Smart Unit Preferences in Intl.NumberFormat

Stage: 1

Champion: Younies Mahmoud @younies

Author: Younies Mahmoud @younies



Representing measurements in localized units is a common need in internationalized software.

In the US and a handful of other countries, temperature is measured in degrees Fahrenheit. The rest of the world uses degrees Celsius. Some countries measure distances and lengths in miles, yards or feet, and inches, others use kilometers, meters, and centimeters. The Scandinavian Mile is a common unit of length in Norway and Sweden.

The measurement units used also depend on what exactly is being measured (roads, people, precipitation), not only on what kind of measurement it is (length, mass, pressure).

We propose adding locale- and usage-aware measurement formatting to ECMAScript's Intl.NumberFormat which will take care of such nuances.


  1. Localized: for each locale, users should be presented with units appropriate for them. For example:

    • Length (distance):
      • en-US: miles
      • fr-FR: kilometers
    • Mass:
      • en-US: pounds
      • fr-FR: kilograms
  2. Usage: the preferred measurement unit is dependent on what is being measured. Consider:

    • Typical length measurements:
      • en-US: miles, feet, inches
      • fr-FR: kilometers, meters, centimeters
    • Rainfall (also a "length" measurement):
      • en-US: inches
      • fr-FR: millimeters
        • Yearly rainfall is still presented in millimeters, even when measurements exceed 1 meter. Consider the rainfall in Cherrapunji: "The highest recorded rainfall in a single year was 22,987 mm (905.0 in) in 1861." (Wikipedia: Rain - Wettest known locations).
  3. The choice of presentation units also depends on the size of the measurement:

    • For en-US, the length of toddlers is typically presented in inches, whereas adults are measured feet and inches. (3 feet seems a reasonable threshold.)
      • The average newborn is 19.5 inches long and weighs 7.25 pounds.
      • Albert Einstein was 5 feet and 9 inches tall.
  4. The precision of the displayed number also depends on usage:

    • For en-US, babies would be measured to a tenth of an inch, whereas measurements presented in feet and inches are rounded to the nearest inch. are measured to the nearest inch only.
    • In metric countries, heights would be measured in centimeters, or in meters presented with two decimal places.

API Design

  • Add a usage string parameter to Intl.NumberFormat that indicates the measurement use case.

    • Examples: person-height, rainfall, snowfall, road.
    • Output units and precision will depend on:
      • Input unit (helps disambiguate the usage)
      • Usage
      • Locale
      • Quantity
    • We propose supporting CLDR's unit preferences, see CLDR's units.xml.
  • A code example adding a usage parameter to Int.NumberFormat:

    const inputValue = 1.8;
    const inputUnit = "meter";
    console.log(new Intl.NumberFormat('en-US', {
        style: 'unit',
        unit: inputUnit,
        usage: 'person-height',  // NEW parameter
        unitDisplay: 'long'
    // expected output: "5 feet and 11 inches"
    // (1.8 meters = 5 feet 10.8661 inches")