Unit conversion guides

Short, factual explanations of units of measurement and the differences between them.

What Is a Newton? The SI Unit of ForceThe newton (N) is the SI unit of force: one kilogram accelerated at one meter per second squared. Definition, real-world sizes and conversions to lbf and kgf.
What Is a Pound-Force (lbf)?The pound-force (lbf) is the force of standard gravity on one pound of mass, exactly 4.4482216152605 N. How it differs from the pound and where it is used.
Kilogram vs Kilogram-Force (kg vs kgf)Kilogram (kg) measures mass; kilogram-force (kgf) measures force, exactly 9.80665 N. Why they get mixed up and how to convert between kgf and newtons.
Newton vs Pound-Force (N vs lbf)Newton (N) is the SI unit of force; pound-force (lbf) is the US customary unit. 1 lbf ≈ 4.448 N. Side-by-side values and a quick mental shortcut.
What Is a Kilonewton (kN)?A kilonewton (kN) is 1000 newtons, about 224.8 lbf or 101.97 kgf. Where kilonewtons are used, from structural loads to climbing gear ratings.
How to Convert Celsius to Fahrenheit and BackConvert Celsius to Fahrenheit with °F = °C × 1.8 + 32, and back with °C = (°F − 32) ÷ 1.8. Reference points, a table, a mental shortcut and common mistakes.
What Is a Kelvin? The SI Unit of TemperatureThe kelvin (K) is the SI base unit of temperature, defined by the Boltzmann constant. Absolute zero, kelvin vs degree Celsius, Rankine and a reference table.
Metric vs Imperial vs US Customary UnitsHow metric (SI), US customary and British imperial units compare, how the inch and pound are defined in metric terms, and where US and UK units differ.
SI Prefixes Explained, From Quecto to QuettaAll 24 SI prefixes with names, symbols and powers of ten, why m and M differ, the kilogram exception, the 2022 additions and the binary prefixes kibi and mebi.
Kilograms vs Pounds (kg vs lb)The pound is defined as exactly 0.45359237 kg, so 1 kg ≈ 2.20462 lb. How to convert both ways, a quick table and a mental shortcut.
How to Convert Inches to CentimetersOne inch is exactly 2.54 cm. How to convert inches and feet to centimeters and back, with tables for screens, rulers and fractions of an inch in millimeters.
Miles vs Kilometers (mi vs km)A mile is exactly 1609.344 meters, so 1 mi ≈ 1.6093 km. Race distances, mph vs km/h speed limits, nautical miles and a mental shortcut.
How to Convert Height in Feet and Inches to cmConvert a height in feet and inches to centimeters and back: total inches times 2.54, worked examples, a height chart from 4 ft 10 in to 6 ft 6 in.
Bar vs PSI and How to Convert Between ThemOne bar is exactly 100,000 Pa, about 14.5 psi. How to convert bar and psi both ways, common values, and gauge vs absolute pressure.
Pressure Units Explained, From Pascal to PSIPascal, kPa, MPa, bar, millibar, hectopascal, atmosphere, torr, mmHg and psi compared, with exact definitions and the fields that use each pressure unit.
Torque Units Explained, N·m, lbf·ft and MoreTorque units compared: newton meter, pound-force foot, pound-force inch and kgf·m. Why torque uses N·m and not joules, with tables and worked examples.
Kilowatts and Horsepower (kW, hp and PS)Kilowatts vs horsepower: mechanical hp is 550 ft·lbf/s, about 745.7 W; metric PS is 735.49875 W. Convert both ways and compare car specs.
US Gallon vs Imperial GallonThe US liquid gallon is exactly 231 cubic inches; the imperial gallon is exactly 4.54609 liters. How pints, quarts, fluid ounces and mpg differ.
Energy Units Explained, From Joules to kWhJoules, calories, kilocalories, watt-hours, kilowatt-hours, BTU and electronvolts: what each measures, where it is used and how they compare.
Knots, mph and km/h ComparedHow km/h, mph, m/s and knots compare, where each speed unit is used, and quick mental shortcuts with their error. One knot is one nautical mile per hour.
Cups, Tablespoons and Teaspoons in MillilitersUS cups, tablespoons, teaspoons and fluid ounces in milliliters, how they relate, and why cups of flour to grams is not a unit conversion.