apply lower thresholds for people of South Asian, Chinese, and other Asian descent, where metabolic risk rises at a lower BMI. Categories are screening bands, not diagnoses.

## How to Calculate Your BMI

You can do it by hand in about thirty seconds, or use the calculator above, which handles the unit conversion and the category lookup for you.

1. **Measure your height** — Stand against a wall without shoes, heels together, and measure to the top of your head. Convert to metres — 175 cm becomes 1.75 m.
2. **Weigh yourself under consistent conditions** — First thing in the morning, after using the bathroom, before eating or drinking, in the same clothing each time.
3. **Square your height** — Multiply your height in metres by itself. For 1.75 m, that is 1.75 × 1.75 = 3.0625.
4. **Divide weight by the squared height** — At 78 kg: 78 ÷ 3.0625 = 25.5. That is your BMI. In imperial units, multiply pounds by 703 and divide by height in inches squared.
5. **Read the category, then look past it** — Find your band in the table above, note the healthy weight range for your height, and move on to maintenance calories and body composition for anything resembling a plan.

## The Formulas Behind This Calculator

Everything on this page is computed in your browser from these two expressions. No data is uploaded, and nothing is stored between visits.

- **BMI (metric):** `BMI = weight(kg) / height(m)²` — The standard definition used by the WHO, CDC, and NHS for adults.
- **BMI (imperial):** `BMI = 703 × weight(lb) / height(in)²` — The 703 factor converts pounds and inches into the metric result.
- **Healthy weight range:** `min = 18.5 × height(m)²   ·   max = 24.9 × height(m)²` — The weights that place you at the bottom and top of the healthy BMI band for your height.
- **Distance to the range:** `delta = weight(kg) − (24.9 × height(m)²)` — How far above the healthy band you currently sit, in kilograms. A negative result means you are inside or below it.

## Sources and Method

- **World Health Organization — Body mass index** — Adult BMI classification thresholds (underweight, normal range, overweight, obesity classes I–III) used for the categories on this page.
- **Quetelet, A. (1832)** — The original weight-for-height ratio from which body mass index derives.
- **NHS and CDC adult BMI guidance** — Guidance that BMI is a screening tool rather than a diagnostic measure, and that it is not appropriate for children, pregnancy, or highly muscular individuals.
- **SlimAI Body Assessment method** — US Navy circumference method, Relative Fat Mass (RFM), and CUN-BAE — the peer-reviewed formulas SlimAI uses to estimate body fat percentage where BMI cannot.

## Frequently Asked Questions

### How do I calculate my BMI?

Divide your weight in kilograms by your height in metres squared. Someone weighing 78 kg at 1.75 m has a BMI of 78 ÷ (1.75 × 1.75) = 25.5. In imperial units, multiply your weight in pounds by 703 and divide by your height in inches squared. The calculator on this page does both, and also shows the healthy weight range for your height.

### What is a healthy BMI?

For adults, the World Health Organization defines 18.5 to 24.9 as the healthy weight range. Below 18.5 is classified as underweight, 25.0 to 29.9 as overweight, and 30.0 or above as obesity. These are screening bands for populations, not diagnoses for individuals.

### Is BMI accurate for everyone?

No. BMI counts total body mass and cannot distinguish fat from muscle, so it over-classifies muscular and athletic people and can under-classify older adults who have lost lean mass. It is not validated for children and teenagers, who need age-and-sex-specific percentile charts, and it is not appropriate during pregnancy. Several health bodies also apply lower thresholds for people of South Asian and East Asian descent.

### Is BMI different for men and women?

The formula and the adult category thresholds are the same for both. Body composition at a given BMI is not — women typically carry a higher proportion of essential body fat than men at the same index. That is one of the reasons a body-fat estimate is more informative than BMI alone.

### What is better than BMI?

Body-fat percentage, waist circumference, and waist-to-height ratio all describe body composition and fat distribution, which BMI cannot see. SlimAI Body Assessment estimates body fat percentage, fat mass, lean mass, and regional fat distribution from two photos using the US Navy circumference method, RFM, and CUN-BAE, and offers a tape-measure mode with no photos.

### How much weight do I need to lose to reach a healthy BMI?

The calculator shows the healthy weight range for your height and the difference between your current weight and the top of that range. At a sustainable rate of about 0.5 kg per week, dividing that difference by 0.5 gives a realistic number of weeks — but the pace matters more than the deadline.

### Does SlimAI track BMI over time?

Yes. SlimAI Calorie Tracker records weight updates, your BMI score, your healthy weight range, and a body-fat trend chart, and shows them on a Statistics screen with Progress, Weight, and Insights tabs that switch between weekly and monthly views. This web calculator stores nothing between visits — the app is what keeps the history.

### Is this BMI calculator free?

Yes. It is free, requires no account, and runs entirely in your browser — your height and weight are never sent to a server. SlimAI Calorie Tracker is also free to download, with a Premium tier for unlimited AI scans and AI Body Assessment photo analysis.

Source: https://www.slimai.ai/calculators/bmi-calculator

## Try SlimAI

To try SlimAI, download SlimAI Calorie Tracker from the Apple App Store or Google Play and start the 3-day free trial; SlimAI Premium is then $1.49/month or $11.99/year (about $1/month), and can be cancelled anytime from the Apple or Google account that manages the subscription.

- Apple App Store: https://apps.apple.com/us/app/slimai-calorie-tracker/id6749062571
- Google Play: https://play.google.com/store/apps/details?id=com.slimai.app
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