BMI Calculator
Calculate adult body mass index from height and weight using metric or imperial units. Compare standard adult BMI categories with lower NICE thresholds where relevant, see the mathematical reference-weight interval for your height, and optionally add waist circumference for a waist-to-height ratio assessment. BMI is a screening measure rather than a diagnosis.
Additional
What BMI measures
Body mass index, or BMI, is a calculation that compares body weight with height. It is commonly used as a quick screening measure for adults.
BMI does not directly measure body fat. Two people with the same BMI can have different proportions of fat, muscle, bone, and body fluid.
For that reason, an adult BMI result is best treated as one piece of health information rather than a diagnosis or complete description of health.
How to use the BMI calculator
Choose Metric to enter height in centimetres and weight in kilograms, or Imperial to enter height in feet and inches and weight in pounds.
Enter height and weight to calculate BMI. The result shows the BMI value, its screening category, the selected threshold set, and a mathematical reference-weight interval for that height.
Open Additional if you need the lower NICE BMI thresholds or want to enter waist circumference. Waist circumference adds a waist-to-height ratio result without changing the BMI calculation.
Switching between Metric and Imperial converts measurements already entered, including an optional waist measurement.
How BMI is calculated
BMI equals weight in kilograms divided by height in metres squared.
When height is entered in centimetres, the calculator converts centimetres to metres before applying the same formula.
For pounds and inches, a commonly written equivalent formula multiplies pounds divided by inches squared by approximately 703. The Olivez calculator converts the measurements internally and uses the same underlying metric definition.
Standard adult BMI categories
CDC adult BMI categories apply to adults aged 20 and older. Underweight is below 18.5, healthy weight is 18.5 to below 25, overweight is 25 to below 30, and obesity begins at 30.
Obesity is further divided into class 1 from 30 to below 35, class 2 from 35 to below 40, and class 3 at 40 or higher.
These are screening categories. A category boundary does not represent a point where health changes suddenly, and the category alone does not diagnose a medical condition.
| Category | Standard adult thresholds | Lower NICE thresholds |
|---|---|---|
| Underweight | Below 18.5 | Below 18.5 |
| Healthy weight | 18.5 to <25 | 18.5 to <23 |
| Overweight | 25 to <30 | 23 to <27.5 |
| Obesity class 1 | 30 to <35 | 27.5 to <32.5 |
| Obesity class 2 | 35 to <40 | 32.5 to <37.5 |
| Obesity class 3 | 40 or higher | 37.5 or higher |
Lower NICE BMI thresholds
NICE recommends lower practical BMI thresholds for adults from South Asian, Chinese, other Asian, Middle Eastern, Black African, and African-Caribbean family backgrounds because cardiometabolic risk can occur at lower BMI values in these populations.
Under this interpretation, overweight begins at BMI 23 and obesity begins at BMI 27.5. Obesity classes 2 and 3 are also shifted downward by 2.5 BMI units.
The BMI formula itself does not change. Only the category boundaries and the resulting reference-weight interval change.
The lower-threshold option is provided as a guideline-based interpretation, not as a substitute for individual clinical assessment.
Why add waist-to-height ratio?
BMI describes weight relative to height but does not show where body fat is carried. Waist-to-height ratio adds a simple measure of central adiposity.
The ratio is calculated by dividing waist circumference by height using the same units. For example, an 82 cm waist and 175 cm height gives a ratio of about 0.47.
NICE recommends using waist-to-height ratio alongside BMI in adults with BMI below 35 as a practical estimate of central adiposity.
A common public-health interpretation is to keep the waist circumference below half of height, corresponding to a ratio below 0.5.
| Waist-to-height ratio | Classification | Interpretation |
|---|---|---|
| Below 0.40 | Below reference band | Below the NICE-defined healthy central-adiposity band; the calculator does not relabel this as a separate health category. |
| 0.40 to <0.50 | Healthy central adiposity | NICE healthy central-adiposity band. |
| 0.50 to <0.60 | Increased central adiposity | NICE increased central-adiposity band. |
| 0.60 or higher | High central adiposity | NICE high central-adiposity band. |
How to measure your waist for waist-to-height ratio
Measurement technique matters. NICE advises finding the bottom of the ribs and the top of the hips, then placing the tape around the waist midway between those points.
Take the measurement after breathing out naturally rather than pulling the abdomen in.
Waist and height must use the same unit before division. Use centimetres for both or inches for both.
Different clinical protocols can use slightly different anatomical landmarks, so use the same measurement method when comparing values over time.
Understanding waist-to-height categories
NICE defines a waist-to-height ratio from 0.40 to below 0.50 as healthy central adiposity, 0.50 to below 0.60 as increased central adiposity, and 0.60 or above as high central adiposity.
NICE does not define values below 0.40 as a separate healthy category, so Olivez labels that region simply as below the reference band.
These categories help provide context about central adiposity. They do not diagnose diabetes, cardiovascular disease, hypertension, or another condition.
