Converting units seems easy until a small mismatch causes a big error. A few habits prevent most mistakes.
Temperature is not a simple multiplication
Most conversions multiply by a constant, but temperature scales have different zero points. Fahrenheit equals Celsius × 9/5 + 32, and Kelvin equals Celsius + 273.15. So 25°C is 77°F and 298.15 K. Doubling a Celsius temperature does not double it in Fahrenheit.
Know the exact factors
One inch is defined as exactly 2.54 centimeters. One pound is exactly 0.45359237 kilograms. Remembering a rounded figure is fine for everyday use, but keep more digits in multi-step work.
US and imperial units differ
A US gallon is about 3.785 liters, while an imperial gallon is about 4.546 liters. Pints, quarts, and fluid ounces also differ between the two systems. When you see gallons, check which country the source is from.
Mixing units inside a formula
Many formulas assume consistent units. BMI needs kilograms with meters, or pounds with inches and a conversion constant. Convert everything first, then calculate.
A famous cost of mixing systems
In 1999, NASA lost the Mars Climate Orbiter after one team used imperial units and another used metric for thruster data. The spacecraft approached Mars too low and was lost.
Sanity-check your result
Ask whether the answer is plausible. A kilometer is about 0.62 miles, so a number of kilometers should be larger than the same distance in miles. A quick estimate catches many typing errors.
Common factors worth remembering
One mile is exactly 1.609344 kilometers. One foot is exactly 0.3048 meters. One ounce is about 28.35 grams. One US fluid ounce is about 29.57 milliliters. One liter is about 0.2642 US gallons. To convert meters per second to kilometers per hour, multiply by 3.6, so 100 km/h is about 27.8 m/s or 62.1 mph.
Areas and volumes need squared and cubed factors
A meter is about 3.28084 feet, but a square meter is about 10.7639 square feet, because the factor is squared. A cubic meter is exactly 1,000 liters. Using the linear factor for an area or volume is one of the most common conversion errors, especially in flooring, paint, and shipping estimates.
Weight versus volume in cooking
Converting between cups and grams depends on the ingredient. A cup of flour weighs much less than a cup of sugar or honey. A volume-to-weight conversion needs a density for that specific ingredient, which is why baking recipes that list weights are more reliable.
Rounding and significant figures
Keep extra digits through the calculation and round only the final answer. Report a result with a precision that matches your input; converting a measurement known to the nearest inch into micrometers does not make it more accurate.
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This guide is for general information only. It is not financial, medical, or legal advice.