Convert across 18 categories using exact defining factors, with the definition behind every number shown
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Convert between MP3, WAV, OGG, AAC, M4A and FLAC with a bitrate, sample rate, channel count and bit depth that all reach the encoder, and a panel that reports what was actually written.
A spec sheet lists a top speed of 100 km/h and your US-facing copy needs mph, while the physics needs m/s.
Input
100 km/h
Converted
62.1371192237 mph 27.7777777778 m/s 91.1344415281 ft/s 1 Kilometer/hour (km/h) = 0.621371192237 Mile/hour (mph) Mile/hour (mph): The exact mile in 3600 s, so 0.44704 m/s exactly
Factors are derived from the defining constants rather than typed in rounded, so the mile is 1609.344 m exactly and the mph factor is 0.44704 m/s exactly. The panel under the result names the factor and the definition it came from, so the arithmetic can be checked against a standard instead of trusted. Where a unit is not exact, such as Mach or a shoe size, the tool says so rather than quoting a number that looks definitive.
Unit conversion looks trivial and has historically caused expensive mistakes. NASA's 1999 Mars Climate Orbiter was lost because thrust data was provided in pound-force-seconds but interpreted as newton-seconds, a 4.45x difference; the probe entered the Martian atmosphere at the wrong altitude and disintegrated. The conversion factor itself was not hard, it is a single multiplication, but trusting that one team's "units" matched another team's assumption is where real systems fail. This converter covers 18 categories: length, mass, temperature, volume, time, speed, area, pressure, energy, power, data storage, angle, frequency, cooking, typography, paper sizes, shoe sizes and fuel economy. Every factor is derived from a defining constant rather than typed in as a rounded decimal. The inch is 0.0254 m exactly and the foot, yard, mile, square foot, cubic inch, psi and mile per hour are all computed from it; the pound is 0.45359237 kg exactly and the ounce, grain, stone, short ton and slug come from that. Each conversion shows the factor it used and the sentence that defines it, so you can check the tool against a reference table instead of trusting it. Where a unit is not exact, the tool says so: Mach is not a fixed speed, an em is whatever the font size happens to be, and shoe sizes are approximations no formula can pin down. Temperature is the one that trips people up most often, because the scales have different zero points as well as different degree sizes. Celsius to Fahrenheit is F = C * 9/5 + 32, not a multiplication. The freezing point of water is 0 C and 32 F and the boiling point is 100 C and 212 F, so a 1 C change is 1.8 F. Kelvin is absolute-zero-based (0 K = -273.15 C) with Celsius-sized degrees; Rankine is absolute-zero-based with Fahrenheit-sized degrees. All five scales here, Reaumur included, convert through their offset as well as their slope, in both directions.
Data storage has a famous dual standard, and this tool treats the two families as the different units they are. SI prefixes are powers of ten: 1 kB = 1,000 bytes, 1 MB = 1,000,000, 1 GB = 1,000,000,000. IEC prefixes are powers of 1,024: 1 KiB = 1,024 bytes, 1 MiB = 1,048,576, 1 GiB = 1,073,741,824. The gap grows with scale, which is why a drive sold as 1 TB shows up as 931.3 GiB in a file manager that labels its GiB as "GB". Convert 1 TB to GiB here and 931.322574615 is exactly what you get. Both families are listed separately, along with kilobits, megabits and gigabits for the units broadband is sold in.
Length and mass are exact rational numbers in the SI system, and the tool keeps them exact. The international foot is 0.3048 m; the US survey foot is 1200/3937 m, about two parts per million longer, and it is a separate unit here because land records that predate 2023 use it. Nautical units run on their own base: one nautical mile is 1,852 m exactly, one minute of latitude, and a knot is one nautical mile per hour. Astronomical distances are exact too, since the au was fixed at 149,597,870,700 m in 2012 and the light year is the exact speed of light times a Julian year.
Area and volume scale as powers of length, which is where dimensional thinking matters. One square mile is not 5,280 square feet, it is 5,280 squared, or 27,878,400 square feet. The acre is 4,840 square yards, so 4046.8564224 square metres exactly. Volume compounds again: the US gallon is 231 cubic inches, so 3.785411784 L exactly, and every US spoon and cup here is a fraction of it, which is why a US teaspoon is 4.92892159375 mL rather than 5. The imperial gallon is a different unit entirely at 4.54609 L, about 20 percent larger, and the imperial fluid ounce is smaller than the US one. Paper sizes convert by sheet area computed from the trimmed ISO 216 and ANSI dimensions, so one A0 covers 16.03 A4 sheets and 16.58 Letter sheets. Fuel economy is a reciprocal family, so more litres per 100 km is worse while more miles per gallon is better; the tool handles the inversion and round trips exactly.
Convert between US spoons and cups, metric cups and millilitres. The US customary cup is 236.59 mL, the nutrition-label cup is 240 mL and the metric cup is 250 mL, and all three are listed separately.
Switch between miles and kilometres, Fahrenheit and Celsius, and US gallons and litres for trips abroad. Imperial gallons are a separate unit, which matters when reading UK fuel figures.
Convert psi, bar and pascals, foot-pounds and joules, or mechanical and metric horsepower, with the exact factor shown so it can be checked against a standard.
Work out why a 1 TB drive reports 931 GB, or convert a 500 Mb/s connection into megabytes per second, with the SI and IEC prefixes kept apart.
Every exact unit is computed from its defining constant, so the factor agrees with the definition to the last bit a double can hold. The inch, pound, gallon, acre, calorie, BTU, electron volt and the rest are exact by international agreement, and the tool marks them "Exact by definition". Results are displayed to 12 significant figures.
Because they are not exact. Mach depends on air temperature, so Mach 1 at cruising altitude is roughly 45 m/s slower than at sea level. An em or a rem is whatever font size applies. Shoe sizes are a stepwise trade convention that varies by brand. The tool converts them anyway but says what the number depends on.
A kilobyte is 1,000 bytes and a kibibyte is 1,024. They are separate units here, so converting between them gives 0.9765625 rather than 1. Drive manufacturers use the SI meaning, and most operating systems display IEC quantities under SI labels, which is the whole source of the confusion.
Because US customary volumes are fractions of the gallon, which is 231 cubic inches exactly. That makes a teaspoon a 768th of a gallon, or 4.92892159375 mL. The 5 mL teaspoon is the separate metric unit used in the UK, and both are listed.
The international foot is 0.3048 m exactly. The US survey foot is 1200/3937 m, about two parts per million longer. Over a mile that is about 3 mm, which does not matter for anything but surveying, where it moves a state plane coordinate by feet. It was retired for new work at the end of 2022 but older land records still use it.
By area. One A0 sheet covers 16.03 A4 sheets, computed from the trimmed dimensions in ISO 216 rather than from the nominal one square metre, so the numbers match what you would get measuring the paper.
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