The Complete Guide to Converting Units
By Arshi Khan, Founder of Just4Marketing · Last reviewed October 2026
Length, weight, and volume conversions all work the same way once you understand the trick: everything routes through a common base unit. Temperature is the one exception, since Celsius, Fahrenheit, and Kelvin don't share a zero point. This guide covers the exact factors and formulas involved.
How base-unit conversion works
For length, weight, and volume, every supported unit has a fixed conversion factor relative to a base unit — meters for length, grams for weight, and milliliters for volume. Converting from one unit to another is a two-step multiplication: your input value is multiplied by the "from" unit's factor to get the equivalent in the base unit, then divided by the "to" unit's factor to land on the target unit. This is why adding a new unit only requires one conversion factor, rather than a separate formula for every possible pair of units.
Exact conversion factors used
Length and weight route through meters and grams:
| Unit | Category | Value in base unit |
|---|---|---|
| Millimeter | Length | 0.001 m |
| Centimeter | Length | 0.01 m |
| Inch | Length | 0.0254 m |
| Foot | Length | 0.3048 m |
| Yard | Length | 0.9144 m |
| Mile | Length | 1,609.344 m |
| Ounce | Weight | 28.3495 g |
| Pound | Weight | 453.592 g |
| Stone | Weight | 6,350.29 g |
| US cup | Volume | 236.588 mL |
| US gallon | Volume | 3,785.41 mL |
Things to watch for
US vs. imperial volume units differ
US cups, pints, quarts, and gallons are all smaller than their UK imperial counterparts of the same name — double check which system a recipe or source is using.
Temperature can't be scaled like other units
Because 0°C, 0°F, and 0K represent different physical temperatures, converting temperature always requires an offset, not just a multiplication factor.
Precision compounds across multiple conversions
Converting through several unit systems in sequence can accumulate small rounding differences — for critical measurements, convert directly between the two units you need rather than chaining conversions.
Stone is a weight unit, not a length or volume
Stone (6.35 kg) is common in the UK and Ireland for body weight but rarely used elsewhere — make sure it's the unit you actually intend before using it.
Frequently asked questions
How does converting between two units actually work?
For length, weight, and volume, every unit has a fixed conversion factor into a common base unit (meters, grams, or milliliters). To convert from one unit to another, the calculator multiplies your value by the 'from' unit's factor to get the base value, then divides by the 'to' unit's factor.
Why is temperature conversion different?
Celsius, Fahrenheit, and Kelvin don't share a common zero point, so a simple multiplication doesn't work. Instead, the value is first converted to Celsius as an intermediate step, then converted from Celsius to the target scale using the standard formulas.
What's the exact formula for Celsius to Fahrenheit?
Fahrenheit = (Celsius × 9 ÷ 5) + 32. To go the other way, Celsius = (Fahrenheit − 32) × 5 ÷ 9. Kelvin is simply Celsius + 273.15, with no multiplication involved.
Are US customary units (like cups and gallons) the same worldwide?
No. This calculator uses US customary volume measurements (US cups, US gallons, etc.), which differ from UK imperial units of the same name — a US gallon (3,785.41 mL) is smaller than a UK imperial gallon (about 4,546 mL).