Core answer: a good unit converter passes four tests: full category coverage (length, weight, area, volume, temperature, speed, data…), complete unit systems (US vs Imperial gallons, short vs long tons), an explicit conversion basis, and bidirectional solving with reasonable precision. The classic traps: a US gallon is 3.785L but an Imperial gallon is 4.546L — 20% apart.
Four quality tests
| Test | Pass | Fail signal |
|---|---|---|
| Coverage | 10+ categories | length/weight only |
| Unit systems | US/Imperial/metric/Chinese all flagged | bare “gallon”, “ton” |
| Basis shown | exact definitions visible | unexplained rounded factors |
| Bidirectional | either direction solvable | one-way only |
The same-name trap table
| Unit | System A | System B | Gap |
|---|---|---|---|
| gallon | US 3.785 L | Imperial 4.546 L | 20% |
| ton (weight) | short ton 907 kg | long ton 1,016 kg | 12% |
| pint | US 473 ml | Imperial 568 ml | 20% |
| mile | statute 1.609 km | nautical 1.852 km | 15% |
| 两 (tael) | modern 50 g | old system 31.25 g | 60% |
Match by scenario
| Need | Tool |
|---|---|
| Height/length | Length converter (cm↔ft/in) |
| Body weight | Weight converter (kg↔lb↔jin) |
| Recipes | Volume converter (cup/oz/tbsp↔ml) |
| Driving abroad | Speed converter (mph↔km/h) |
| Phone storage | Data converter (GB↔TB, MiB note) |
| Oven settings | Temperature converter (°F↔°C) |
Example: importing a 65-inch TV
The “65-inch” is the diagonal: 65×2.54 = 165.1cm. A 16:9 panel is then about 143.9×80.9cm. Check the wall: the mount zone is 150cm wide, so it fits with 3cm margin per side. Same flow converts the listed 55 lb shipping weight: 55×0.4536 ≈ 25kg — two-person lift.
Example: baking with an American recipe
“2 cups flour, 1 cup milk, bake at 350°F.” Cups: 2×236.6 = 473ml flour (about 250g by density), 237ml milk. Oven: (350−32)×5/9 ≈ 177°C, bake as directed. The two conversions — volume and temperature — are exactly where unit tools earn their keep; guessing “350°C” would carbonize the cake.
Common mistakes
- “A gallon is a gallon”: US and Imperial gallons differ 20% — fuel-economy figures from the UK and US are not comparable directly.
- “1 GB = 1000 MB exactly”: vendors use decimal (1GB=10⁹ bytes), operating systems binary (1GiB=2³⁰) — that is why a “1TB” drive shows 931GB.
- “Chinese units are folklore”: modern definitions are exact — 1斤=500g, 1亩≈666.67㎡, 1里=500m; old-system 两 (31.25g) survives in herbal medicine.
- “Temperature converts by scaling”: °F→°C needs the offset: (F−32)×5/9; 100°F is 37.8°C, not a proportional rescale.