Unit Converter

Convert length, weight, volume, and temperature between metric (SI) and imperial or US Customary units using exact factors published by NIST.

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Calculation Examples

Calculation Case Result
10 km to miles 10 ÷ 1.609344 = 6.2137 miles (1 mile = 1,609.344 m, exact)
25°C to Fahrenheit 25 × (9/5) + 32 = 45 + 32 = 77°F
500 g to ounces (avoirdupois) 500 ÷ 28.3495 = 17.637 oz (1 oz = 28.3495 g)
1 m² to ft² (area, not linear) 1 × (3.28084)² = 1 × 10.7639 = 10.7639 ft²
Mihail Doncenco
Mihail Doncenco Site Creator & Calculator Expert

Founder and developer of CalcMate. Builds and maintains every calculator on this site, focusing on formula accuracy and clear, practical results.

How to Use the Unit Converter

Select the measurement category (Length, Weight/Mass, Volume, or Temperature). Choose the source unit in the first dropdown and the target unit in the second, enter the value, and click "Convert." The result appears with the conversion factor or formula that produced it.

Most conversions multiply by a fixed ratio. Temperature is the exception: Celsius and Fahrenheit have different zero points (0°C = 32°F), so conversion needs a linear equation: \(T_F = T_C \times \dfrac{9}{5} + 32\) and \(T_C = (T_F - 32) \times \dfrac{5}{9}\). Example: 25°C gives \(25 \times \dfrac{9}{5} + 32 = 77\) °F.

Conversion Factors and Standards

The factors in this tool follow NIST Special Publication 811 (2008 edition, Appendix B) and the BIPM SI Brochure (9th edition, 2019). Several are defined exactly, not approximated:

Length: 1 inch = 25.4 mm and 1 foot = 0.3048 m (exact, International Yard and Pound Agreement, 1959), so 1 m = 3.280839895... ft. 1 mile = 1,609.344 m and 1 nautical mile = 1,852 m are also exact. NIST retired the US survey foot on January 1, 2023, so "foot" now means the international foot.

Mass: 1 pound (avoirdupois) = 0.45359237 kg (exact), so 1 kg = 2.20462262... lb. Three different tons share one word: 1 US short ton = 2,000 lb, 1 UK long ton = 2,240 lb, and 1 metric tonne = 1,000 kg.

Volume: 1 US liquid gallon = 231 cubic inches = 3.785411784 litres (exact), and 1 UK imperial gallon = 4.54609 litres, about 20% larger. Using the US factor for a UK gallon understates the volume by 16.7%, and using the UK factor for a US gallon overstates it by 20.1%.

Metric capacity conversion chart: millilitres, centilitres, decilitres, litres and their relationships

Useful Tips 💡

  • To convert area by hand, square the linear factor, and for volume, cube it. 1 m² = 3.28084² = 10.7639 ft², and 1 m³ = 3.28084³ = 35.3147 ft³. Using the plain linear factor 3.28084 understates an area by 3.28 times and a volume by 10.76 times.
  • US Customary and UK Imperial units share names but differ in value for gallons, pints, and fluid ounces. Check whether a recipe or specification is US or UK before choosing units. A US gallon is 3.785 L and a UK gallon is 4.546 L, a gap of about 20%.
  • Temperature needs an equation, not a ratio. Multiplying Celsius by 9/5 without adding 32 is off by exactly 32°F at every temperature. Only the full formula puts the two scales on the same number, at -40 (-40°C = -40°F).

📋Steps to Calculate

  1. Select the measurement category: Length, Weight/Mass, Volume, or Temperature.

  2. Choose the source unit (e.g., Celsius) and the target unit (e.g., Fahrenheit) from the dropdowns.

  3. Enter the numeric value to convert and click "Convert" to see the result with the applied factor or formula.

Mistakes to Avoid ⚠️

  1. Applying a linear factor to area or volume. Square metres to square feet needs 3.28084² = 10.7639, and cubic metres to cubic feet needs 3.28084³ = 35.3147. The linear factor alone understates an area by 3.28 times and a volume by 10.76 times.
  2. Confusing US and UK volume units. 1 US gallon = 3.785 L and 1 UK gallon = 4.546 L. 1 US fluid ounce = 29.574 mL and 1 UK fluid ounce = 28.413 mL. Mixing them shifts results by 4% to 20%, depending on the unit and direction.
  3. Confusing the three tons. 1 short ton (US) = 2,000 lb = 907.185 kg, 1 long ton (UK) = 2,240 lb = 1,016.047 kg, and 1 metric tonne = 1,000 kg = 2,204.62 lb. In international trade, "tonne" means 1,000 kg, so confirm which unit a specification needs.
  4. Treating Celsius to Fahrenheit as a simple ratio. The formula is F = C × 1.8 + 32. Skipping the + 32 gives a result that is wrong by exactly 32°F every time.

