Conversion Methodology and Sources

How the calculations work

For linear conversions, every unit is stored as a factor relative to one base unit for its category. The entered value is multiplied by the source factor and divided by the destination factor:

result = entered value × source factor ÷ destination factor

Length uses the meter as its base, mass uses the kilogram, area uses the square meter, volume uses the liter, speed uses meters per second, pressure uses the pascal, energy uses the joule, power uses the watt and force uses the newton. Angles are normalized through radians and time through seconds.

Standards and conversion constants

Metric and SI unit definitions follow the International System of Units. Non-SI and US customary relationships are checked against published conversion-factor tables. Exact defined relationships are stored at their defined precision; calculated relationships retain additional internal precision before display rounding.

• BIPM — The International System of Units (SI Brochure)
• NIST Special Publication 811 — Guide for the Use of the SI
• NIST SP 811 Appendix B.8 — Conversion factors by unit
• NIST — Prefixes for binary multiples
• IEC 80000-13 — Quantities and units for information science
• NIST — Gregorian calendar year definition

Temperature and fuel-consumption formulas

Temperature scales are not simple multiples because Celsius and Fahrenheit have different zero points. Everyday Units converts through Celsius using °C = (°F − 32) × 5/9 and K = °C + 273.15.

Fuel economy includes both distance-per-volume units and volume-per-distance units. Conversions between those two forms are reciprocal, so doubling miles per gallon does not double liters per 100 kilometers. A value of zero is displayed as zero by the calculator rather than attempting division by zero.

US customary and Imperial differences

Units with similar names are labeled separately whenever their sizes differ. A US liquid gallon is 3.785411784 liters, while an Imperial gallon is 4.54609 liters. US and Imperial miles per gallon therefore produce different results. A US short ton is 2,000 pounds; an Imperial long ton is 2,240 pounds. The international inch, foot, yard and pound use the same defined values in modern US customary and Imperial measurement.

Data-storage conventions

Decimal storage units use powers of 1,000: 1 kB = 1,000 bytes, 1 MB = 1,000,000 bytes and 1 GB = 1,000,000,000 bytes. Binary units use powers of 1,024: 1 KiB = 1,024 bytes, 1 MiB = 1,048,576 bytes and 1 GiB = 1,073,741,824 bytes. Keeping the decimal and binary names separate avoids a common source of ambiguity.

Average months and years

Calendar months and years do not have one fixed duration. For approximate duration conversion, the site uses the mean Gregorian calendar year of 365.2425 days, equal to 31,556,952 seconds. An average month is defined as one twelfth of that year: 30.436875 days, or 2,629,746 seconds. Do not use these averages to calculate calendar dates or deadlines.

Precision and rounding

Calculations use JavaScript double-precision numbers and retain more digits internally than are normally shown. Results are displayed with up to ten significant digits, unnecessary trailing zeros are removed, and very large or very small results may use scientific notation. Display rounding does not change the underlying conversion factor.

For safety-critical, medical, legal, laboratory or commercial work, verify the result against the standard and rounding policy required for that specific use.