New BMI Calculator 2026
The Oxford/Trefethen formula (2013) is more accurate than the 200-year-old Quetelet formula — especially if you are taller than 5'9" or shorter than 5'3".
Calculate My New BMI →Free · No signup · Both formulas side by side
What Is the New BMI Formula?
Professor Nick Trefethen of Oxford University proposed a corrected BMI formula in 2013:
The original 1832 Quetelet formula uses height² — which overstates BMI for tall people and understates it for short people. Trefethen's exponent of 2.5 corrects this bias because body volume scales with height at a power between 2 and 3, not exactly 2.
Old vs New BMI Formula
| Old BMI (Quetelet, 1832) | New BMI (Trefethen, 2013) | |
|---|---|---|
| Formula | weight ÷ height² | 1.3 × weight ÷ height²·⁵ |
| Proposed | Lambert Quetelet, 1832 | Nick Trefethen, Oxford, 2013 |
| Bias for tall adults | Overstates BMI ↑ | Corrected |
| Bias for short adults | Understates BMI ↓ | Corrected |
| WHO Standard? | ✅ Yes | Not yet — academic use |
| Best for | Average heights (5'3″–5'9″) | All adult heights |
Worked Examples
5'4" (163cm)
65 kg
Average height — small difference
6'2" (188cm)
85 kg
Tall adult — old formula overstates by 2.1 points
5'0" (152cm)
55 kg
Short adult — old formula understates by 1.7 points
New BMI vs Old BMI — Results Across Heights
All examples use 75 kg (165 lb) to isolate the height effect. Notice how the gap widens at extreme heights.
| Height | Old BMI (÷h²) | New BMI (÷h²·⁵) | Difference | Verdict |
|---|---|---|---|---|
| 4'11" (150 cm) | 33.3 | 36.4 | +3.1 | Old understates — short person flagged as overweight, new shows obese |
| 5'2" (157 cm) | 30.4 | 32.2 | +1.8 | Old understates BMI for shorter adults |
| 5'5" (165 cm) | 27.5 | 28.3 | +0.8 | Near-average height — minimal difference |
| 5'7" (170 cm) | 26.0 | 26.4 | +0.4 | Closest match between formulas |
| 5'10" (178 cm) | 23.7 | 23.4 | −0.3 | Formulas nearly equal — old slightly overstates |
| 6'0" (183 cm) | 22.4 | 21.7 | −0.7 | Old overstates BMI — tall adult misclassified |
| 6'3" (190 cm) | 20.8 | 19.8 | −1.0 | Old overstates by a full point |
| 6'6" (198 cm) | 19.1 | 17.9 | −1.2 | Old formula falsely keeps tall adult as "Normal" |
* New BMI = 1.3 × 75 ÷ height(m)^2.5. Old BMI = 75 ÷ height(m)². Categories: Underweight <18.5, Normal 18.5–24.9, Overweight 25–29.9, Obese ≥30.
Why the Old Formula Falls Short — The Science
The Quetelet formula (weight ÷ height²) was never designed as a medical measure. Lambert Quetelet was a Belgian statistician studying the average person in the 1830s — before germ theory, modern nutrition, or the concept of body composition. He used height² as the divisor because it produced a convenient constant for people of average height. It was never validated for tall or short individuals.
The Geometry Problem
A person who is 10% taller than average is not just 10% taller — they are also wider, deeper, and proportionally heavier. Body volume scales with height at a power between 2 and 3, not exactly 2. Dividing by height² therefore systematically underestimates the BMI of tall people.
The Statistical Evidence
A 2016 PLOS ONE study confirmed that the optimal height exponent across large populations is approximately 2.5, supporting Trefethen's formula. A 2002 BMJ study of diverse ethnic populations also found that the height² divisor introduces systematic error across groups with different body proportions.
The Clinical Impact
A 6'3" man classified as "Normal" (BMI 20.8) by the old formula may actually carry excess body fat that the old tool misses. Conversely, a 4'11" woman classified as "Obese" (BMI 33.3) may be appropriately proportioned for her height. These misclassifications affect insurance, eligibility for weight-loss surgery, and clinical risk assessments.
Trefethen's original 2013 argument (published in a letter to The Economist and on the Oxford Mathematical Institute website): “If you are short, BMI overestimates how fat you are. If you are tall, BMI underestimates how fat you are. It is time we stopped using it.” His proposed correction — multiplying by 1.3 and using height^2.5 — produces a distribution that matches body fat percentage data far more closely across the height spectrum.
Limitations of BMI — Both Old and New
The Trefethen formula corrects the height bias, but both formulas share the fundamental limitation of all BMI calculations: they measure weight relative to height, not actual body fat. Neither formula accounts for age, sex, ethnicity, or muscle mass.
Athletes and muscular individuals
Muscle weighs more than fat. A competitive athlete or bodybuilder can have a BMI of 28–30 (overweight by BMI) while carrying very low body fat. Neither formula can distinguish muscle from fat.
Older adults
After age 60, people typically lose muscle and gain fat even at a stable weight. A 70-year-old with a Normal BMI may carry more visceral fat than a 30-year-old with the same BMI — neither formula captures this.
Ethnic body composition differences
Asian populations tend to carry more visceral fat at lower BMI values. The WHO has published separate BMI cut-points for Asian adults (overweight at ≥23 rather than ≥25). The Trefethen formula does not address this.
Pregnancy
BMI is not a valid health measure during pregnancy. Expected weight gain varies by trimester and starting BMI.
Children and adolescents
BMI-for-age percentiles (not fixed cut-points) apply to people under 20. Neither adult formula should be applied to children.
Very short and very tall adults
The Trefethen formula improves accuracy but still breaks down at extreme heights (under 4'8" or over 6'10") where body proportions differ significantly from the average adult used to calibrate the exponent.
When to Go Beyond BMI for Body Composition
BMI — even the improved Trefethen version — is a screening tool, not a diagnostic one. If your BMI falls outside the Normal range, or if you are athletic, older, or from a population with different body composition norms, consider these more direct measures:
Body Fat Percentage
DEXA scan (gold standard), hydrostatic weighing, Bod Pod, bioelectrical impedance, or the Navy tape method (waist + neck circumference)
Healthy ranges
Men: 10–20% | Women: 18–28% (general fitness ranges)
Why it matters
Directly measures fat mass vs lean mass. A muscular person with BMI 28 may have 15% body fat — perfectly healthy.
Waist Circumference
Measure at the navel (or at the narrowest point of the torso) with a flexible tape
Healthy ranges
Men: <40 in (102 cm) | Women: <35 in (88 cm)
Why it matters
Strongly predicts visceral (abdominal) fat and metabolic risk. Waist-to-height ratio (waist ÷ height < 0.5) is another simple marker.
Waist-to-Hip Ratio
Measure waist at narrowest, hip at widest, divide
Healthy ranges
Men: <0.90 | Women: <0.85 (WHO thresholds)
Why it matters
Apple-shaped fat distribution (high W:H ratio) carries more metabolic risk than pear-shaped (low W:H). BMI cannot distinguish these.
Bottom line: Use the new BMI as a quick first screen. If your result is borderline, or if you are particularly tall, short, muscular, older, or of Asian descent, pair it with waist circumference and — if possible — a body fat measurement. For any clinical decision, consult a healthcare provider.
Try the Full BMI Calculator
Our BMI calculator shows both the old Quetelet result and the new Trefethen result side by side — with WHO category, healthy weight range, and metric/imperial support.
Calculate My BMI →Frequently Asked Questions
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