Unit definitions
Each converter turns a value into a base unit and back. The factors below are the exact values the code uses. Inch, foot, yard, mile, pound and ounce follow the 1959 international yard and pound agreement (1 yd = 0.9144 m, 1 lb = 0.45359237 kg); the US gallon is 231 cubic inches; the imperial gallon is 4.54609 liters; the nautical mile is 1,852 m; the standard atmosphere is 101,325 Pa. Values were checked against NIST Special Publication 811, Appendix B, and the BIPM SI Brochure. Where a value is a convention rather than a definition (millimeters and inches of mercury), it is marked.
Length: base unit m| Unit | 1 unit in m | Note |
|---|
| Millimeters (mm) | 0.001 | |
| Centimeters (cm) | 0.01 | |
| Meters (m) | 1 | |
| Kilometers (km) | 1,000 | |
| Inches (in) | 0.0254 | |
| Feet (ft) | 0.3048 | |
| Yards (yd) | 0.9144 | |
| Miles (mi) | 1,609.344 | |
| Nautical miles (nmi) | 1,852 | |
Mass (weight): base unit kg| Unit | 1 unit in kg | Note |
|---|
| Milligrams (mg) | 0.000001 | |
| Grams (g) | 0.001 | |
| Kilograms (kg) | 1 | |
| Metric tonnes (t) | 1,000 | |
| Ounces (oz) | 0.028349523125 | |
| Pounds (lb) | 0.45359237 | |
| Stone (st) | 6.35029318 | 14 lb, used in the UK and Ireland |
| US short tons (ton) | 907.18474 | 2,000 lb |
| Imperial long tons (long ton) | 1,016.0469088 | 2,240 lb |
Volume: base unit L| Unit | 1 unit in L | Note |
|---|
| Milliliters (mL) | 0.001 | |
| Liters (L) | 1 | |
| Cubic meters (m³) | 1,000 | |
| Cubic inches (in³) | 0.016387064 | |
| Cubic feet (ft³) | 28.316846592 | |
| US fluid ounces (US fl oz) | 0.0295735295625 | |
| US cups (US cup) | 0.2365882365 | |
| US pints (US pt) | 0.473176473 | |
| US quarts (US qt) | 0.946352946 | |
| US gallons (US gal) | 3.785411784 | |
| Imperial fluid ounces (imp fl oz) | 0.0284130625 | |
| Imperial pints (imp pt) | 0.56826125 | |
| Imperial gallons (imp gal) | 4.54609 | |
Cooking measures: base unit mL| Unit | 1 unit in mL | Note |
|---|
| Milliliters (mL) | 1 | |
| Liters (L) | 1,000 | |
| US teaspoons (US tsp) | 4.92892159375 | |
| US tablespoons (US tbsp) | 14.78676478125 | |
| US fluid ounces (US fl oz) | 29.5735295625 | |
| US customary cups (US cup) | 236.5882365 | |
| US legal cups (nutrition labels) (legal cup) | 240 | FDA labeling cup |
| Metric teaspoons (tsp) | 5 | |
| Metric tablespoons (tbsp) | 15 | UK, Canada, New Zealand |
| Australian tablespoons (AU tbsp) | 20 | |
| Metric cups (metric cup) | 250 | Australia, New Zealand, Canada, South Africa |
| Japanese cups (JP cup) | 200 | |
| Imperial fluid ounces (imp fl oz) | 28.4130625 | |
| Imperial pints (imp pt) | 568.26125 | |
Area: base unit m²| Unit | 1 unit in m² | Note |
|---|
| Square millimeters (mm²) | 0.000001 | |
| Square centimeters (cm²) | 0.0001 | |
| Square meters (m²) | 1 | |
| Hectares (ha) | 10,000 | |
| Square kilometers (km²) | 1,000,000 | |
| Square inches (in²) | 0.00064516 | |
| Square feet (ft²) | 0.09290304 | |
| Square yards (yd²) | 0.83612736 | |
| Acres (ac) | 4,046.8564224 | International acre, 4,840 yd² |
| Square miles (mi²) | 2,589,988.110336 | |
Speed: base unit m/s| Unit | 1 unit in m/s | Note |
|---|
| Kilometers per hour (km/h) | 0.277777777777778 | |
| Miles per hour (mph) | 0.44704 | |
| Meters per second (m/s) | 1 | |
| Knots (kn) | 0.514444444444445 | |
| Feet per second (ft/s) | 0.3048 | |
Pressure: base unit Pa| Unit | 1 unit in Pa | Note |
|---|
| Pounds per square inch (psi) | 6,894.75729316836 | |
| Bar (bar) | 100,000 | |
