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5 miles is 8.0467 km. 10 kg is 22.046 lb. 98.6°F is exactly 37°C. Exact conversion factors, six categories, zero rounding surprises.
Unit conversion errors sink recipes, workouts, construction orders, and at least one Mars orbiter. The fix is always the same: convert through a base unit with exact definitions — an inch is exactly 2.54 cm, a pound exactly 0.45359237 kg, a US gallon exactly 3.785411784 liters. This converter covers the six categories that come up daily — length, weight, volume, area, speed, and temperature — using exact factors, so 6 feet is 182.88 cm every single time.
Everyone uses it mid-task: 5 miles is 8.0467 km, 10 kg is 22.046 lb, 98.6°F is exactly 37°C — length, weight, volume, area, speed, temperature.
Rounding in the factors. Many converters use shortcuts like 1 mile ≈ 1.6 km or 1 kg ≈ 2.2 lb — fine for conversation, wrong for work. The exact definitions: 1 mile = 1.609344 km exactly, 1 kg = 2.2046226 lb, 1 inch = 2.54 cm exactly (by international agreement since 1959), 1 US gallon = 231 cubic inches = 3.785411784 L. The errors compound: a 100-mile trip converted at 1.6 is 160 km, actually 160.93 km — nearly a kilometer off. The notorious real-world case is the Mars Climate Orbiter, lost in 1999 because one team output pound-force seconds and another expected newton-seconds — a $327 million unit-conversion failure.
Temperature cannot convert by simple multiplication because the scales have different zeros — that is why °C = (°F − 32) × 5/9 has a subtraction first. The anchors worth memorizing: 32°F = 0°C (water freezes), 212°F = 100°C (boils), 98.6°F = 37°C (body), and −40° is the one point where Fahrenheit and Celsius agree. Kelvin shares Celsius's degree size but starts at absolute zero: K = °C + 273.15, so room temperature (~22°C) is about 295 K. The quick mental trick for °F→°C weather: subtract 30 and halve (68°F → (68−30)/2 = 19°C, true answer 20°C — close enough for a jacket decision).
The daily-driver set: 1 inch = 2.54 cm, 1 mile ≈ 1.61 km (a 10k race is 6.2 miles), 1 kg ≈ 2.2 lb (so a 150 lb person is ~68 kg), 1 liter ≈ 1.06 US quarts, 1 US gallon ≈ 3.79 L, 1 acre = 43,560 sq ft (a football field without end zones is ~1.1 acres), and 1 m/s = 2.237 mph (which is why a 100 km/h highway is 62 mph). One warning on gallons: the US gallon (3.785 L) and the imperial gallon (4.546 L) differ by 20% — British fuel economy figures use imperial, which is why UK mpg numbers always look better than US ones for the same car.