How to Convert Units
6 min read · Updated September 26, 2026 · by the SolveCalcPro editorial team
Converting units means expressing the same quantity in a different unit. The trick is to multiply by a conversion factor that equals 1, such as 1.609 km per mile, so the value stays the same while the unit changes.
The conversion-factor method
To convert 5 miles to kilometres, multiply by (1.609344 km ÷ 1 mi). The miles cancel and you are left with 5 × 1.609344 = 8.04672 km. Writing the units and cancelling them helps you catch errors.
The metric prefix ladder
| Prefix | kilo (k) | hecto | deca | base | deci | centi (c) | milli (m) |
|---|---|---|---|---|---|---|---|
| Factor | 1,000 | 100 | 10 | 1 | 0.1 | 0.01 | 0.001 |
Moving one step down the ladder multiplies by 10, so 2.5 km = 2,500 m = 250,000 cm.
Common factors
| Quantity | Factor |
|---|---|
| Length | 1 in = 2.54 cm · 1 ft = 0.3048 m · 1 mi = 1.609 km |
| Mass | 1 lb = 0.4536 kg · 1 oz = 28.35 g |
| Volume | 1 US gal = 3.785 L · 1 US cup = 236.6 mL |
| Speed | 1 mph = 1.609 km/h · 1 m/s = 3.6 km/h |
Temperature is different
Because the scales have different zero points, you must use formulas, not just a factor. °F = °C × 9/5 + 32 and °C = (°F − 32) × 5/9. Kelvin is Celsius plus 273.15. Body temperature 98.6 °F converts to (98.6 − 32) × 5/9 = 37 °C = 310.15 K.
Chained conversions
Convert 60 mph to m/s: 60 × 1.609344 = 96.56 km/h, then ÷ 3.6 = 26.82 m/s. Use the unit converter to do it in one step.
Practice questions
Test yourself, then tap to check. Answers are calculated by the tools linked below.
- How many kilometres is a 26.2-mile marathon?
Show answer
26.2 mi in km: 42.1648128
- Write 45,000,000 in scientific notation.
Show answer
Scientific notation: 4.5 × 10^7
- Area of a circle with radius 7.
Show answer
Area: 153.93804
Frequently asked questions
What is the easiest way to convert miles to km?
Multiply by 1.609, or roughly by 1.6.
Why do US and UK gallons differ?
They are different historical units: 1 US gallon = 3.785 L and 1 imperial gallon = 4.546 L.
How do I convert Celsius to Fahrenheit quickly?
Double the Celsius value, subtract 10%, then add 32. It is close for everyday temperatures.
Are pounds a unit of mass or weight?
In everyday use pounds measure mass (like kilograms); in physics the pound-force is a unit of weight.
Try it yourself
Unit ConverterConvert length, mass, temperature, volume, area and speed between metric and imperial units.
Scientific Notation ConverterConvert numbers to scientific, engineering and standard notation.Geometry CalculatorArea, perimeter, circumference, volume and surface area of common shapes.Keep learning
Written and reviewed by the SolveCalcPro editorial team. Spot a mistake? Tell us. See our editorial policy.