Unit input must accept a familiar system and convert

What it is

Length, mass, temperature, and area each have more than one everyday system. People report the one they measured in: feet and inches, kilograms, Fahrenheit. An interface that only accepts the other system demands mental arithmetic, and errors scale with the conversion factor. Familiar-unit entry with conversion lets input travel in the user’s system and converts before storage. The issue is which ruler the number is written with, not which currency the money is, and not comma versus point.

Why it happens

Bodily quantities are encoded in familiar units: “I’m 175 cm” or “five nine.” Conversion is extra arithmetic plus mixed radices (12 inches to a foot, 2.54 cm to an inch), and carrying is where mental sums roll over. A metric-only product in an imperial region will receive numbers already multiplied wrong that still “look reasonable,” because people force a kilogram figure that seems in the right ballpark. System conversion takes the arithmetic off the person, but must also show the converted result so they can check “my 5'9" was taken as 175 cm.” If the system follows the OS locale, a traveling user is suddenly handed an unfamiliar ruler.

Where it stops holding

Science, engineering, and customs that legally take only SI can offer a familiar system only as an aid; the submitted value stays the legal unit and must say so. A cooking “cup” is not a fixed volume; conversion would fake precision. Mixing two systems’ numbers in one field without a label (typed in cm, switched to ft, number not converted) is a coupling error. Dimensionless counts (people, items) need no system.

Applying it

• Offer the region’s usual system for input, and show the conversion into the stored unit beside it.
• Allow an in-moment switch; convert the already-typed number on switch rather than leaving old digits on a new unit.
• Give dual imperial (feet + inches) two slots or a clear separator; do not let 5.9 mean 5 feet 9 inches in one box.
• Verify by filling height or weight in the unit you actually use and checking stored value plus the shown conversion. Offering only the unfamiliar unit, or switching systems without converting the digits, still leaves arithmetic on the person.

Related

• Within the group: E2.19.1 Money input must name the currency; do not let people assume the current one · E2.19.3 Grouping and decimal marks vary by locale and must adapt · E2.19.4 Number–unit coupling errors are hard to notice before submit
• Adjacent: E2.11 Numeric input and steppers · E2.14 Time and range pickers
• Search terms: familiar-unit entry · metric imperial · unit conversion