Why waist-to-height interpretation is limited at BMI 35 or higher
NICE specifically recommends waist-to-height ratio alongside BMI as a practical estimate of central adiposity when adult BMI is below 35.
At BMI 35 or higher, waist circumference adds less useful information for this form of risk classification.
Oliv ez therefore still calculates the mathematical ratio if waist circumference is entered, but it does not present the NICE central-adiposity band as though it had the same interpretive value in that situation.
What the reference weight interval means
A BMI boundary can be converted back into a weight for a fixed height using weight = BMI × height squared.
The calculator uses the lower and upper healthy-BMI boundaries from the selected threshold set to calculate a corresponding weight interval.
Under the standard thresholds, that interval corresponds to BMI 18.5 through just below 25. Under the lower NICE interpretation, the upper boundary is BMI 23.
This is mathematical context only. It is deliberately called a reference-weight interval rather than an ideal weight because it is not a personalised goal or treatment recommendation.
BMI can be misleading when muscle mass is high
BMI uses total body weight, so it cannot tell whether that weight comes from body fat, muscle, bone, or fluid.
A person with substantial muscle mass can therefore have a high BMI without having the same body-fat profile as another person with the same BMI.
NICE advises interpreting BMI cautiously in adults with high muscle mass because BMI may be a less accurate indicator of adiposity in that situation.
BMI in adults aged 65 and older
The mathematical BMI formula does not change at age 65, but interpretation may require more context.
NICE advises caution in older adults because health conditions, functional capacity, body composition, and the possible protective effect of a slightly higher BMI can affect what the number means for an individual.
For an older adult, BMI is therefore particularly important to interpret alongside wider clinical information rather than by category alone.
Does BMI differ for men and women?
For adults, the standard BMI formula and CDC category thresholds do not change according to sex.
That is why this calculator does not ask for sex before calculating adult BMI.
Body composition and fat distribution can still differ substantially between individuals, so identical BMI values do not imply identical body-fat percentage or health risk.
BMI for children and teenagers
This calculator is intentionally for adults aged 20 and older.
Children and teenagers aged 2 through 19 are assessed using BMI-for-age measures that account for age and sex because growth and body composition change during development.
Do not apply the fixed adult categories shown here to a child or teenager.
BMI and pregnancy
The adult BMI categories in this calculator are not designed to assess pregnancy-related weight change.
Pregnancy changes body weight and body composition in ways that require pregnancy-specific clinical guidance.
The reference-weight interval should not be used as a pregnancy weight-gain target.
Why measurements near a category boundary need context
Height and weight measurements are not perfectly precise, and BMI categories use fixed numerical boundaries.
A small measurement difference can therefore change the displayed BMI or category when a result is very close to a threshold.
The calculator displays BMI to one decimal place. Treat a value near a category boundary as a screening estimate rather than a sharply different biological state.
What BMI does not tell you
BMI cannot determine body-fat percentage, muscle mass, fat distribution, blood pressure, blood glucose, cholesterol, fitness, diet quality, or whether a person has a medical condition.
CDC recommends considering BMI with other information such as medical history, health behaviours, physical examination findings, and laboratory results when assessing an individual's health.
Waist-to-height ratio adds information about central adiposity, but it also remains a screening measure rather than a diagnosis.
| Area | Why it matters |
|---|---|
| Body composition | BMI cannot distinguish body fat from muscle, bone, or body fluid. |
| Fat distribution | BMI alone does not show where body fat is carried. Waist-to-height ratio can add information about central adiposity. |
| High muscle mass | BMI can be less informative when high muscle mass contributes substantially to body weight. |
| Older adults | BMI needs cautious interpretation in older adults because body composition, health conditions, and functional status can change its meaning. |
| Children and teenagers | People aged 2 through 19 need age- and sex-specific BMI-for-age interpretation rather than fixed adult thresholds. |
| Pregnancy | Adult BMI categories are not designed to assess pregnancy-related weight change. |
| Individual health | Medical history, physical findings, blood pressure, laboratory results, health behaviours, and other factors can change individual interpretation. |
| Result | Meaning |
|---|---|
| BMI | Weight relative to height, displayed to one decimal place |
| BMI category | Screening category under the selected adult threshold set |
| Reference weight interval | Weights mathematically corresponding to the selected healthy-BMI boundaries at the entered height |
| Waist-to-height ratio | Optional waist circumference divided by height, with both measurements in the same units |
BMI and waist-to-height formulas
BMI compares weight with height squared. Waist-to-height ratio compares two measurements of length using the same units.
- Weight in kilograms
- Weight in pounds
- Height in metres
- Height in centimetres
- Height in inches
- Waist circumference
- Height measured in the same length unit as waist circumference
- Waist-to-height ratio
Examples
Adult BMI in metric units
Height: 175 cm; weight: 72 kg; standard adult thresholds.
BMI is 23.5, which falls within the standard healthy-weight category. The mathematical reference-weight interval is approximately 56.7 to less than 76.6 kg.
The reference interval describes BMI boundaries and is not a personalised target.
