Productive ToolboxBMI Calculator
Calculate your Body Mass Index (BMI) instantly with metric or imperial units. Get your BMI category, healthy weight range, and ideal weight estimates — free, no sign-up required.
Enter valid height and weight to view comprehensive BMI scaling and ideal ranges.
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What Is a BMI Calculator?
A BMI calculator is a free health screening tool that computes your Body Mass Index from your height and weight. BMI is the most widely used metric for classifying adult weight status — it is referenced by the World Health Organization, the CDC, and healthcare providers in every country as a first-pass indicator of whether a person's weight relative to their height falls within a healthy range.
The challenge with most online BMI calculators is that they return a single number and stop there. That number — say, 27.4 — is not actionable on its own. You need context: What category does that place you in? How far is it from the healthy range? How much would you need to weigh to reach a healthy BMI at your height? What does that translate to in actual kilograms or pounds?
This calculator answers all of those questions in one place. Beyond the BMI score, it shows your BMI category, the healthy weight range for your exact height, and ideal weight estimates using the clinically established Devine and Robinson formulas. It supports both metric (kg/cm) and imperial (lb/ft/in) units and requires no account or sign-up.
How BMI Is Calculated
BMI uses one of two equivalent formulas depending on your unit system. Both produce the same result — the calculator converts internally so switching units never changes your BMI value.
Metric Formula
BMI = weight (kg) ÷ height (m)²
Example: 70 kg ÷ (1.75 m)² = 22.9
Imperial Formula
BMI = (weight (lb) × 703) ÷ height (in)²
Example: (154 lb × 703) ÷ 69² = 22.7
The healthy weight range is derived by back-solving the formula: the minimum healthy weight at your height equals 18.5 × height(m)², and the maximum equals 24.9 × height(m)². The ideal weight estimates use the Devine and Robinson formulas — both are based on height in inches above 5 feet and are widely used in clinical pharmacology for drug dosing calculations.
How to Use the BMI Calculator
Step-by-Step Guide
- 1Select your unit system: Choose Metric (kg and cm) or Imperial (lb, ft, in) using the toggle at the top of the calculator. The inputs and outputs update instantly — you can switch at any time without re-entering values.
- 2Enter your height: Type your height in the height field. In metric mode, enter centimetres (e.g. 175). In imperial mode, enter feet and inches separately (e.g. 5 ft 9 in).
- 3Enter your weight: Type your current weight. In metric mode, enter kilograms. In imperial mode, enter pounds. The BMI result updates as soon as both fields have valid values.
- 4Read your BMI result: Your BMI score appears immediately alongside your category (Underweight, Normal Weight, Overweight, or Obese) and a color-coded visual scale showing where you sit in the range.
- 5Check your healthy weight range: Below the BMI score, the calculator shows the minimum and maximum weight that corresponds to a healthy BMI (18.5–24.9) for your exact height — in whichever unit system you selected.
- 6Review ideal weight estimates: The Devine and Robinson formula results give you a clinical reference point for ideal weight. These are used in medical settings for drug dosing and provide a more nuanced target than a simple BMI midpoint.
- 7Use the weight simulator: Drag the simulator slider to explore how your BMI would change at different weights. This is useful for setting a realistic weight goal — you can see the BMI impact of losing or gaining 5, 10, or 20 kg/lbs.
What This Calculator Provides
- ✓BMI score to 1 decimal place
- ✓WHO category: Underweight, Normal, Overweight, Obese
- ✓Color-coded visual scale showing your position
- ✓Healthy weight range (min–max kg or lb) for your height
- ✓Ideal weight via Devine formula (clinical reference)
- ✓Ideal weight via Robinson formula (clinical reference)
- ✓Weight simulator slider — preview BMI at target weights
- ✓Local history log for tracking changes over time
- ✓Metric and imperial — switch anytime without data loss
- ✓Private: your inputs are not collected or stored
Real-World Use Cases
Setting a Realistic Weight-Loss Goal
A 5 ft 8 in (173 cm) person weighing 210 lbs (95 kg) has a BMI of 31.9 — Class I Obesity. They use the healthy weight range output to find that reaching a BMI of 24.9 requires a weight of 164 lbs (74 kg). The weight simulator shows that losing just 20 lbs (9 kg) would bring them to 190 lbs — a BMI of 28.9, out of the obese range and into overweight. That intermediate milestone is psychologically more achievable than the full 46-lb target.
