How to convert units with conversion factors
A conversion factor is a fraction equal to 1 because its numerator and denominator are the same quantity in different units (for example, 1 km / 1000 m). Multiplying by it does not change the value, only the unit. Arrange the factors so the unit you want to get rid of appears once on top and once on the bottom, and cancel it.
Example · 90 km/h to m/s
90 km/h · (1000 m / 1 km) · (1 h / 3600 s) = 90,000 / 3600 = 25 m/s. Shortcut: from km/h to m/s divide by 3.6; the other way, multiply by 3.6.
Example · square and cubic units
If 1 m = 100 cm, then 1 m² = 100² cm² = 10,000 cm² and 1 m³ = 100³ cm³ = 1,000,000 cm³. The factor is raised to the same power as the unit. Also remember that 1 dm³ = 1 L and 1 cm³ = 1 mL.
Metric to imperial: the conversions you need most
| From | To | Multiply by |
|---|---|---|
| inches | centimeters | 2.54 |
| feet | meters | 0.3048 |
| miles | kilometers | 1.609344 |
| pounds | kilograms | 0.45359237 |
| ounces | grams | 28.3495 |
| US gallons | liters | 3.78541 |
| mph | km/h | 1.609344 |
| psi | kPa | 6.89476 |
Temperature is special
Temperature scales do not share the same zero, so multiplying is not enough: K = °C + 273.15 and °F = °C · 9/5 + 32. However, a temperature difference of 1 °C is the same as 1 K.
SI prefixes
| Prefix | Symbol | Factor | Prefix | Symbol | Factor |
|---|---|---|---|---|---|
| tera | T | 10¹² | deci | d | 10⁻¹ |
| giga | G | 10⁹ | centi | c | 10⁻² |
| mega | M | 10⁶ | milli | m | 10⁻³ |
| kilo | k | 10³ | micro | µ | 10⁻⁶ |
| hecto | h | 10² | nano | n | 10⁻⁹ |
| deca | da | 10¹ | pico | p | 10⁻¹² |
Frequently asked questions
Why does my 1 TB drive show 931 GB?
The manufacturer counts 1 TB = 10¹² bytes, but many operating systems show sizes in binary units (1 GiB = 2³⁰ bytes) while still writing "GB". 10¹² / 2³⁰ ≈ 931. Check it in the "Data" category.
Is a food calorie the same as a calorie?
No: the "calories" on food labels are kilocalories (kcal). One kcal is 4,184 J.
What is the difference between horsepower and metric horsepower?
Mechanical horsepower (hp) is about 745.7 W; metric horsepower (CV in Spanish, PS in German) is 735.5 W, about 1.4% less. Car specifications in Europe often use the metric one.