Unit Conversion: Exact Definitions and Display Precision

Before converting a measurement, identify the unit and its definition. “Pound” can describe mass or force, and “gallon” has different regional meanings. A calculator needs that choice before it can apply a factor.

Inches and centimetres

The international inch is exactly 0.0254 metres, so one inch is exactly 2.54 centimetres. The Inches to Centimetres Converter uses that definition. Ten inches becomes 25.4 cm.

For mixed feet and inches, first reduce the measurement to inches: 5 feet 6 inches is 5 × 12 + 6 = 66 inches. Enter 66 in the converter. This page accepts a single decimal value, so enter one and a half inches as 1.5 rather than typing a mixed fraction.

Pounds and kilograms

The international avoirdupois pound is exactly 0.45359237 kilograms. Its reciprocal gives approximately 2.20462262 pounds per kilogram. Use Kilograms to Pounds for mass; this is not a pound-force calculation or a troy-unit conversion. The definitions and exact-versus-rounded convention are documented in the NIST conversion-factor tables.

Display rounding is separate

Changing the displayed precision should change how many digits you see, not prematurely round the conversion itself. A direct conversion followed by its reverse can return the original value within ordinary floating-point tolerance. Copying an already rounded result into a different calculation can introduce a small difference.

More digits also do not improve the original measurement. If a ruler reading is approximate, an exact conversion factor does not make that reading exact. Save the unit and the measurement precision together rather than recording a bare number.

Read regional and digital labels

The general Unit Converter labels its gallon as US. Do not reuse a US gallon result for an Imperial gallon. Likewise, decimal data prefixes and binary data prefixes have different meanings: MB is a million bytes; MiB is 1,048,576 bytes. Only choose labels that the tool actually offers.

Temperature requires an offset as well as a scale factor. For example, 0°C becomes 32°F and −40°C becomes −40°F. A ratio alone would fail these cases. Use the dedicated Celsius to Fahrenheit page to see the formula and a pre-rendered common-values table.

If an input is empty, non-finite or outside what can be represented, a clear error is preferable to a displayed zero. Very small results may use scientific notation so that a nonzero value is not hidden by formatting.