How Unit Conversion Works — The Conversion Factor Method

Unit conversion is multiplication by a ratio. That’s it. The entire field of unit conversion — length, weight, volume, temperature, speed — boils down to multiplying by the right conversion factor.

The universal method

Result = Value × (Desired Unit / Current Unit)

Example: 5 miles to kilometers. The conversion factor is 1 mile = 1.60934 km:

5 miles × 1.60934 = 8.0467 km

To go the other way, flip the ratio:

8 km × (1 / 1.60934) = 4.971 miles

Common conversion factors

Length

From → To	Factor
Inches → cm	× 2.54
Feet → metres	× 0.3048
Miles → km	× 1.60934
Yards → metres	× 0.9144

Weight

From → To	Factor
Pounds → kg	× 0.45359
Ounces → grams	× 28.3495
Stones → kg	× 6.35029

Volume

From → To	Factor
US gallons → litres	× 3.78541
Cups (US) → ml	× 236.588
Fluid oz (US) → ml	× 29.5735

Temperature: the exception

Temperature conversion isn’t a simple ratio because the scales have different zero points:

°C to °F: F = C × 9/5 + 32
°F to °C: C = (F − 32) × 5/9
°C to K:  K = C + 273.15

You can’t just multiply — you need both multiplication and addition.

Chaining conversions

Need to go from feet to millimetres? Chain:

Feet → metres → millimetres
5 ft × 0.3048 = 1.524 m × 1000 = 1524 mm

Or multiply the factors: 0.3048 × 1000 = 304.8 mm/ft.

Common mistakes

• Confusing US gallons (3.785 L) with Imperial gallons (4.546 L)
• Using UK fluid ounces (28.4 ml) vs US fluid ounces (29.6 ml)
• Forgetting that “cups” vary by country
• Mixing up nautical miles (1.852 km) with statute miles (1.609 km)

Convert between 8 categories with the OurDailyCalc unit converter — length, weight, area, volume, temperature, speed, time, and data storage.

TL;DR

• Multiply by the conversion factor (a ratio where numerator = desired unit)
• Temperature is special — requires formula with addition, not just multiplication
• Chain factors for multi-step conversions
• Watch out for US vs Imperial vs metric variations