Pre-Appointment Health Screening
A person scheduling their annual physical checks their BMI the day before so they can have an informed conversation with their doctor. They calculate a BMI of 26.2 — borderline overweight — and note their healthy weight range is about 115–154 lbs for their 5 ft 6 in height. Armed with this context, they ask their doctor about waist circumference and cholesterol as additional risk indicators rather than just hearing their BMI read out in the appointment.
Fitness Plan Progress Tracking
Someone starting a 12-week training program checks their BMI at the start (29.1), at week 4 (28.3), week 8 (27.6), and week 12 (26.8). Using the built-in history log, they can see the trend clearly. The simulator shows they need to drop another 8 kg to reach a healthy BMI — which becomes the goal for their next training block.
Clinical Drug Dosing Reference
A pharmacist or medical student uses the Devine and Robinson ideal weight outputs as a quick cross-reference when verifying weight-based drug dosing calculations. Both formulas are well-established in pharmacology — the calculator surfaces both so the clinician can apply whichever protocol their institution uses, with the values available in both kg and lb.
Evaluating Underweight Risk
A parent notices their adult child looks noticeably thin after a stressful year. They use the calculator together — at 5 ft 4 in and 100 lbs, the BMI comes out at 17.2, classified as Underweight. The healthy weight range shows they need to be between 108 and 145 lbs. This concrete range helps frame a constructive conversation about nutrition and prompts a GP visit.
Insurance and Health Assessment Forms
Many health insurance applications, employer wellness programs, and gym membership intake forms ask for BMI. Rather than calculating manually or using a formula from memory, someone fills in their height and weight here, gets the BMI to one decimal place, and copies it directly into the form — confident the number is accurate.
BMI Categories & Healthy Weight Reference
| BMI Range | Category | Risk of related disease (WHO) |
|---|---|---|
| Below 18.5 | Underweight | Low, but the risk of other clinical problems rises |
| 18.5 – 24.9 | Normal weight | Average |
| 25.0 – 29.9 | Overweight | Increased |
| 30.0 – 34.9 | Obesity class I | Moderate |
| 35.0 – 39.9 | Obesity class II | Severe |
| 40.0+ | Obesity class III | Very severe |
Source: World Health Organization adult BMI classification (Technical Report Series 894); the US CDC uses the same cut-offs for adults. These categories do not apply to children and teens, who are assessed with BMI-for-age percentiles.
Healthy Weight Range by Height
| Height (cm) | Height (ft/in) | Min Healthy Weight | Max Healthy Weight |
|---|---|---|---|
| 155 cm | 5 ft 1 in | 44.4 kg / 97.9 lb | 59.8 kg / 131.8 lb |
| 160 cm | 5 ft 3 in | 47.4 kg / 104.4 lb | 63.7 kg / 140.4 lb |
| 165 cm | 5 ft 5 in | 50.3 kg / 110.9 lb | 67.7 kg / 149.3 lb |
| 170 cm | 5 ft 7 in | 53.5 kg / 117.9 lb | 71.9 kg / 158.5 lb |
| 175 cm | 5 ft 9 in | 56.7 kg / 125.0 lb | 76.3 kg / 168.2 lb |
| 180 cm | 5 ft 11 in | 59.9 kg / 132.1 lb | 80.7 kg / 177.9 lb |
| 185 cm | 6 ft 1 in | 63.3 kg / 139.5 lb | 85.2 kg / 187.8 lb |
| 190 cm | 6 ft 3 in | 66.8 kg / 147.2 lb | 89.8 kg / 197.9 lb |
* Healthy weight range calculated using WHO BMI thresholds of 18.5 (minimum) and 24.9 (maximum). Values rounded to one decimal place.