Practical Applications📊

  1. Engineering and construction: US specifications use inches, feet, and pounds, while international standards use SI. Rounded factors add up. For a 20,000 lb load, 1 lb ≈ 0.45 kg gives 9,000 kg, but the exact factor gives 9,071.8 kg, a gap of 72 kg.

  2. Cooking: US recipes use cups and ounces, most others use grams and millilitres. One US cup is 236.6 mL, but a cup of flour weighs about 120 g and varies with how you scoop it, so weigh dry ingredients when you can.

  3. Fuel and shipping: 500 US gallons is 1,892.7 L, while 500 imperial gallons is 2,273.0 L. Mixing the two on shipping paperwork shifts the volume by about 380 L.

  4. Oven temperatures: a US recipe at 350°F equals (350 - 32) × 5/9 = 176.7°C, so most European ovens are set to 175°C or 180°C.

Questions and Answers

Can the calculator be used for multiple conversions at once?

No, the converter handles one conversion at a time, so each result shows the correct source unit, target unit, and factor. For a long column of values, a spreadsheet is faster: the formula =A1*0.0254 converts inches to metres, and fill-down applies it to thousands of rows. The factor 0.0254 is exact, so the spreadsheet result matches this converter.

How do I convert Celsius to Fahrenheit (and back)?

To convert Celsius to Fahrenheit, multiply by 9/5 and add 32: \(T_F = T_C \times \dfrac{9}{5} + 32\). To go back, subtract 32 and multiply by 5/9: \(T_C = (T_F - 32) \times \dfrac{5}{9}\). Examples: 100°C = 212°F, 0°C = 32°F, and 350°F = 176.7°C. Kelvin uses the same step size as Celsius: \(T_K = T_C + 273.15\), and 0 K is absolute zero (-273.15°C). The two scales show the same number only at -40°.

How do I convert units manually using conversion factors?

Multiply the source value by a fraction that has the target unit on top and the source unit on the bottom: \(\text{Target} = \text{Source} \times \dfrac{\text{Target unit}}{\text{Source unit}}\). Example: \(5 \text{ kg} \times \dfrac{2.20462 \text{ lb}}{1 \text{ kg}} = 11.023\) lb. For area, square the linear factor, and for volume, cube it: \(1 \text{ m}^2 = (3.28084 \text{ ft})^2 = 10.7639 \text{ ft}^2\).

What are the most common unit conversions and where do the factors come from?

The most common conversions link SI (metric) units to US Customary or imperial units for length, mass, volume, and temperature. The key factors are defined by law and treaty: 1 inch = 25.4 mm and 1 pound = 0.45359237 kg (both exact, International Yard and Pound Agreement, 1959), and 1 US gallon = 3.785411784 litres (exact, 231 cubic inches). NIST publishes these in Special Publication 811 (2008 edition, Appendix B), and legal metrology, science, and trade documents use the same values.

What is a unit converter and how does it work?

A unit converter multiplies a value by a defined conversion factor, the exact ratio between two units of the same quantity, so the measurement changes units but not size. Example: \(10 \text{ km} \div 1.609344 = 6.2137\) miles. Temperature is the exception, because the scales have different zero points, so it uses the linear formula \(T_F = T_C \times 1.8 + 32\).

What standards do the conversion factors follow?

The conversion factors follow NIST Special Publication 811 (2008 edition, Appendix B) and the BIPM SI Brochure (9th edition, 2019). Exact definitions include 1 inch = 25.4 mm, 1 foot = 0.3048 m, 1 pound = 0.45359237 kg, 1 US gallon = 231 in³ = 3.785411784 L, and 1 nautical mile = 1,852 m. Exact means the value is a definition with no rounding. Ratios that cannot be written exactly, such as metres to feet (1/0.3048), are rounded only for display.

Why are there different types of gallons, pints, and ounces?

The US Customary and British Imperial systems developed separately and kept the same unit names with different values. 1 US liquid gallon = 3.785411784 litres, while 1 UK imperial gallon = 4.54609 litres, about 20% larger. 1 US fluid ounce = 29.5735 mL, while 1 UK fluid ounce = 28.4131 mL. For mass, 1 US short ton = 2,000 lb (907.185 kg) and 1 UK long ton = 2,240 lb (1,016.047 kg). Always check whether a source uses US or UK units.
Disclaimer: This calculator is designed to provide helpful estimates for informational purposes. While we strive for accuracy, financial (or medical) results can vary based on local laws and individual circumstances. We recommend consulting with a professional advisor for critical decisions.