Conversions
Exact conversion factors where they exist, stated ones where they do not. Cooking conversions are the interesting case: a cup of flour and a cup of sugar are different masses, so anything volumetric asks what you are measuring rather than pretending it does not matter.
Measurement
Length ConverterLength and distance between metric and imperial units, on the exact defined factors. An inch is exactly 0.0254 metres, not an approximation.Weight and Mass ConverterMass between metric and imperial, including the two different tons. A short ton and a long ton differ by about 10%, which is not a rounding difference.Temperature ConverterTemperature between Celsius, Fahrenheit and Kelvin. Unlike every other converter on the site this one has an offset as well as a scale, which is why you cannot convert a temperature *difference* the same way.Area ConverterArea across metric, imperial and Indian land units. The Indian units are regional by nature and the page says which convention it uses rather than implying a national standard.Volume ConverterVolume and capacity across metric, US and imperial units. A US gallon and an imperial gallon differ by about a fifth, which is the single most expensive confusion in this dimension.Speed ConverterSpeed between kilometres per hour, miles per hour, metres per second and knots. A knot is one nautical mile an hour, and a nautical mile is a minute of latitude, which is why it is not a round number in either system.Pressure ConverterPressure across pascals, bar, psi, atmospheres and millimetres of mercury. Tyre pressure, blood pressure and weather pressure all live in different units for historical reasons rather than technical ones.Energy ConverterEnergy across joules, calories, kilowatt hours and BTU. The food calorie is a kilocalorie, so a 2,000 calorie diet is two million of the calories a physicist means.Power ConverterPower across watts, horsepower and BTU per hour. Mechanical horsepower and metric horsepower differ by about one and a half percent, which is why the same engine is quoted differently in different markets.Force ConverterForce across newtons, kilogram force and pound force. A kilogram is a mass and a kilogram force is a weight, and treating them as the same thing is the mistake this dimension exists to prevent.Torque ConverterTorque across newton metres, pound feet and kilogram force metres. Pound feet and foot pounds are the same magnitude used for different quantities, which is why the order of the words is not arbitrary.Density ConverterDensity across kilograms per cubic metre, grams per cubic centimetre and pounds per cubic foot. Water is one gram per cubic centimetre, which makes it the reference everything else is judged against.Flow Rate ConverterFlow rate across litres per minute, cubic metres per hour, US gallons per minute and cubic feet per minute. Pump and compressor specifications rarely use the same unit as the pipe standard they attach to.Frequency ConverterFrequency across hertz, kilohertz and revolutions per minute. Hertz and revolutions per minute measure the same thing, so a motor at 3,000 rpm is running at 50 Hz.Time Unit ConverterTime across milliseconds to years, with the two units that are approximations flagged: a month here is thirty days and a year is 365, because neither has a fixed length.
Cooking
Cooking Measurement ConverterCooking measures converted by volume, with an approximate weight. A US cup is 236.6 ml and a metric cup is 250, and a cup of flour weighs about half what a cup of water does.Oven Temperature ConverterOven temperatures across Celsius, Fahrenheit and gas mark, with the fan oven adjustment. A fan oven runs about twenty degrees Celsius hotter in effect, so recipes written for conventional ovens need reducing.Baking Pan Size ConverterConverting a recipe between tin sizes and shapes. Scale by base area rather than diameter, and a round 20 cm tin is not equivalent to a square 20 cm one: the square holds about 27 percent more.Recipe Scaling CalculatorRecipe quantities scaled with the right bakeware size. Ingredients scale linearly and tin diameter does not: it scales by the square root, because it is area that holds the mixture.
Apparel
Shoe Size ConverterShoe sizes across UK, US and EU, anchored to foot length. Sizes are national conventions rather than measurements, so any conversion is approximate and brands vary within it.Clothing Size ConverterClothing sizes from body measurements. Menswear is usually sized on the chest measurement itself, which is why it converts cleanly; womenswear uses arbitrary numbers that have drifted downwards for decades.Ring Size ConverterRing sizes across US, UK and EU systems. The EU size is just the inside circumference in millimetres, which makes it the one worth measuring in.
Vehicle
Fuel Economy ConverterFuel economy across km/l, both mpg conventions and l/100km. The imperial gallon is about twenty percent larger than the US one, so the same car reads noticeably better in UK mpg.Tire Size CalculatorWhether a different tyre size fits, and what it does to the speedometer. The aspect ratio is a percentage of the width rather than a fixed height, which is why a wider tyre with the same number is also taller.
Technical
Wire Gauge ConverterWire gauge to diameter, with the voltage drop over a run. AWG is a geometric series running backwards, so a larger number is a thinner wire and every six gauges halves the diameter.Sheet Metal Gauge ConverterSheet metal gauge to thickness, by material. Gauge is not a unit: 16 gauge steel and 16 gauge aluminium are different thicknesses, and the same number means different things in each standard.Pipe Size ConverterPipe nominal size to actual dimensions. Nominal size is a label, not a measurement: a 2 inch pipe has an outside diameter of 60.3 mm, and the schedule sets the wall thickness and therefore the bore.Thread and Bolt Size ConverterThread sizes between metric pitch and imperial threads per inch, with tapping drill sizes. Metric states the distance between threads and imperial states how many fit in an inch, so the two are reciprocals.
Science
Weight on Other Planets CalculatorYour weight on other worlds. Weight is a force that changes with gravity while mass does not, which is the distinction the question is really about.Astronomical Distance ConverterAstronomical distances across light years, parsecs and astronomical units. A parsec is not a science-fiction invention: it is the distance at which one astronomical unit subtends one arcsecond.Water Hardness ConverterWater hardness across ppm, grains per gallon and the German, French and Clark degree scales. All measure the same calcium carbonate concentration and differ only in units.Sound Level (Decibel) CalculatorCombining sound levels and the effect of distance. Decibels are logarithmic, so two identical sources add three decibels rather than doubling, and ten decibels is roughly a doubling of perceived loudness.