Frequently Asked Questions
What is BMI?
BMI (Body Mass Index) is a numerical value derived from a person's weight and height. It is calculated by dividing weight in kilograms by the square of height in metres (kg/m²). The World Health Organization uses BMI as a population-level screening tool to classify underweight, normal weight, overweight, and obesity in adults. It does not directly measure body fat, but it correlates with body fat percentage in most adults and is widely used because it requires only two measurements and no specialist equipment.
What is a healthy BMI range?
For adults aged 18 and over, the WHO defines a healthy BMI as 18.5 to 24.9. Below 18.5 is classified as underweight, 25.0 to 29.9 as overweight, and 30.0 and above as obese. These thresholds are population-level guidelines — individual health depends on many additional factors including muscle mass, bone density, age, sex, and ethnicity.
Is BMI the same for men and women?
The BMI formula itself is identical for men and women. However, the same BMI value may represent different body fat percentages between sexes — women naturally carry more body fat than men at the same BMI. This is why some clinicians complement BMI with waist circumference or body fat percentage measurements when assessing health risk.
Does BMI apply to children and teenagers?
Standard adult BMI categories do not apply to children and adolescents. For people under 18, clinicians use BMI-for-age percentile charts that account for normal growth patterns and sex differences at different ages. This calculator is designed for adults aged 18 and over.
Can BMI be inaccurate?
Yes, BMI has well-documented limitations. Athletes and heavily muscular individuals often fall into the overweight or obese category despite having low body fat. Conversely, older adults can have a normal BMI while carrying too little muscle and too much visceral fat. BMI is best used as a starting point for investigation, not a definitive health verdict.
What are the Devine and Robinson ideal weight formulas?
The Devine formula (1974) and Robinson formula (1983) are two of the most commonly cited medical formulas for estimating ideal body weight from height, applied in clinical pharmacology for drug dosing. Devine: males = 50 kg + 2.3 kg per inch over 5 ft; females = 45.5 kg + 2.3 kg per inch over 5 ft. Robinson: males = 52 kg + 1.9 kg per inch over 5 ft; females = 49 kg + 1.7 kg per inch over 5 ft. The two formulas typically differ by 2–5 kg for the same height.
How do I calculate BMI in imperial units?
In imperial units, the formula is: BMI = (weight in pounds × 703) divided by (height in inches squared). For example, someone who is 5 ft 9 in (69 inches) and weighs 170 lbs: BMI = (170 × 703) divided by (69 squared) = 119,510 divided by 4,761, which equals approximately 25.1. This calculator handles the conversion automatically — select Imperial, enter your height in feet and inches and your weight in pounds.
What BMI is considered obese?
A BMI of 30.0 or above is classified as obese by the WHO and most national health authorities. Obesity is further subdivided into Class I (30.0–34.9), Class II (35.0–39.9), and Class III or severe obesity (40.0 and above). Each class carries progressively higher risks of type 2 diabetes, cardiovascular disease, and hypertension. This calculator's healthy weight range output shows exactly how much weight would bring you into the normal range.
Is my data stored when I use this BMI calculator?
We do not collect or store what you enter. Any history the tool keeps is visible only to you.
What should I do if my BMI is outside the healthy range?
A BMI result outside the 18.5–24.9 healthy range is a prompt to investigate further, not a diagnosis. If your BMI indicates underweight, overweight, or obesity, the most useful next step is to consult a doctor or registered dietitian who can assess your full health picture — including body composition, blood markers, blood pressure, and lifestyle factors.
Medical Disclaimer
BMI is a population-level screening indicator, not a clinical diagnosis. Results from this calculator are for informational purposes only and do not constitute medical advice. If your BMI result is outside the normal range, or if you have any health concerns, consult a qualified healthcare professional before making changes to your diet, exercise routine, or treatment plan.
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