Reliable measurement unit conversion
Unit conversion changes the numerical representation of a quantity without changing the quantity itself. Linear conversions multiply through a common base unit; temperature scales require an offset as well as a scale factor.
Unit names matter. This tool distinguishes U.S. and imperial gallons, uses binary kibibytes through tebibytes for digital storage, and states the assumed speed used for Mach conversion.
How to use the unit converter
- Choose a category: Select units that describe the same kind of quantity.
- Choose source and target units: Read qualifiers such as U.S., imperial, 365-day, or assumed Mach speed.
- Enter a value: The converted result updates immediately.
- Round appropriately: Match output precision to the uncertainty of the original measurement.
Formula and variables
Most categories convert through a common base unit. Temperature uses scale-specific functions through kelvin.
value₂ = value₁ × factor₁ / factor₂- value₁ — Input value
- Quantity expressed in the source unit (source unit)
- factor₁ — Source factor
- Source unit value in the category base unit (base/source)
- factor₂ — Target factor
- Target unit value in the category base unit (base/target)
- value₂ — Converted value
- Same quantity expressed in the target unit (target unit)
Miles to kilometers
Convert 5 international miles to kilometers.
- Input
- 5 mi
- 1 mi = 1,609.344 m
- 5 × 1,609.344 / 1,000
Result: 5 mi = 8.04672 km.
The numerical value changes because the kilometer is smaller than the mile.
Understanding your results
Keep precision and definitions visible
A displayed conversion can contain more digits than the source measurement justifies.
- Exact definitions do not make an inexact measurement exact.
- U.S. liquid and imperial volume units are different.
- Mach number depends on local speed of sound; this converter uses the stated 343 m/s assumption.
- KiB, MiB, GiB, and TiB are binary multiples rather than decimal kilobytes through terabytes.
Assumptions
- Source and target units belong to the selected measurement category.
- The international foot and avoirdupois pound definitions are intended.
- Mach conversion uses an assumed sound speed of 343 m/s.
Limitations
- Does not propagate measurement uncertainty or significant figures automatically.
- Does not convert compound dimensions such as viscosity, torque, or fuel economy.
- Temperature below absolute zero is rejected.
- Calendar year length and local speed of sound can differ from the stated assumptions.
Common mistakes
- Mixing units from different physical dimensions.
- Confusing U.S. gallons with imperial gallons.
- Treating binary storage units as decimal SI prefixes.
- Reporting every displayed digit as experimentally significant.
Practical use cases
Everyday measurement
Convert travel, mass, volume, temperature, area, and time values.
Technical checking
Verify a conversion factor before using a value in a larger calculation.
Frequently asked questions
Are the conversions exact?
Many factors are exact definitions, but some scientific values and assumptions are approximate. Input measurement uncertainty still applies.
Why are U.S. and imperial gallons separate?
They are different volumes: the U.S. liquid gallon is about 3.785 L and the imperial gallon is 4.54609 L.
Why can I not enter a temperature below absolute zero?
The converter treats temperatures as physical thermodynamic values and rejects a negative kelvin equivalent.
Sources and review
- NIST Guide to the SI, Appendix B: Conversion Factors — National Institute of Standards and Technology. Accessed 2026-07-13.
Reviewed 2026-07-13.