Accuracy, formulas and methodology
ConvertUnits.cc uses deterministic conversion factors in your browser. International definitions include 1 lb = 0.45359237 kg, 1 inch = 0.0254 meter, and 1 international foot = 0.3048 meter. The U.S. liquid gallon is 3.785411784 liters; it is not the imperial gallon.
Definitions and reference sources
Our definitions follow the International System of Units (SI) and recognized U.S. customary measurement factors. See NIST's Guide to SI conversion factors and the BIPM SI Brochure.
How we calculate
For linear units, the original value is converted into a common base unit and then into the target unit. Temperature uses offset formulas, such as °F = (°C × 9/5) + 32. Fuel efficiency is a reciprocal relationship: US mpg and L/100 km cannot be converted by a constant multiplier.
Precision and limitations
Source factors are stored at their defined precision. Display rounding is adjustable, and calculations do not use the displayed rounded output as an intermediate. Floating-point calculations may incur small machine rounding errors. For regulatory, medical, industrial, or safety-critical use, verify values against the applicable authoritative standard.
How calculations are checked
Unit definitions and numerical examples are validated by automated tests, and core browser interactions are checked in Chromium during the repository's build workflow. These checks identify software regressions; they are not independent calibration, accreditation or formal accuracy certification. Review the public automated test cases.
How result pages are produced
Numeric answer pages are generated by a static build from specified query values, and use the same conversion engine as the interactive tools. They are computational references, not separate expert-authored articles. Conversion factors, caveats and rounding are explained here so readers can verify the result instead of relying on an unsupported claim of precision.
Checkable conversion facts across 16 measurement types
Every value below is calculated by the same deterministic engine used in the live converters, not transcribed from a keyword list. The linked standards describe the underlying definitions and conventions; they have not certified or audited ConvertUnits.cc. An equals sign means the displayed decimal matches the computed result at the chosen precision; it is not a claim of independent calibration.
| Measurement | Calculated example | Definition or limitation | External reference |
|---|---|---|---|
| Weight & mass | 1 lb = 0.45359237 kg | An international avoirdupois pound is a mass unit (lb); pound-force (lbf) is a different quantity. | NIST SI conversion factors |
| Length | 1 in = 2.54 cm | The international inch is defined as exactly 2.54 centimeters; do not substitute a rounded ruler reading. | NIST SI conversion factors |
| Volume & cooking | 1 US gal = 3.785411784 L | This is a US liquid gallon, not a UK imperial gallon. Fluid ounces are units of volume, not mass. | NIST SI conversion factors |
| Temperature | 0 °C = 32 °F | Temperature needs an offset: °F = (°C × 9/5) + 32, not a simple multiplication. | BIPM SI Brochure |
| Pressure | 101.325 kPa = 1 atm | A standard atmosphere is 101.325 kPa by definition. This changes units, not gauge pressure to absolute pressure. | NIST SI conversion factors |
| Torque | 1 ft·lbf ≈ 1.355817948 N·m | Foot-pound-force measures torque, not pound mass; the output is not a manufacturer's tightening specification. | NIST SI conversion factors |
| Time | 60 min = 1 hr | A decimal duration of 1.5 hours is one hour and 30 minutes, not one hour and 50 minutes. | BIPM SI Brochure |
| Speed | 60 mph = 96.56064 km/h | International statute miles differ from nautical miles; converting speed does not determine travel time. | NIST SI conversion factors |
| Area | 1 m² ≈ 10.763910417 ft² | Area factors are squared length factors; 1 square foot is 0.09290304 square meters. | NIST SI conversion factors |
| Force | 1 kN ≈ 224.8089431 lbf | The symbol lbf means pound-force, not the lb unit of mass; no gravitational acceleration is assumed. | NIST SI conversion factors |
| Digital storage | 1 GB = 1,000 MB | The calculator uses decimal GB and MB (powers of 1000). Binary GiB and MiB use powers of 1024. | IEC 80000-13 overview |
| Density | 1000 kg/m³ = 1 g/cm³ | Density relates mass and volume. A material's volume cannot be inferred from its mass without density. | NIST SI conversion factors |
| Power | 1 kW ≈ 1.34102209 hp | This calculator uses mechanical horsepower; metric and electrical horsepower have other factors. | NIST SI conversion factors |
| Acceleration | 1 m/s² ≈ 3.280839895 ft/s² | The foot here is the international foot, exactly 0.3048 m; acceleration is not itself a force. | NIST SI conversion factors |
| Fuel economy | 30 US mpg ≈ 7.840486111 L/100 km | US miles per gallon and liters per 100 km are reciprocals, not a constant multiplier; zero is not a valid finite conversion. | NIST SI conversion factors |
| Flow rate | 1 L/min = 0.06 m³/h | Volumetric flow measures volume per time; converting it does not tell you the total volume without a duration. | BIPM SI Brochure |
Do not mix unit conventions
US versus UK fluid ounces: 1 US fl oz ≈ 29.573529563 mL; 1 UK imperial fl oz = 28.4130625 mL. These are different volume standards; neither is the mass ounce. Use the US fluid-ounce converter only for the US convention.
Decimal versus binary storage: 1 GB = 1,000 MB; 1 GiB = 1,024 MiB. The GB to MB tool uses decimal prefixes. See the IEC binary-prefix standard overview for naming conventions.
Mass versus force: lb is pound mass, whereas lbf is pound-force. They cannot be interchanged by a bare unit conversion. See mass and force tools for compatible units.
Display precision: An approximate symbol (≈) identifies a rounded decimal result; the actual conversion retains its underlying factor. Measurement tolerance, survey conventions, regulatory rounding, and manufacturer requirements must be assessed independently.
For further details, see NIST Guide to the SI and the BIPM SI Brochure. These external references provide definitions, not third-party certification of our software.