Add waist circumference
Height: 175 cm; weight: 72 kg; waist circumference: 82 cm.
BMI is 23.5 and waist-to-height ratio is 0.47. Because BMI is below 35, the ratio falls within the NICE healthy central-adiposity band.
82 ÷ 175 is approximately 0.47.
Imperial BMI calculation
Height: 5 ft 9 in; weight: 160 lb; standard adult thresholds.
BMI is approximately 23.6, within the standard healthy-weight category.
Same BMI, different threshold interpretation
Height: 170 cm; weight: 70 kg.
BMI is 24.2. It is within the standard healthy-weight category but falls within the overweight category when the lower NICE thresholds are selected.
Under the lower threshold set, the healthy-weight interval at 170 cm ends below approximately 66.5 kg.
Waist-to-height interpretation at BMI 35 or higher
Height: 170 cm; weight: 105 kg; optional waist circumference entered.
BMI is 36.3. The calculator still reports the mathematical waist-to-height ratio, but it does not apply the NICE central-adiposity category as though it had the same interpretive usefulness.
NICE recommends waist-to-height ratio as an additional practical estimate of central adiposity when adult BMI is below 35.
Frequently Asked Questions
What is BMI?
Body mass index is weight in kilograms divided by height in metres squared. It is commonly used as a screening measure of weight relative to height.
What is a healthy BMI for adults?
Under standard adult thresholds, the healthy-weight category is BMI 18.5 to below 25 for adults aged 20 and older.
What BMI is considered overweight?
Under standard adult thresholds, overweight is BMI 25 to below 30. Under the lower NICE thresholds available in this calculator, overweight begins at BMI 23.
What BMI is considered obesity?
Under standard adult thresholds, obesity begins at BMI 30. NICE uses a lower practical obesity threshold of BMI 27.5 for the specified family-background groups covered by that guidance.
Why does the calculator include lower NICE thresholds?
NICE recommends lower practical BMI thresholds for adults from South Asian, Chinese, other Asian, Middle Eastern, Black African, and African-Caribbean family backgrounds because cardiometabolic risk can occur at lower BMI values. The BMI formula itself remains unchanged.
What is waist-to-height ratio?
Waist-to-height ratio is waist circumference divided by height using the same units. It is an additional practical measure of central adiposity.
What is a healthy waist-to-height ratio?
NICE defines 0.40 to below 0.50 as healthy central adiposity, 0.50 to below 0.60 as increased central adiposity, and 0.60 or above as high central adiposity.
Why does the calculator say to keep waist below half of height?
A waist circumference below half of height corresponds to a waist-to-height ratio below 0.50, which is the boundary between NICE's healthy and increased central-adiposity bands.
How should I measure my waist?
NICE advises finding the bottom of the ribs and the top of the hips, placing the tape midway between them, and taking the measurement after breathing out naturally.
Why is waist-to-height interpretation limited when BMI is 35 or higher?
NICE recommends waist-to-height ratio alongside BMI as a practical estimate of central adiposity in adults with BMI below 35. The calculator therefore shows the ratio at higher BMI values but does not apply the category in the same way.
Is the reference weight interval my ideal weight?
No. It is a mathematical range corresponding to the selected healthy-BMI boundaries at the entered height. It is not a personalised goal, treatment target, or medical recommendation.
Does BMI directly measure body fat?
No. BMI cannot directly distinguish body fat from muscle, bone, or body fluid and does not show where body fat is distributed.
Is BMI accurate for muscular people?
BMI can be less informative when high muscle mass contributes substantially to body weight. NICE advises interpreting BMI with caution in adults with high muscle mass.
Does BMI work differently for men and women?
The adult BMI formula and standard numerical categories do not change according to sex. Individual body composition and fat distribution can still differ.
Does age change the adult BMI formula?
The formula is unchanged for adults, but interpretation may need additional context in older adults. NICE advises caution when interpreting BMI in people aged 65 and older.
Can children use this BMI calculator?
No. This calculator uses fixed adult categories and is intended for adults aged 20 and older. Children and teenagers aged 2 through 19 require age- and sex-specific BMI-for-age interpretation.
Can I use this BMI calculator during pregnancy?
Adult BMI categories are not designed to assess pregnancy-related weight change, and the reference-weight interval should not be used as a pregnancy weight-gain target.
Why can a small height or weight change alter the category?
BMI categories use fixed numerical thresholds. Measurement differences and rounding can therefore change the displayed category when a result is close to a boundary.
Does BMI diagnose obesity-related disease?
No. BMI is a screening measure. It does not diagnose diabetes, cardiovascular disease, hypertension, or another condition.
What else should be considered alongside BMI?
A fuller health assessment can include medical history, health behaviours, physical examination findings, blood pressure, laboratory results, body composition, and other relevant information.
What is the BMI formula in kilograms and centimetres?
BMI equals weight in kilograms multiplied by 10,000 and divided by height in centimetres squared.
What is the BMI formula in pounds and inches?
A commonly used imperial shortcut is approximately 703 multiplied by weight in pounds divided by height in inches squared.