The essentials

BMI (body mass index) is your weight in kilograms divided by your height in meters squared. For adults, 18.5 to 24.9 is the healthy range, 25 to 29.9 is overweight and 30 or more is obesity; many Asian countries use lower cut-offs. BMI is a quick screening number, not a diagnosis: your waist size and a few blood tests (blood sugar, cholesterol, liver enzymes) tell your doctor much more.

How do I read the BMI line on my health check?

In this fictitious checkup, BMI sits with the vital signs, next to blood tests often ordered when it is high.

Fictitious example of a checkup summary, man aged 52 (United States format)
Measurement or testResultReference rangeFlag
Height5 ft 7 in (170 cm)--
Weight190 lb (86.2 kg)--
BMI29.8 kg/m²18.5-24.9H Overweight
Waist circumference39 in (99 cm)40 in or less (men)-
Hemoglobin A1c5.9%Below 5.7%H
Glucose, fasting108 mg/dL70-99 mg/dLH
Triglycerides172 mg/dLBelow 150 mg/dLH
ALT38 U/L7-55 U/L-
TSH2.1 mIU/L0.4-4.0 mIU/L-
  1. BMI 29.8: 86.2 ÷ (1.70 × 1.70) = 29.8. Overweight on the US chart[1], "high risk" in Singapore (from 27.5)[5].
  2. Waist 39 in: under the risk line of 40 inches for men (35 for women)[8].
  3. A1c 5.9% and fasting glucose 108: prediabetes range, the key finding[9].
  4. ALT 38: "normal" for the lab, but above the 29 to 33 U/L liver specialists use for men[10].

BMI is one line; your blood results tell the rest. BloodSense reads your whole report, explains each flag in plain words and prepares your questions for the visit.

Get my results explained

What is BMI and how is it calculated?

BMI compares weight with height. The formula is weight (kg) ÷ height (m)²[1]. In US units, multiply your weight in pounds by 703, then divide by your height in inches, twice[13]. Example: 150 lb at 5 ft 5 in (65 in) gives 150 × 703 ÷ 65 ÷ 65 = 25.0.

It is a quick screening tool: it flags weight that may affect health, but it does not measure body fat or health itself[1][6]. Like the blood pressure chart by age, it is a starting point.

What is a normal BMI? The adult chart

For adults 20 and older, the cut-offs are the same for men and women of every age[1], and WHO uses the same thresholds[3].

BMI categories for adults (CDC and WHO)
BMI (kg/m²)Category
Below 18.5Underweight
18.5 to 24.9Healthy weight
25 to 29.9Overweight
30 to 34.9Class 1 obesity
35 to 39.9Class 2 obesity
40 or moreClass 3 (severe) obesity

Source: CDC[1].

Why is the BMI range lower for Asian people?

At the same BMI, many Asian people carry more body fat and face diabetes and heart disease earlier. A WHO expert group found risk rising from a BMI of 22 to 25 depending on the population, and proposed action points at 23, 27.5, 32.5 and 37.5[4].

BMI risk bands used in Singapore (Health Promotion Board)
BMI (kg/m²)Health risk
18.5 to 22.9Low (healthy)
23.0 to 27.4Moderate
27.5 or moreHigh

Source: Health Promotion Board, Singapore[5].

What is a healthy BMI for children and teens?

From age 2 to 19, BMI is compared with children of the same age and sex on growth charts, as a percentile[2].

BMI-for-age categories, ages 2 to 19 (CDC)
PercentileCategory
Below the 5thUnderweight
5th to below the 85thHealthy weight
85th to below the 95thOverweight
95th or higherObesity
120% of the 95th or higher, or BMI 35 or moreSevere obesity

Source: CDC[2]. WHO uses its own growth reference instead[3], so categories can differ between countries.

What does a high or low BMI mean, and when is BMI wrong?

A high BMI usually means extra body fat, but not always. A 2025 international commission advises confirming it with another measure, such as waist size, unless BMI is above 40[6].

  • Common Extra body fat, raising the risk of type 2 diabetes, high blood pressure and heart disease[5].
  • Common Belly fat with a "normal" BMI. Fat around the waist carries more risk, and BMI can miss it[7].
  • Possible Muscle, not fat. Athletes and very muscular people can have a high BMI with little fat[5][13].
  • Possible Muscle loss with age. After 65, standard charts are less reliable[13].
  • Rare A low BMI from illness. A BMI below 18.5 is linked to malnutrition and weak bones[13].

Which blood tests go with a high BMI?

