Unit Conversion: A Practical Guide
Unit conversion is just multiplication by cleverly chosen ones, but the stakes are real: a famous Mars orbiter was lost to a metric-imperial mix-up. In daily life the cost is smaller, usually a ruined recipe or a wrong dose estimate.
This guide gives you the essential factors, fast mental shortcuts, and the traps that catch even careful people.
How conversion really works
Every unit has an exact factor to a base unit. To convert, multiply by the factor to the base, then divide by the target's factor. 5 km to miles: 5 x 1000 = 5000 m, / 1609.34 = 3.11 miles.
The inch is defined as exactly 2.54 cm, which anchors the whole imperial-metric bridge. From there everything is arithmetic.
Temperature is the exception: it needs an offset too. F = C x 9/5 + 32. You cannot convert temperature by multiplication alone.
The factors worth memorizing
Length: 1 m = 3.28 ft, 1 km = 0.621 mi, 1 in = 2.54 cm. Weight: 1 kg = 2.205 lb, 1 oz = 28.35 g. Volume: 1 L = 0.264 US gal, 1 cup = 237 ml.
Area scales by the square: 1 sq m = 10.76 sq ft. Volume by the cube: 1 cubic m = 35.3 cubic ft. Forgetting to square or cube is a classic error.
Speed: 1 m/s = 3.6 km/h = 2.237 mph. Energy: 1 kWh = 3.6 MJ = 3412 BTU.
Mental shortcuts
Km to miles: multiply by 0.6 (close) or 5/8 (closer). Celsius to Fahrenheit: double, subtract 10%, add 32. Pounds to kg: halve, then subtract 10% of the half.
Feet to meters: divide by 3 and subtract a bit. These get you within a few percent, fine for daily life.
For money-level precision, always use the calculator, not the shortcut.
Costly mistakes
US vs imperial gallons differ by 20%: 3.785 L vs 4.546 L. Fuel economy and recipes both suffer from this confusion.
Fluid ounces (volume) vs ounces (weight) share a name but measure different things. A cup of flour weighs about 120 g, not 8 oz.
Troy ounces for precious metals are 31.1 g, not 28.35 g. Small difference, big money at gold prices.
Nautical miles, statute miles and kilometers are three different things; aviation and maritime contexts specify which.
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Try the Length calculatorFrequently asked questions
How many feet in a meter?
3.28084 feet, exactly by definition.
Why is temperature conversion different?
The scales have different zero points, so the formula needs an offset plus scaling.
US vs imperial gallon?
US is 3.785 L, imperial is 4.546 L. Check which your source means.
How do I convert square units?
Square the linear factor: sq ft to sq m uses 0.0929, not 0.3048.
What is the fastest km to miles trick?
Multiply by 0.6 for a quick estimate.
Are conversion factors exact?
Most are exact by definition; a few traditional units vary by country.