The New BMI Formula (Trefethen)
Oxford mathematician Nick Trefethen’s fix for a known flaw in classic BMI — it makes tall people look heavier and short people lighter than they are.
The new BMI is 1.3 × weight ÷ height^2.5 (weight in kilograms, height in metres) — proposed by Oxford mathematician Nick Trefethen in 2013 to correct classic BMI, which divides by height squared and so overstates the tall and understates the short. Near average height the two agree; at the extremes they part.
Here the two BMIs differ by 0. Near average height the two nearly agree; drag height to the extremes to see them part. Both read the same WHO categories — Trefethen changes the number, not the cut-points.
Why height² is the problem
Classic BMI divides weight by height squared — a choice from the 1830s that’s easy to compute but doesn’t match how bodies actually scale. Real people don’t get proportionally heavier by an exact square as they get taller, so height² overstates a tall person’s BMI and understates a short person’s.
Trefethen’s exponent of 2.5 fits the height-to-mass relationship more faithfully, and the 1.3 factor keeps the numbers close to the BMI everyone knows for people of average height.
Same categories, a different number
The new BMI is read against the same WHO cut-points as classic BMI — 18.5, 25, 30, and up. It changes the number, not the thresholds. That’s exactly why showing both is useful: when they land in different bands, that’s the honest uncertainty a single BMI number hides.
Good for
A fairer BMI for the very tall or very short, and a quick reality-check on how much your classic BMI depends on the height formula.
Keep in mind
It’s a proposal, not an official standard — health bodies still use classic BMI — and it inherits every BMI blind spot: it can’t tell muscle from fat or see fat distribution.
One BMI number hides its own uncertainty. See the range.
The BMI calculator runs classic and new BMI together and reads them against your waist — an honest range and a second opinion, not one decimal pretending to be exact.
Open the BMI calculatorNick Trefethen (Oxford) (2013). Proposed in The Economist (letter) / Oxford page — the original write-up. A widely-cited proposal, not an official standard. The factor 1.3 and exponent 2.5 are checked against the source and covered by our test suite.
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FAQ
What is the new BMI formula?
new BMI = 1.3 × weight ÷ height^2.5, with weight in kilograms and height in metres. It was proposed by Oxford mathematician Nick Trefethen in 2013 as a correction to the classic BMI (weight ÷ height²).
Why did Trefethen propose a new BMI?
Classic BMI divides by height squared, but bodies don't scale as a perfect square. So the old formula makes tall people look heavier and short people lighter than they really are. The exponent 2.5 (and the 1.3 factor to keep familiar numbers) fits real body shapes better across heights.
Is the new BMI better than the old one?
It's gentler at the height extremes, and many find it fairer — but it's a proposal, not an official standard. Health bodies still use classic BMI. WIMSTA shows both so the gap between them is visible: that gap is the honest signal that no single BMI number is exact.
Does the new BMI use different categories?
No. It reads the same WHO cut-points as classic BMI (18.5 / 25 / 30 / 35 / 40). Trefethen changes how the number is computed, not how the categories are drawn.
How every formula is sourced: the method page and the formula registry.