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Measurement tool

Unit Conversion Calculator

Convert common measurements with explicit SI and U.S. customary definitions, documented boundaries, and one final rounding step.

Free · Private · Runs in your browser

Five measurement categories

Convert a measurement

Choose a category and two supported units. Values and results stay in your browser.

Quantity and units

Ordinary decimals from −1,000,000,000,000 through 1,000,000,000,000 are accepted. Exponent notation is not.

The converted quantity will appear here after you submit a valid value and supported units.

Formula and calculation method

Length, area, volume, and mass conversions use one base unit per category. The calculator multiplies the entered value by its starting-unit factor, then divides by the ending-unit factor. This is the conversion-ratio method described by NIST: the numerical representation changes while the quantity does not.

Temperature is different because its scales have offsets. Fahrenheit converts to Celsius with (°F − 32) ÷ 1.8; Kelvin converts to Celsius with K − 273.15. The Celsius value then converts to the requested scale.

Internal calculations use JavaScript double-precision numbers and the declared decimal constants without intermediate display rounding. The result is rounded once to at most 12 significant digits; very large or small values use scientific notation.

Inputs, outputs, and boundaries

  • Inputs: one measurement category, one ordinary decimal value, one starting unit, and one ending unit.
  • Outputs: the converted value, ending-unit symbol, selected direction, and conversion relationship.
  • Supported categories: length, area, volume, mass, and thermodynamic temperature.
  • Input bounds: finite values from −1,000,000,000,000 through 1,000,000,000,000. Exponent notation is not accepted.
  • Zero and negative values: zero and negative values are accepted. A temperature below absolute zero is invalid. Negative zero is displayed as zero.
  • Fixed versus changing information: calculations use fixed unit relationships and user input only. Authority editions, URLs, and explanatory guidance can change and are reviewed annually.

Aletheia conventions

Foot, inch, yard, mile, acre, square-foot, and cubic-foot values use the international foot definition, not the retired U.S. survey foot. Gallon, quart, pint, and fluid ounce mean U.S. liquid measure, not British Imperial or U.S. dry measure. Ounce and pound mean avoirdupois mass. Labels state these distinctions wherever ambiguity is likely.

Limitations and disclaimer

This calculator does not convert currency, time, speed, pressure, energy, power, data, fuel economy, angles, cooking measures, Imperial capacity, survey-foot legacy records, or compound units. It does not propagate measurement uncertainty, significant figures, tolerances, or calibration status.

A rounded display does not increase the precision of the original measurement. This general-purpose tool is not a calibrated instrument, legal-metrology determination, engineering certification, laboratory result, or professional recommendation. Verify consequential conversions against the controlling standard and applicable procedure.

Worked examples

One international mile equals 1,609.344 meters, so dividing by 1,000 produces exactly 1.609344 kilometers. One U.S. liquid gallon is exactly 231 cubic inches; applying the exact inch definition produces 3.785411784 liters.

For 68 °F, subtract 32 and divide by 1.8. The result is 20 °C. Adding 273.15 produces 293.15 K.

Common questions

Why are some familiar units excluded?

Names such as gallon, ounce, ton, and foot can represent different systems or historical definitions. This release includes only relationships that can be labeled and sourced without ambiguity.

Does the result preserve significant figures?

No. The calculator provides up to 12 significant digits for convenience, but only the user knows the measurement's uncertainty and appropriate reporting precision.

Are the conversion factors live data?

No. The tool uses a reviewed, versioned registry of fixed relationships. It does not request rates, location, account data, or external conversion services.

Use the Fraction Calculator for exact rational arithmetic, or the Percentage Calculator for part-to-whole and relative-change questions.

Version and verification

Calculation version 1.0.0; factors, equations, and deterministic fixtures last verified 2026-08-11. Authority pages were retrieved 2026-08-11. Automated tests cover each category, bounds, absolute zero, metadata, privacy, keyboard use, responsive layout, contrast, and Axe behavior. Manual VoiceOver remains waived and is not claimed as passed.