Common unit conversions: exact factors for length, weight, volume, area and temperature

Common unit conversions: exact factors for length, weight, volume, area and temperature

One inch is 2.54 cm, one mile is 1.609344 km, one pound is 0.45359237 kg, and one US gallon is 3.785411784 L. These are the factors behind the table below, generated by running Mathify's conversion code rather than copied from memory. Values are rounded to 10 significant digits for display.

Where do these factors come from?

Every value on this page was produced by calling the converters in the Mathify repository (lib/calculations/conversion.ts) with a value of 1, by the script scripts/content-data/unit-conversion-factors.ts. The repository's constants file marks the pound, ounce, US short ton, US gallon, kilometre-per-hour and knot factors as exact. Mathify offers separate converters for length, weight, volume, area, speed and temperature.

Assumptions: US customary volumes (gallon, quart, pint, cup, fluid ounce, tablespoon, teaspoon) unless a line says UK. The stone is 14 pounds. The ton is the US short ton of 2,000 pounds.

What are the length conversions?

• 1 inch = 25.4 mm
• 1 inch = 2.54 cm
• 1 foot = 30.48 cm
• 1 foot = 0.3048 m
• 1 yard = 0.9144 m
• 1 mile = 1.609344 km
• 1 mile = 5280 ft
• 1 mile = 1760 yd
• 1 km = 0.6213711922 mi
• 1 m = 3.280839895 ft
• 1 m = 39.37007874 in
• 1 cm = 0.3937007874 in

For fractional inches, the Fractional Inches Converter is handy.

What are the weight conversions?

• 1 ounce = 28.34952313 g
• 1 pound = 0.45359237 kg
• 1 pound = 453.59237 g
• 1 pound = 16 oz
• 1 stone = 14 lb
• 1 stone = 6.35029318 kg
• 1 US short ton = 2000 lb
• 1 US short ton = 907.18474 kg
• 1 kg = 2.204622622 lb
• 1 kg = 35.27396195 oz
• 1 g = 0.03527396195 oz

What are the volume conversions?

• 1 US gallon = 3.785411784 L
• 1 US gallon = 4 qt
• 1 US quart = 946.352946 mL
• 1 US pint = 473.176473 mL
• 1 US cup = 236.5882365 mL
• 1 US fluid ounce = 29.57352956 mL
• 1 US tablespoon = 14.78676478 mL
• 1 US teaspoon = 4.928921594 mL
• 1 US cup = 8 fl oz
• 1 US cup = 16 tbsp
• 1 US tablespoon = 3 tsp
• 1 UK gallon = 4.54609 L
• 1 litre = 0.2641720524 gal
• 1 litre = 33.8140227 fl oz

A UK gallon is about 1.2009 times a US gallon, so recipe or fuel figures labelled "gallon" need a check on which one is meant.

What are the area conversions?

• 1 square foot = 0.09290304 m^2
• 1 square yard = 0.83612736 m^2
• 1 acre = 4046.856422 m^2
• 1 acre = 43560 ft^2
• 1 hectare = 2.471053815 acre
• 1 square mile = 640 acre
• 1 square mile = 2.58998811 km^2
• 1 m2 = 10.76391042 ft^2

What are the speed conversions?

• 1 mph = 1.609344 km/h
• 1 km/h = 0.6213711922 mph
• 1 knot = 1.852 km/h
• 1 m/s = 3.6 km/h

How do you convert Celsius and Fahrenheit?

Temperature scales have different zero points, so there is no single multiplying factor. The formulas are F = C x 9/5 + 32 and C = (F - 32) x 5/9.

From Celsius:

• -40 C = -40 F = 233.15 K
• 0 C = 32 F = 273.15 K
• 20 C = 68 F = 293.15 K
• 37 C = 98.6 F = 310.15 K
• 100 C = 212 F = 373.15 K

From Fahrenheit:

• 0 F = -17.78 C
• 32 F = 0.00 C
• 50 F = 10.00 C
• 68 F = 20.00 C
• 72 F = 22.22 C
• 98.6 F = 37.00 C
• 212 F = 100.00 C
• 350 F = 176.67 C
• 400 F = 204.44 C

At -40 the two scales read the same number.

How do you reverse a conversion?

Divide by the factor instead of multiplying, or multiply by its reciprocal. Going from kilometres to miles uses 1 / 1.609344 = 0.6213711922, which matches the 1 km line above. Going from centimetres to inches uses 1 / 2.54 = 0.3937007874. Chaining conversions works the same way: feet to inches to centimetres is 12 x 2.54 = 30.48 cm per foot, which matches the foot line. Keep full precision until the last step, then round once.

Worked example: converting a recipe and a trip

• 5 miles x 1.609344 = 8.04672 km.
• 180 lb x 0.45359237 = 81.65 kg.
• 2.5 US gallons x 3.785411784 = 9.46 L.
• 350 F is 176.7 C.
• 6.5 in x 2.54 = 16.51 cm.
• 1.5 US cups = 354.8823547 mL.
• A 26.2 mile marathon is 42.1648128 km.

How much error do rounded shortcuts add?

Common mental shortcuts differ from the exact factors:

• Using 1.6 km per mile is off by -0.58%.
• Using 0.45 kg per pound is off by -0.79%.
• Using 2.5 cm per inch is off by -1.57%.

For quick estimates that is fine. For construction, dosing or anything priced by weight, use the exact factor or a converter.

Common mistakes

• Mixing US and UK gallons or pints.
• Using ounces (weight) and fluid ounces (volume) interchangeably.
• Applying the Celsius-to-Fahrenheit formula without adding 32.
• Rounding too early in multi-step conversions.
• Confusing the US short ton with the metric tonne (1,000 kg).

Key takeaways

• Use exact factors when accuracy matters and rounded ones for estimates.
• Volume names differ between US and UK systems.
• Temperature conversions need an offset as well as a multiplier.
• When in doubt, check the result with the matching Mathify converter.

FAQ

How many cm are in an inch?

2.54 cm.

How many kilometres are in a mile?

1.609344 km.

How many pounds are in a kilogram?

About 2.204622622 lb, which is 1 / 0.45359237.

How many mL are in a US cup?

236.5882365 mL.

What is 350 F in Celsius?

176.7 C.