BMI Calculator: How to Calculate BMI?
BMI as a screening tool, then discuss concerns with a healthcare provider when needed.
BMI is a simple ratio of weight to height² that helps classify underweight, healthy weight, overweight, or obesity. It is one piece of the puzzle and should be considered with other health factors. BMI is a measurement of leanness or corpulence based on height and weight, and it estimates tissue mass. It is widely used as a general indicator of healthy body weight for height.
What the calculator shows
A BMI calculator returns your BMI value (often shown in kg/m²) and your BMI category. Many tools also show a BMI table, BMI chart, or BMI graph with height–weight combinations.
BMI Calculator Inputs and Units
Standard vs Metric
Most tools offer Standard and Metric tabs. Standard uses Feet, Inches, and Pounds, while Metric uses centimeters and kilograms.
Metric Units and Other Units
Some calculators label Metric Units for SI and Other Units for USC. They may also include unit conversion so you can switch formats easily.
How Do I Calculate a BMI?
BMI formula in metric units
BMI formula (SI): BMI = mass (kg) ÷ height² (m). If your height is in centimeters, convert it to meters first.
BMI formula in US units
BMI formula (USC): BMI = 703 multiplier × mass (lbs) ÷ height² (in). This aligns pounds and inches with BMI units.
Step-by-step example
- Convert height to meters if needed.
- Square the height value.
- Divide weight by height squared.
This gives the BMI value you see in the Calculated Results area.
What Is the Formula for BMI Calculator in kg?
Metric BMI formula
Use: BMI = weight (kg) / height² (m). This is the simplest formula for most users.
If your height is in centimeters
Convert centimeters to meters by dividing by 100. Then use the same BMI formula in kg.
What Is the BMI of 70 kg 170 cm?
Step-by-step math
Height is 170 centimeters, which equals 1.70 m. Then BMI = 70 kg ÷ (1.70 × 1.70) = 70 ÷ 2.89 ≈ 24.2.
What that result means
A BMI of about 24.2 falls in the Healthy Weight range for adults. Your result can still vary by body composition and health context.
BMI Categories and Ranges
| BMI Category | BMI Range |
| Underweight | Below 18.5 |
| Healthy Weight | 18.5 to 24.9 |
| Overweight | 25.0 to 29.9 |
| Obesity | 30.0 or greater |
WHO classification table
Some tools also show WHO classes like Severe Thinness, Moderate Thinness, Mild Thinness, plus Obese Class I, Obese Class II, and Obese Class III.
BMI for Children and Teens
For children and teens (often age 2–20 or 2 through 19 years), BMI uses BMI-for-age percentiles instead of adult ranges. These come from growth charts and are split by boys chart and girls chart.
Percentile categories
Common percentile groups include Underweight percentile, Healthy weight percentile, At risk of overweight, and Overweight percentile. Many calculators link to a BMI Calculator for Children and Teens.
What Is the Correct BMI for My Age?
Adults 20 and older
For adults 20 and older, the standard BMI ranges usually apply. Still, BMI is one potential health indicator, not a full health assessment.
Children and teens
Age and growth patterns matter, so percentiles work better than adult categories. Height changes can shift BMI meaningfully.
Older adults
Older adults may carry more body fat at the same BMI than younger adults. A provider may consider medical history, health behaviors, physical exam findings, and laboratory findings.
What BMI Can and Cannot Tell You
BMI is a screening measure
BMI is a screening measure and is not intended to diagnose disease or illness. It helps flag when additional testing or action may be useful.
Why BMI can be misleading
BMI does not account well for muscle mass, bone density, or fat distribution. Athletes and bodybuilders can show higher BMI due to muscle, not excess fat. Women have more body fat than men at the same BMI, and ethnicity can influence risk patterns.
Risks Associated With Being Overweight or Obese
Higher BMI is linked with risks such as high blood pressure, higher LDL cholesterol, lower HDL cholesterol, and higher triglycerides. It is also associated with Type II diabetes, coronary heart disease, stroke, gallbladder disease, osteoarthritis, sleep apnea, and some cancers like breast, colon, kidney, liver, endometrial, and gallbladder cancers. Mental health concerns like clinical depression and anxiety can also occur, along with body pains and reduced quality of life.
If your BMI is high, aim to maintain a healthy weight, follow a heart-healthy diet, prepare healthy recipes, and increase physical activity. If you have symptoms or risk factors, consult your clinician about next steps.
Risks Associated With Being Underweight
Low BMI can be linked to malnutrition, vitamin deficiencies, and anemia. It may increase risk of osteoporosis, low immune function, and growth issues in younger people. Some women may face hormonal imbalances, menstrual cycle disruption, and higher miscarriage risk in early pregnancy.
Underweight can also reflect an underlying condition, including anorexia nervosa. If low BMI seems unexplained, seek medical guidance.
FAQs
How do I calculate a BMI?
Use weight divided by height squared. In metric, use kg and meters, then divide kg by m².
What is the BMI of 70 kg 170 cm?
It is about 24.2, which sits in the Healthy Weight category for adults.
What is the correct BMI for my age?
Adults use standard ranges, while children use BMI-for-age percentiles. For older adults, BMI works best alongside other health indicators.
What is the formula for BMI calculator in kg?
BMI = weight (kg) / height² (m), after converting centimeters to meters if needed.