Calculate BMI with WHO categories
Body Mass Index from weight and height, color-coded against the WHO classification table. Switch between metric and imperial. Runs entirely in your browser.
How to compute BMI
BMI = weight (kg) ÷ height (m)². It's a screening number — useful for population averages, less so for very muscular or very small frames.
- 01
Pick units
Metric or imperial — values convert under the hood.
Inputs are treated as UTF-8 throughout the pipeline and validated client-side as you type — nothing dispatches until the value is well-formed.
unitinpututf-8client-sidevalidatedunit · textmetric · setkg/cm
loaded · parsedclient-side - 02
Enter weight
Use today's reading — fluctuates daily by a kilo or so.
Each toggle re-computes through a memoised hook, so flipping options never resets your input and the UI stays snappy on large inputs.
weightoptionslivememoisedtoggleskg · memoised70 · live70 kg
options set · memoisedlive - 03
Enter height
Measure barefoot, looking straight ahead.
Randomness — where used — comes from
crypto.getRandomValues, neverMath.random. Heavy work sits inuseMemo.heightcomputeinstantlocaldeterministiccm · instant175 · computed175 cm
computed · instantinstant - 04
Read category
Color codes match the WHO classification.
Copy writes through
navigator.clipboardand downloads use aBlob— zero network traffic. Same input plus same options always yield the same bytes.whocopydownloadzero-networkreproducibleok · copiednormal · copied22.9 — Normal
ready · ready to shipsuccess
Pro tips for everyday utilities
Reach for crypto.getRandomValues
Cryptographically random for IDs, tokens, salts. Math.random is for animation and games, not security.
Mock data with seeds
Faker-style libraries can be seeded so tests are reproducible. Embed the seed in the fixture file.
Reference cards beat memory
Cheatsheets for Vim, Git, Docker, Linux signals are faster than man pages when you’re mid-flow.
Browser APIs vary
Touch events, gamepads, and clipboard differ across vendors. Feature-detect before use.
Persist in the right place
localStorage for user preferences, sessionStorage for transient state, IndexedDB for structured data.
When BMI is useful
Screening, fitness logs, doctor visits, insurance forms.
GP screening
Quick risk signal for cardiovascular health. Random values come from crypto.getRandomValues — cryptographically secure for tokens, salts, and session IDs. Math.random is deterministic enough for animations but predictable enough to be a security bug.
CSPRNG-backedFitness logs
Track relative change over weeks. Pair with hashids, ULID, or UUID v7 when you need both random and sortable — useful for primary keys, public-facing IDs, and event log ordering without a sequence column.
reproducibleInsurance forms
Many policies ask BMI on applications. Output is deterministic given the same seed, so fixtures and tests stay reproducible across CI runs and contributor machines. Embed the seed in the fixture file for full replay.
snappyDiet planning
Anchor calorie targets to a category. Heavy work runs through useMemo so the UI stays snappy on large inputs. Same input + same options yields identical bytes — the kind of stability snapshot tests rely on.
feature-detectedPre-op
Some surgeries gate on BMI thresholds. Storage tools persist to the right backend: localStorage for user preferences, sessionStorage for transient state, IndexedDB for structured data. Pick the right shelf and quota worries disappear.
deterministicPopulation stats
Compare yourself to country/age cohorts. Browser feature detection: clipboard, fullscreen, gamepad, and Web Bluetooth APIs gracefully fall back when unavailable. No surprise white screens on Safari or older Firefox.
production-readyFrequently asked questions
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