Convert Within a Defined Measurement Family

Convert through one declared base unit

An online unit converter should first confirm that the source and target measure the same kind of quantity. Length can convert to length, area to area, and mass to mass. Length cannot convert to mass without a separate physical relationship, and volume cannot convert to mass without density. This page organizes units into six families and blocks cross-family selections.

For multiplicative families, the converter changes the entered value to a declared base unit and then from that base to the target. Temperature uses an affine formula because its scales have different zero points as well as different interval sizes. The result shows the path, source note, and reverse-check residual.

Supported conversion families

Length uses meter as base and supports millimeter, centimeter, meter, kilometer, international inch, international foot, yard, and international mile. Mass uses kilogram and supports milligram, gram, kilogram, metric tonne, avoirdupois ounce, and avoirdupois pound.

Area uses square meter and the squared versions of millimeter, centimeter, meter, kilometer, international inch, and international foot, plus hectare. Volume uses cubic meter and supports cubic centimeter, milliliter, liter, cubic meter, U.S. liquid fluid ounce, and U.S. liquid gallon. The U.S. labels matter because Imperial fluid units are different.

Time uses second and supports millisecond, second, minute, hour, day defined as 86,400 seconds, and seven-day week. It does not include calendar months or years because their lengths vary. Temperature supports degrees Celsius, kelvin, and degrees Fahrenheit.

The catalog deliberately excludes nautical, survey, historical, typography, cooking-spoon, data-storage, pressure, energy, currency, and many other families. Absence is preferable to a conversion whose definition is unclear.

Worked length example

The international foot is exactly 0.3048 meter. Convert 1 meter to feet by dividing one by 0.3048, producing approximately 3.280839895 feet. The reverse check multiplies the unrounded result by 0.3048 and should return a value close to one meter.

An international inch is exactly 0.0254 meter, and a foot contains twelve inches under that definition. The tool labels these as international units. It does not offer the discontinued U.S. survey foot, whose historical conversion differs slightly.

For 2.5 kilometers to miles, convert to 2,500 meters, then divide by 1,609.344 meters per international mile. The result is approximately 1.553428 miles. Display rounding does not change the stored intermediate.

Worked mass example

The avoirdupois pound is exactly 0.45359237 kilogram. Convert 10 pounds to kilograms by multiplying, giving 4.5359237 kilograms. One avoirdupois ounce is one sixteenth of that pound, exactly 0.028349523125 kilogram under the definition.

These labels do not refer to troy mass. A jewelry or precious-metal context can use troy ounces, which are outside this catalog. The converter refuses to infer the intended ounce from an unlabeled value.

Temperature is not a single factor

For Celsius to Fahrenheit:

°F = °C × 9/5 + 32

For Fahrenheit to Celsius:

°C = (°F − 32) × 5/9

Kelvin and Celsius have the same interval size, with K = °C + 273.15. Convert 20°C to obtain 68°F and 293.15 K. The page distinguishes temperature values from temperature intervals; adding 32 applies to values, not differences.

A kelvin value below zero is rejected because the kelvin scale does not extend below absolute zero. The tool does not interpret sensor accuracy or physical feasibility beyond that domain guard.

Area and volume require powered factors

One international foot equals 0.3048 meter, but one square foot equals 0.3048 squared, or 0.09290304 square meter. One cubic foot would use the cube, though it is not included in the volume list. Applying a length factor directly to area is a common error.

One liter is 0.001 cubic meter, and one milliliter is one cubic centimeter or 0.000001 cubic meter. The U.S. liquid gallon and fluid ounce use their defined U.S. liquid relationships. Result labels never shorten them to an ambiguous “gallon” or “oz.”

Exact definitions and displayed precision

The conversion catalog records source, definition version, and whether a factor is exact. SI prefixes are powers of ten. International inch/foot relationships and the avoirdupois pound-to-kilogram definition are exact in the cited NIST tables. A decimal result can still repeat and require display rounding.

The converter uses ordinary browser numbers, so it does not promise arbitrary precision. A reverse residual near zero is a software consistency check, not measurement uncertainty. The significant figures appropriate to a measurement depend on the source instrument and context.

Limits and responsible use

The converter does not update exchange rates, calculate dimensional formulas, transform coordinates, handle timezones, count business days, distinguish calendar duration, or select regulatory units. Unit symbols are case-sensitive in scientific writing even when the interface uses friendly labels.

Medical dosing, engineering, aviation, surveying, trade measurement, laboratory work, and regulated commerce can require approved standards, traceability, tolerances, and specialized systems. Verify a consequential conversion using the controlling current source. This page applies only the conversion families and constants shown in its visible table.

Frequently asked questions

Choose the measurement family first, convert, and review the base-unit path and source note before copying the result and unit.