A high BMI is mainly a reason to look for silent problems in the blood:

  • Blood sugar. Hemoglobin A1c or fasting glucose. The ADA advises testing adults with a BMI of 25 or more (23 with Asian ancestry) plus one risk factor, and everyone from 35, at least every 3 years[9]. See the blood sugar chart.
  • Cholesterol. A lipid panel: every 4 to 6 years for most healthy adults, more often with diabetes or heart disease[12]. It includes triglycerides.
  • Liver enzymes. An ALT test can hint at fatty liver disease. A normal ALT does not rule it out: specialists consider about 29 to 33 U/L normal for men and 19 to 25 U/L for women, below most lab ranges[10]. With diabetes or obesity plus metabolic problems, doctors may calculate a FIB-4 score[10].
  • TSH, if you have signs of an underactive thyroid, which can add weight[11].

Children and teens with overweight and a risk factor may also be tested for diabetes from puberty or age 10[9]. See also diabetes.

How do I measure height, weight and waist correctly?

  • Weigh yourself in light clothing, without shoes, on the same scale and at the same time of day.
  • Measure your height against a wall, heels down, rather than guessing it.
  • Measure your waist on bare skin, just above the hip bones, after breathing out normally.
  • Do not use BMI during pregnancy: the ranges do not apply[5].

When should I see a doctor about my BMI?

When to see a doctor

  • Emergency: call 911 for chest pain, sudden breathlessness, fainting, or confusion with intense thirst.
  • Within a few days if you are losing weight without trying, or have strong thirst and frequent urination.
  • At your next appointment if your BMI is 25 or more (23 with Asian ancestry) or below 18.5, or your waist is above 40 inches (men) or 35 inches (women), and your blood sugar and cholesterol have not been checked.

Questions to ask your doctor

  1. Is my BMI a fair picture of my health, given my build, age and ancestry?
  2. Should we also measure my waist or waist-to-height ratio?
  3. Which blood tests do I need now: A1c, lipid panel, ALT?
  4. Are my liver enzymes truly normal for someone with my weight?

Frequently asked questions

Is BMI different for men and women?

No. For adults 20 and older, the formula and cut-offs are the same for both sexes and all ages[1]. It does not show where the fat is stored, which is why waist size helps[7].

Can I have a normal BMI and still have high blood sugar?

Yes. Belly fat, family history, age and ancestry all count, which is why everyone is tested from age 35[9].

What BMI counts as obese?

30 or more for adults on the WHO and US charts[1][3]; Singapore treats 27.5 or more as high risk[5].

  1. Centers for Disease Control and Prevention. Adult BMI categories. March 19, 2024. cdc.gov (accessed November 9, 2026)
  2. Centers for Disease Control and Prevention. Child and teen BMI categories. June 28, 2024. cdc.gov (accessed November 9, 2026)
  3. World Health Organization. Obesity and overweight. Fact sheet, October 7, 2026. who.int (accessed November 9, 2026)
  4. WHO Expert Consultation. Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. Lancet. 2004;363(9403):157-163. doi:10.1016/S0140-6736(03)15268-3 (accessed November 9, 2026)
  5. Health Promotion Board, Singapore. Weight, BMI and health problems. HealthHub, reviewed May 10, 2023. healthhub.sg (accessed November 9, 2026)
  6. Rubino F, Cummings DE, Eckel RH, et al. Definition and diagnostic criteria of clinical obesity. Lancet Diabetes Endocrinol. 2025;13(3):221-262. doi:10.1016/S2213-8587(24)00316-4 (accessed November 9, 2026)
  7. Ross R, Neeland IJ, Yamashita S, et al. Waist circumference as a vital sign in clinical practice: a Consensus Statement from the IAS and ICCR Working Group on Visceral Obesity. Nat Rev Endocrinol. 2020;16(3):177-189. doi:10.1038/s41574-019-0310-7 (accessed November 9, 2026)
  8. National Heart, Lung, and Blood Institute. Assessing your weight and health risk. National Institutes of Health. nhlbi.nih.gov (accessed November 9, 2026)
  9. American Diabetes Association Professional Practice Committee. 2. Diagnosis and classification of diabetes: Standards of Care in Diabetes—2026. Diabetes Care. 2026;49(Suppl 1):S27. doi:10.2337/dc26-S002 (accessed November 9, 2026)
  10. Rinella ME, Neuschwander-Tetri BA, Siddiqui MS, et al. AASLD Practice Guidance on the clinical assessment and management of nonalcoholic fatty liver disease. Hepatology. 2023;77(5):1797-1835. doi:10.1097/HEP.0000000000000323 (accessed November 9, 2026)
  11. MedlinePlus. Hypothyroidism. US National Library of Medicine. medlineplus.gov (accessed November 9, 2026)
  12. Centers for Disease Control and Prevention. Testing for cholesterol. May 15, 2024. cdc.gov (accessed November 9, 2026)
  13. Cleveland Clinic. Body mass index (BMI). clevelandclinic.org (accessed November 9, 2026)
  14. Medicare.gov. Obesity behavioral therapy. Centers for Medicare & Medicaid Services. medicare.gov (accessed November 9, 2026)

This page is general information. It does not replace the advice of a doctor, who interprets your BMI and results with your history, examination and other measurements.