How to calculate BMI
Calculating BMI by hand takes about 30 seconds once you've measured your weight and height. This page walks through the five steps, the two formulas, three worked examples, and the common mistakes that lead to wrong answers.
Step 1 — measure your weight
The most accurate reading is in the morning, after using the bathroom, before eating or drinking, and without clothes. A 2017 study in PLOS ONE found that daily weight can fluctuate by 0.5–2 kg from water alone [1], so consistent timing matters if you are tracking change over time.
For the BMI calculation itself, accuracy to the nearest 0.5 kg (1 lb) is fine. Use a calibrated digital scale; analogue bathroom scales drift quickly.
Step 2 — measure your height
Stand against a wall without shoes, with heels, buttocks and shoulder blades touching the wall. Look straight ahead (Frankfurt plane: the lower border of the eye socket in line with the top of the ear canal). Have someone mark the wall at the top of your head with a flat object, then measure from the floor to the mark.
A 2012 study in BMJ comparing self-reported height with measured height found men over-report by an average of 1.3 cm and women by 0.7 cm — enough to change a BMI by 0.5 units at average heights [2]. Always measure.
Step 3 — pick the right formula
Use the form that matches the units you used in steps 1 and 2:
- Metric (kg and m):
BMI = weight (kg) / height (m)² - Imperial (lb and in):
BMI = 703 × weight (lb) / height (in)²
A complete derivation of where the 703 comes from is on the BMI formula page.
Step 4 — calculate
The calculation has three operations:
- Square the height.
- Divide the weight by the squared height.
- (Imperial only) multiply by 703.
Step 5 — read your band
Compare your BMI to the WHO adult bands in the categories guide, or, for children and teens, enter your age and sex in the child calculator to get the CDC percentile.
Worked example 1 — metric adult
A 70 kg adult, 1.75 m tall.
1. 1.75² = 3.0625
2. 70 / 3.0625 = 22.857
3. (no third step)
BMI 22.86 → "Normal weight" (18.5–24.9).
Worked example 2 — imperial adult
The same person: 154.32 lb, 68.90 in.
1. 68.90² = 4747.21
2. 154.32 / 4747.21 = 0.03251
3. 0.03251 × 703 = 22.85
BMI 22.85 — the 0.01 difference vs Example 1 is rounding in the unit conversion.
Worked example 3 — child with CDC LMS
A 10-year-old boy, 140 cm (1.40 m), 45 kg.
1. 1.40² = 1.96
2. 45 / 1.96 = 22.96
The BMI number is 22.96, but the interpretation requires
percentiles, not cut-offs. Using the CDC 2022 extended LMS
reference (data file
Official docs/bmi-age-2022.csv in this
repository), 22.96 at age 10 for a boy falls at the
96th percentile, which is the "Obesity" band
for children. This is computed via the Box–Cox LMS
formula. See the worked
example on the formula page for the full z-score
derivation.
Common mistakes
Forgetting the 703
The single most common error. BMI = lb / in²
(no 703) produces a number ~14× too small; a BMI 22.86 person
would "score" 0.0325. If your BMI looks like a small decimal,
suspect a missing 703.
Using centimetres where the formula expects metres
The metric formula needs metres. 170² = 28,900
(centimetres) is a 10,000× error vs 1.70² = 2.89
(metres). Convert before squaring.
Self-reported height
Men over-report by an average 1.3 cm, women by 0.7 cm [2]. Always measure, especially if your BMI is near a category boundary.
Weighing at the wrong time
A post-meal weight, a clothed weight, or a post-exercise weight can each add 0.5–1.5 kg. Pick one consistent time and stick to it.
Mixing up children's interpretation
A BMI of 22 is "overweight" for a 6-year-old and "underweight" for a 17-year-old. Always use the percentile-based child calculator for ages 2–19 — the WHO adult bands do not apply.
Use the calculator instead
If you have a smartphone, the calculator on our home page does the math in less than 100 ms, handles the unit conversion for you, and for children returns the exact CDC percentile from the same official LMS data referenced in step 4 above. The by-hand version is useful to understand the formula, but for day-to-day tracking the calculator is faster and harder to get wrong.
FAQ
What is the fastest way to calculate BMI?
Divide your weight (kg) by your height (m) squared, or use the 703 conversion for imperial. The home-page calculator does it for you.
Do I need to know my exact height?
Within 1 cm is fine. A 1 cm error at average height changes BMI by about 0.3 units, which is well inside any category band.
How often should I recalculate?
The CDC and NHS both suggest rechecking every few years for adults at a stable weight, or every 1–2 months if you are actively working on weight change.
References
- Díaz VO, Guallar-Castillón P, López-García E. Daily weight fluctuations and their determinants in a free-living population. PLOS ONE. 2017;12(2):e0171490. https://doi.org/10.1371/journal.pone.0171490
- Johnson W, Engelman L, Smythe J, et al. How much does self-reported weight and height underestimate obesity in adults? Cross-sectional analysis of the Health Survey for England. BMJ Open. 2012;2(6):e001370. (Reports the 1.3 cm / 0.7 cm over-reporting figures for men and women.) https://doi.org/10.1136/bmjopen-2012-001370
-
U.S. Centers for Disease Control and Prevention, National
Center for Health Statistics. CDC Extended BMI-for-Age
Growth Chart Percentiles (LMS parameters), 2–20 years,
2022 release. Bundled as
Official docs/bmi-age-2022.csv. https://www.cdc.gov/growthcharts/extended-bmi.htm
Last updated: 6 June 2026. No content on this page constitutes medical advice.
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Use the calculator with your own numbers, read the formula in detail, or see where BMI falls short.