You buy a 1 TB hard drive, plug it in, and your computer says 931 GB. It's not faulty and you haven't been cheated. It's one of several everyday unit traps that catch people out. Here are the most common ones and how to avoid them.
Decimal versus binary storage
Drive makers count in decimal: 1 TB is 1,000,000,000,000 bytes. Many operating systems count in binary, where 1 "GB" really means 1 gibibyte, or 1,073,741,824 bytes. Divide the drive's bytes by the binary figure and you get about 931. Same drive, two ways of counting.
The fix is simply to know which one you're looking at. The binary units have their own names (KiB, MiB, GiB, TiB), and when you see those, you know the larger divisor is in use.
US and imperial units share names but not sizes
A US gallon is about 3.785 litres. An imperial gallon is about 4.546 litres. That's a difference of roughly 20 per cent, under the same name. The same goes for pints and fluid ounces. If a recipe or fuel figure comes from the UK and you're reading it in the US (or the other way round), check which gallon or pint it means.
There are three kinds of ton
A short ton (US) is 2,000 lb. A long ton (UK) is 2,240 lb. A metric tonne is 1,000 kg, or about 2,205 lb. In freight and shipping, mixing these up costs real money.
Area factors are squared
If 1 metre is about 3.28 feet, then 1 square metre is about 3.28 × 3.28, or roughly 10.76 square feet, not 3.28. People convert length correctly and then forget to square it for area. This is where floor-plan and flooring estimates go wrong.
Temperature needs an offset, not just a multiplier
Most conversions are a single multiplication. Celsius to Fahrenheit isn't: you multiply by 9/5 and then add 32. That's why 0 °C is 32 °F, not 0 °F. Celsius and Fahrenheit meet at exactly one point, −40, where both scales read the same.
"Knots per hour" is wrong
A knot is already a speed: one nautical mile per hour, or exactly 1.852 km/h. Saying "knots per hour" describes acceleration, not speed.
See the scale, not just the answer
Most online converters give you one number and stop. That's fine for a single conversion, but it hides how the two units relate, which is exactly where these traps live.
Calculate Conversion is a site I built to take a different approach. Each converter shows both scales side by side, so when you convert 1 inch to 2.54 cm you can also read off 0.5, 1.5 and 2 inches at a glance. It lists the exact conversion factor it uses, covers length, weight, temperature, volume, area, speed, pressure, energy, time and data storage, and runs entirely in your browser.
The short version
- Storage: decimal (GB) versus binary (GiB) explains the "missing" space.
- Gallons, pints and fluid ounces differ between US and imperial.
- Short ton, long ton and metric tonne are three different weights.
- Square the factor for area.
- Temperature needs an offset as well as a multiplier.
Once you know where the traps are, unit conversions stop being a source of surprises.
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