ToolCraft
Education ·Jul 5, 2026 ·1 min read

The Complete Guide to Unit Conversion

Length, mass, temperature, volume, area, speed and data — every conversion rule with the traps that cause errors.

Marcus Webb

Tools & Productivity Writer

Unit conversion is simple in principle — multiply by a factor — and dangerous in practice, because a wrong factor looks exactly like a right one. This guide covers the categories, the rules and the traps.

The general rule

Every conversion reduces to: value × factor. The work is finding the right factor and applying it the right way round. When unsure, sanity-check: is the answer bigger or smaller than the input, and does that direction make sense?

Linear categories

  • Length: 1 inch = 2.54 cm exactly; 1 mile = 1.609 km.
  • Mass: 1 lb = 0.454 kg; 1 oz = 28.35 g.
  • Volume: 1 US gallon = 3.785 L; 1 imperial gallon = 4.546 L (they differ!).
  • Area: square the length factor. 1 sq mile = 2.59 sq km.
  • Speed: 1 mph = 1.609 km/h; 1 knot = 1.852 km/h.
  • Data: 1 KB = 1024 B, 1 MB = 1024 KB (binary).

Temperature: the exception

Temperature has an offset, so it is not a pure factor. °F = °C × 9/5 + 32. The offset (32) is why you cannot just multiply. Water: 0°C = 32°F, 100°C = 212°F.

The three real-world traps

  • US vs imperial gallons: a 20% difference that breaks recipes and fuel math.
  • Binary vs decimal: "1 GB" is 1,000,000,000 bytes in marketing and 1,073,741,824 bytes in operating systems.
  • Cooking volume vs mass: a cup of flour and a cup of sugar weigh differently; mass-based recipes are more precise.

When a number is used for money or safety, always double-check the factor and the direction with a converter.

How do I convert area when I know the length factor?

Square the linear factor. If 1 m = 3.281 ft, then 1 m² = 3.281² = 10.76 ft².

Tools for this task

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