# How do I convert a measurement?

Converts a measurement of length, weight, volume, temperature, area, or speed from any unit to any other.

- Page: https://www.acalculator.org/convert/unit
- JSON spec: https://www.acalculator.org/convert/unit.json
- Version: 31ed7e204ead

## Default answer

Example with the default inputs (Measure Length, Amount 1, From Inches (in), To Centimeters (cm)): 1 in = 2.54 centimeters (cm).

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| measure | Measure | What kind of measurement to convert. |
| amount | Amount | The measurement to convert, in the "From" unit. |
| lengthFrom | From | The unit of length the amount is in. |
| lengthTo | To | The unit of length to convert to. |
| weightFrom | From | The unit of weight the amount is in. |
| weightTo | To | The unit of weight to convert to. |
| volumeFrom | From | The unit of volume the amount is in. |
| volumeTo | To | The unit of volume to convert to. |
| temperatureFrom | From | The unit of temperature the amount is in. |
| temperatureTo | To | The unit of temperature to convert to. |
| areaFrom | From | The unit of area the amount is in. |
| areaTo | To | The unit of area to convert to. |
| speedFrom | From | The unit of speed the amount is in. |
| speedTo | To | The unit of speed to convert to. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| result | Converted amount | The amount in the "To" unit. |
| given | From | The amount and its unit, as entered. |
| into | To | The unit the amount is converted to. |
| formula | Formula | How to convert from the one unit to the other. |

## Method

converted = amount × (base units per "From" unit) ÷ (base units per "To" unit); temperatures add an offset

## Assumptions

- Every factor is the exact definition in NIST SP 811 (2008), Appendix B.8: 1 in = 2.54 cm, 1 lb = 0.45359237 kg, 1 US gallon = 231 in³.
- Cups, pints, quarts, gallons, and fluid ounces are US customary units.
- Temperatures convert through kelvins: K = °C + 273.15 = (°F + 459.67) × 5 ÷ 9. Nothing is colder than 0 K.
- A length, weight, volume, area, or speed cannot be negative.
- The answer shows at most 6 decimal places; the calculation keeps full precision.

## Worked examples

1. measure = length, amount = 1, lengthFrom = in, lengthTo = cm gives result = 2.54, formula = cm = in × 2.54. Source: NIST SP 811 (2008) B.8: 1 in = 2.54 cm exactly.
2. measure = weight, amount = 150, weightFrom = lb, weightTo = kg gives result = 68.038856. Source: NIST SP 811 (2008) B.8: 1 lb = 0.45359237 kg exactly.
3. measure = volume, amount = 1, volumeFrom = gal, volumeTo = L gives result = 3.785412. Source: NIST SP 811 (2008) B.8: 1 US gallon = 231 in³ = 3.785411784 L.
4. measure = temperature, amount = 98.6, temperatureFrom = F, temperatureTo = C gives result = 37, formula = °C = (°F − 32) × 5 ÷ 9. Source: NIST SP 811 (2008) 4.2.1.1: t/°C = (t/°F − 32) ÷ 1.8.
5. measure = area, amount = 1, areaFrom = acre, areaTo = ft2 gives result = 43,560. Source: NIST SP 811 (2008) B.8: 1 acre = 43,560 ft².
6. measure = speed, amount = 60, speedFrom = mph, speedTo = kmh gives result = 96.56064. Source: NIST SP 811 (2008) B.8: 1 mi = 1.609344 km exactly.

## FAQ

### Which units can I convert?

Length: millimeters to miles, nautical miles, and mils. Weight: micrograms to short tons, including grains and ounces. Volume: milliliters, liters, cubic units, and US teaspoons, tablespoons, fluid ounces, cups, pints, quarts, and gallons. Temperature: Celsius, Fahrenheit, and kelvins. Area: square millimeters to square miles, hectares, and acres. Speed: m/s, km/h, mph, ft/s, and knots.

### Are the conversions exact?

Yes, for every unit here. Since 1959 the inch is exactly 2.54 cm and the pound exactly 0.45359237 kg, and the US gallon is exactly 231 cubic inches. The calculator uses these definitions (NIST SP 811) and rounds only the number it shows.

### Why can a temperature be negative but a length cannot?

Celsius and Fahrenheit put zero at a chosen temperature, so colder temperatures are negative. Nothing is colder than absolute zero, 0 K, which is −273.15 °C and −459.67 °F. A length, weight, volume, area, or speed starts at zero.

### Are cups and gallons US or UK units?

US customary units. A US cup is 236.5882365 mL and a US gallon 3.785411784 L. A UK (imperial) gallon is larger, 4.54609 L, and is not in this list.

## Sources

- NIST SP 811 (2008), Guide for the Use of the International System of Units, section 4.2.1.1 and Appendix B.8. https://www.nist.gov/pml/special-publication-811