This page explains where the numbers on this site come from, so you can decide for yourself whether to trust them. Modern Converts is an independent project with no vendor or manufacturer behind it, and the unit table, the formulas and this check are maintained together; see About.
Every measurement category has one anchor unit, normally the SI unit: the pascal for pressure, the cubic metre per second for volume flow rate, the volt-ampere for apparent power. Each unit in the category is defined by a single ratio to that anchor. A conversion from unit A to unit B is therefore A → anchor → B, and the factor shown on a page is the product of those two ratios. Categories that mix metric and imperial units carry one extra ratio between the metric anchor and the imperial anchor, for example 1 in = 0.0254 m, which is exact by definition.
The unit table is built on the open-source convert-units library and used as-is; the site adds the display names, formulas, tables and explanatory text.
The ratios are chained with arbitrary-precision decimal arithmetic (the decimal.js library), not with binary floating point, so a factor such as 1 m³/h = 1/3600 m³/s is carried as an exact fraction until it is printed. Factors quoted in formulas, in the step-by-step guides and in page descriptions are rounded to 6 significant digits. The calculator shows more digits; the underlying value is the same.
The table below is not a static list: it is computed from the live unit table every time the site is built, and an automated test with the same rows must pass before any release. Right now 21 of 21 factors agree with the reference at 6 significant digits. Where a unit has more than one legitimate definition, the note says which one the site uses.
| Conversion | Reference | This site | Source and definition |
|---|---|---|---|
| 1 in → m | 0.0254 | 0.0254 | NIST SP 811 B.8: exact; international inch |
| 1 ft → m | 0.3048 | 0.3048 | NIST SP 811 B.8: exact; international foot |
| 1 mi → km | 1.609344 | 1.60934 | NIST SP 811 B.8: exact; international mile |
| 1 lb → kg | 0.45359237 | 0.453592 | NIST SP 811 B.8: exact; avoirdupois pound |
| 1 oz → g | 28.34952 | 28.3495 | NIST SP 811 B.8: avoirdupois ounce |
| 1 psi → Pa | 6894.757 | 6894.76 | NIST SP 811 B.8: pound-force per square inch |
| 1 mmHg → Pa | 133.3224 | 133.322 | NIST SP 811 B.8: conventional millimeter of mercury |
| 1 inHg → Pa | 3386.389 | 3386.39 | NIST SP 811 B.8: conventional inch of mercury |
| 1 bar → Pa | 100000 | 100000 | NIST SP 811 B.8: exact |
| 1 hp → W | 745.6999 | 745.7 | NIST SP 811 B.8: mechanical horsepower, 550 ft·lbf/s |
| 1 kWh → J | 3600000 | 3600000 | NIST SP 811 B.8: exact |
| 1 cal → J | 4.184 | 4.184 | NIST SP 811 B.8: thermochemical calorie (the International Table calorie is 4.1868 J) |
| 1 gal → l | 3.785412 | 3.78541 | NIST SP 811 B.8: US liquid gallon, not the imperial gallon |
| 1 lbf → N | 4.448222 | 4.44822 | NIST SP 811 B.8: pound-force |
| 1 lbf-ft → Nm | 1.355818 | 1.35582 | NIST SP 811 B.8: pound-force foot |
| 1 knot → m/s | 0.5144444 | 0.514444 | NIST SP 811 B.8: international knot |
| 1 mph → km/h | 1.609344 | 1.60934 | NIST SP 811 B.8: exact |
| 1 ac → m2 | 4046.856 | 4046.86 | NIST SP 811 B.8: international acre (the US survey acre is 4046.873 m²) |
| 1 ha → m2 | 10000 | 10000 | NIST SP 811 B.8: exact |
| 1 kB → byte | 1000 | 1000 | IEC 80000-13: decimal kilobyte |
| 1 KiB → byte | 1024 | 1024 | IEC 80000-13: binary kibibyte |
It is a reference for converting between units. It is not a calibrated instrument and not a source for legal metrology; if a contract, a regulation or a safety calculation depends on a conversion, check it against the primary source listed below.
If a factor, a formula or a description is wrong, email [email protected] with the page URL. Corrections ship in the next deployment, and the sitemap date of the affected page is updated so search engines re-crawl it.