Every unit conversion in the world is one of two operations: multiply by a factor, or, for a stubborn handful like temperature, multiply and then shift. That is the entire subject. What makes conversions feel hard is not the math but the bookkeeping: which factor, which direction, and which of two similarly named units a label means. This pillar collects the factors worth knowing, the traps worth avoiding, and links every dedicated guide and converter on the site, starting with the length converter and its siblings.
In this guide
The one principle behind all conversions
A unit is a ruler, and converting only swaps rulers: the quantity never changes. Five kilometers of road is one length, whether the sign reads 5 km or 3.11 miles. So almost every conversion is value × factor, and the inverse direction divides by the same factor. Chaining works too: meters to feet to inches is one multiplication after another, which is why a converter can offer dozens of units from a small table of factors. Keep that picture and you can sanity-check any result: if a number got bigger when the unit got bigger, something is upside down.
Exact by law: the 1959 agreement
A detail that surprises people: the basic imperial-metric factors are not measured approximations, they are definitions. In 1959 the English-speaking countries signed the International Yard and Pound Agreement, defining the yard as exactly 0.9144 meters and the pound as exactly 0.45359237 kilograms. Everything cascades from there: the inch is exactly 2.54 cm, the mile exactly 1.609344 km. There is no “more precise” value waiting in a lab; the digits end. This is why a good converter can be exact for length and weight while honest converters round only for display.
The factors worth knowing by heart
| Conversion | Factor | Status |
|---|---|---|
| 1 inch → cm | 2.54 | exact |
| 1 pound → kg | 0.45359237 | exact |
| 1 mile → km | 1.609344 | exact |
| 1 US gallon → liters | 3.785411784 | exact |
| 1 UK gallon → liters | 4.54609 | exact, and not the US gallon |
| 1 bar → psi | 14.50 | rounded |
| 1 kWh → megajoules | 3.6 | exact |
| 1 mechanical horsepower → watts | 745.7 | rounded |
For everyday mental work, two-digit versions are fine: 100 km/h is about 62 mph, a kilogram is about 2.2 pounds. The dedicated guides below give each pair its shortcuts together with the shortcut’s error, so you know when mental math is good enough and when it is not.
Temperature: the exception that shifts
Temperature scales do not share a zero, so a factor alone cannot convert them; the formula multiplies and shifts: °F = °C × 9/5 + 32. Room temperature of 20°C is 68°F, a 100°F summer day is 37.8°C. The shift is why doubling a Celsius value does not double the Fahrenheit one, and why temperature is the unit people most often convert wrong by hand. The temperature converter handles Celsius, Fahrenheit, and Kelvin in all directions.
The five classic traps
- Two gallons exist. The US gallon is 3.785 liters, the UK (imperial) gallon 4.546. Fuel economy and recipes cross this line constantly; the fuel economy converter keeps them apart.
- GB is not GiB. Storage marketing counts in powers of ten, operating systems in powers of two, which is the whole story of the “missing” disk space: a 500 GB drive is 465.7 GiB with nothing lost. The data storage converter speaks both dialects.
- Weight units fork at the ounce. Fluid ounces measure volume, avoirdupois ounces measure mass, and a troy ounce of gold is heavier than a kitchen one. Label vigilance beats memory here.
- Tons come in three sizes. Metric tonne 1,000 kg, US short ton about 907 kg, UK long ton about 1,016 kg. Shipping documents care which.
- Rounding too early. Convert first, round last. Chaining two rounded conversions stacks their errors, which is how a recipe drifts from 350°F to “about 180°C” to 355°F on the way back.
Every converter and guide
- Length: converter, with the cm-inches-feet math in the cm to inches guide.
- Weight: converter, with kg-lbs shortcuts in the kg to lbs guide.
- Temperature: converter, with the formula and the landmark values in the Celsius to Fahrenheit guide.
- Volume: converter for liters, gallons, cups and the rest, with the two-gallon and cup traps in the liters, gallons and cups guide.
- Speed: converter, with the mph-kmh shortcuts in the mph to km/h guide and the pace side covered in the pace guide.
- Area: converter for square meters, feet, acres, hectares.
- Data: storage converter for the GB vs GiB world, explained fully in the MB, GB and TB guide.
- Pressure: converter for psi, bar, pascal, atmospheres, with the landmark pressures in the pressure units guide.
- Energy and power: energy and power converters.
- Time, angles, frequency: time, angle, and frequency converters, with degrees, radians and gradians untangled in the angle units guide.
The wider collection, including encoding and file converters beyond physical units, lives in the converters hub.
Frequently asked questions
Why do different sites give slightly different answers?
Rounding choices, almost always. The underlying factors for length and weight are exact by definition, so any disagreement past the displayed decimals is a display decision, not a physics dispute. Suspicion is warranted only when answers differ in the first two or three digits, which usually means one site grabbed the wrong unit (US vs UK gallon is the classic).
What does “exact” really mean for a conversion factor?
That the factor is a definition with finitely many digits, not a measurement. 1 inch = 2.54 cm has no further decimals to discover. Contrast that with physical constants, which are measured and carry uncertainty.
Is there a trick for converting in my head?
Per pair, yes, and each dedicated guide lists its shortcut with the error attached, like ×2.2 for kg to pounds at a 0.2% cost. The general trick is directional sense: memorize which unit is bigger, and you will catch nine out of ten mistakes before they leave your head.
Why does the US still use customary units?
Inertia and infrastructure: road signs, deeds, recipes, tooling. Ironically, US customary units have been legally defined in metric terms since 1959, so the US is metric underneath and imperial on the surface.