| Kilopascals (kPa) | 1,000 | |
| Pascals (Pa) | 1 | |
| Hectopascals (millibars) (hPa) | 100 | |
| Standard atmospheres (atm) | 101,325 | |
| Kilograms-force per square centimeter (kgf/cm²) | 98,066.5 | |
| Millimeters of mercury (mmHg) | 133.322387415 | conventional |
| Inches of mercury (inHg) | 3,386.389 | conventional |
Data storage: base unit B| Unit | 1 unit in B | Note |
|---|
| Bits (bit) | 0.125 | |
| Bytes (B) | 1 | |
| Kilobytes (kB) | 1,000 | |
| Megabytes (MB) | 1,000,000 | |
| Gigabytes (GB) | 1,000,000,000 | |
| Terabytes (TB) | 1,000,000,000,000 | |
| Kibibytes (KiB) | 1,024 | |
| Mebibytes (MiB) | 1,048,576 | |
| Gibibytes (GiB) | 1,073,741,824 | |
| Tebibytes (TiB) | 1,099,511,627,776 | |
Fuel economy: base unit km/L| Unit | Factor | Note |
|---|
| Liters per 100 kilometers (L/100 km) | 100 | km/L = 100 ÷ value |
| Miles per US gallon (mpg (US)) | 0.425143707430272 | |
| Miles per imperial gallon (mpg (UK)) | 0.354006189934647 | |
| Kilometers per liter (km/L) | 1 | |
Temperatures convert through kelvin: K = °C + 273.15, and °F = °C × 9/5 + 32. Fuel economy converts through kilometers per liter; liters per 100 km is its reciprocal (100 ÷ km/L).
Numbers you type
- A point or a comma is accepted as the decimal mark. When both appear, the last one is the decimal mark (1,234.5 and 1.234,5 are both 1234.5). A single comma followed by exactly three digits is read as a thousands separator (1,500 is 1500).
- Results show up to ten significant digits with thousands separators, so floating-point artifacts such as 0.30000000000000004 never appear.
- Fractions are computed exactly with whole-number arithmetic: 1/3 + 1/6 is exactly 1/2.
- The rounding calculator works on the digits you typed, so 1.005 rounded to two places is 1.01, the answer you would get on paper.
Dates and times
- Dates use the Gregorian calendar extended backwards, as in ISO 8601. Leap years are years divisible by 4, except century years not divisible by 400.
- Date arithmetic counts whole calendar days, so daylight saving changes never shift a result.
- Adding months keeps the day of the month when it exists and otherwise uses the last day: January 31 plus one month is February 28, or 29 in a leap year. A February 29 birthday is counted on February 28 in common years.
- Week numbers follow ISO 8601: weeks start on Monday and week 1 is the week containing the year's first Thursday.
- Working days are Monday to Friday. Public holidays differ by country and region, so the tool skips only the dates you list.
- Time zone conversions use the IANA time zone database built into your browser, so they follow the daylight saving rules your device knows. Times that are skipped or repeated when clocks change are flagged.
Testing
The calculation modules have automated tests against reference values: the exact definitions above, known calendar facts (2000 was a leap year, 1900 was not; 3 January 2021 belongs to ISO week 53 of 2020), daylight saving transitions in New York, exact fraction and rounding cases, and everyday examples. The tests run before every release, together with a check that every internal link works.
Limits
The tools do exact arithmetic on the values you enter; they cannot know whether those values are right. Measurements carry their own uncertainty, so round results to a precision that matches your input. For anything with legal, medical, financial or safety consequences, confirm the figure with the responsible authority.
Sources