Krill KitsKrill Kits// A swarm of small, sharp tools for letters, numbers, and units.
§ 01 / TOOL

BMI + Body Fat.

STATUS ACTIVESYSTEM METRICREF WHO + DEURENBERG
> INPUT
SEX
AGE
yrs
HEIGHT (FT / IN)
ft
in
WEIGHT (LB)

ENTER WEIGHT AND HEIGHT.

BMI
// READY

Sex, age, height, weight.

Sex + age unlock a body fat % estimate (Deurenberg formula). BMI works without them.

§ 02 / ABOUT

How to use this.

BMI — Body Mass Index — is the ratio of your weight to the square of your height. It’s a fast health screen used in clinical settings worldwide. The thresholds are the same for men and women and don’t change with age.

Traditional BMI ignores body composition — it can’t distinguish muscle from fat. That limitation is why we added body fat % estimation via the Deurenberg formula, the most-cited population equation for estimating BF% from BMI + age + sex. It’s not as precise as a DEXA scan but gets you within ±5% for most adults, which is enough to know if your BMI is telling the right story.

// THE FORMULAS

  • BMI = weight(kg) ÷ height(m)² or (weight(lb) ÷ height(in)²) × 703.
  • Body fat % (Deurenberg) = 1.20·BMI + 0.23·Age − 10.8·Sex − 5.4 (Sex = 1 for male, 0 for female).

// WHAT THE NUMBERS MEAN

  • Under 18.5 — Underweight. Can indicate under-eating, illness, or age-related muscle loss.
  • 18.5 – 24.9 — Normal. The WHO "healthy" range for most adults.
  • 25 – 29.9 — Overweight. Elevated metabolic risk for most people. False positives are common in muscular builds.
  • 30+ — Obese. Significantly elevated risk across most health markers.

// WHEN BMI LIES

BMI is a population statistic, not a personal diagnosis. It misclassifies muscular athletes as "overweight," underestimates risk in skinny-fat body types, and doesn’t work for children, pregnant people, or elderly adults with muscle loss. Body fat %, waist circumference, and training history all give more accurate individual pictures.

For a deeper dive on what BMI does and doesn’t tell you, see What is TDEE — and why it matters more than BMI.

§ 03 / FAQ

Questions. Answered.

How is BMI calculated?+
Metric: BMI = weight (kg) ÷ height (m)². Imperial: BMI = (weight (lb) ÷ height (in)²) × 703. Both formulas give the same BMI number — it's unit-agnostic on the output side.
How is body fat % calculated?+
Via the Deurenberg (1991) formula: BF% = 1.20·BMI + 0.23·Age − 10.8·Sex − 5.4, where Sex = 1 for male and 0 for female. It’s the most-cited population formula for estimating body fat from BMI alone, accurate to within ±5% for most adults. DEXA, hydrostatic weighing, and BodPod are more accurate but require lab equipment.
Why does BMI need sex and age for body fat?+
Classic BMI doesn't — WHO thresholds are identical for men and women. But at the same BMI, women carry more essential body fat than men (for reproductive function), and everyone gains a small amount of fat with age even if weight stays the same. Deurenberg accounts for both so the body fat estimate is meaningful for individuals, not just populations.
What are the BMI categories?+
The WHO standard for adults: underweight (< 18.5), normal (18.5–24.9), overweight (25–29.9), obese (≥ 30). Same thresholds for men and women.
What are the body fat % categories?+
American Council on Exercise standards. Men: essential 2–5%, athletes 6–13%, fitness 14–17%, average 18–24%, obese ≥25%. Women: essential 10–13%, athletes 14–20%, fitness 21–24%, average 25–31%, obese ≥32%. Women always run higher — that's normal biology, not a health issue.
Is BMI a reliable measure of health?+
It's a rough screening tool, not the whole picture. BMI doesn't account for muscle mass (athletes often read as overweight), bone density, or fat distribution. That's why we added body fat % — it fixes some of the muscle/fat confusion. Waist circumference, DEXA scans, and blood work give even fuller pictures if you want them.
Does BMI work for children or pregnant people?+
Not really. Children have their own age- and sex-adjusted BMI percentiles (not the adult cutoffs). Pregnant people need pregnancy-specific charts. For both, consult a professional rather than a generic web calculator.
What's the healthy weight range on my result?+
We back-calculate the weight range that would give you a BMI between 18.5 and 24.9 at your height. It's not a goal or prescription — just context for the BMI number.
Is my data saved anywhere?+
No. Math happens in your browser. The URL syncs locally so you can share or bookmark; nothing is logged server-side.
§ 04 / TOOLS

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§ 05 / READING

Deeper dives.