Convert between 150+ units across 10 categories, length, weight, volume, temperature, area, speed, time, data, pressure, and energy. Live results as you type.
The US uses a dual system in practice, metric in science, medicine, military, and many industries; imperial in everyday life, construction, and food. This means Americans encounter both systems constantly and need to convert between them. NASA's Mars Climate Orbiter was lost in 1999 because one engineering team used metric units and another used imperial, a $327.6 million mistake caused entirely by a unit conversion error. The cost of unit confusion in everyday life is smaller but constant. In nutrition and fitness, food labels list serving sizes in both grams and ounces. Tracking macros accurately requires understanding grams, a 28g serving of almonds is roughly 1 ounce. In international e-commerce, product specifications in one unit system must be converted for buyers in another.
The world’s measurement systems split primarily into two camps: the International System of Units (SI), known informally as the metric system, and the Imperial / US Customary system. Understanding both, and how to move between them, is an essential everyday skill for science, travel, cooking, construction, and commerce.
The metric system was developed in France in the late 18th century and has become the global standard for science, medicine, and commerce in nearly every country. Its genius is simplicity: every unit is derived from a base unit by multiplying or dividing by powers of 10. Kilo = 1,000×, Centi = 1/100, Milli = 1/1,000, Micro = 1/1,000,000. This means converting 1.5 km to meters is just 1.5 × 1,000 = 1,500 m, no memorization of arbitrary factors required.
The imperial system grew organically from historical units based on body parts, agricultural measures, and royal edicts. It uses seemingly arbitrary conversion factors: 12 inches in a foot, 3 feet in a yard, 5,280 feet in a mile. 16 ounces in a pound. 8 pints in a gallon. Despite its complexity, it persists in everyday US life for height, weight, cooking, and road distances, and in the UK for a few holdovers like miles and pints of beer.
The meter was originally defined as one ten-millionth of the distance from the North Pole to the equator. Since 1983, it is defined as the distance light travels in a vacuum in 1/299,792,458 of a second. The inch is defined as exactly 2.54 cm, a precise relationship established by international agreement in 1959. All other imperial length units flow from this definition.
The kilogram is the SI base unit of mass, defined since 2019 by fixing Planck’s constant. An important distinction: mass is the amount of matter in an object (constant everywhere); weight is the gravitational force on that mass (varies on the Moon or in space). In everyday use these terms are used interchangeably. The pound (lb) is defined as exactly 0.45359237 kg.
| US Unit | mL | Relationship |
|---|---|---|
| 1 teaspoon | 4.929 mL | 3 tsp = 1 tbsp |
| 1 tablespoon | 14.787 mL | 16 tbsp = 1 cup |
| 1 fluid ounce | 29.574 mL | 8 fl oz = 1 cup |
| 1 cup | 236.588 mL | 2 cups = 1 pint |
| 1 pint (US) | 473.176 mL | 2 pints = 1 quart |
| 1 quart (US) | 946.353 mL | 4 quarts = 1 gallon |
| 1 gallon (US) | 3,785.41 mL | 3.785 liters |
Miles per hour (mph) is used in the US and UK for vehicle speeds. Kilometers per hour (km/h) is used everywhere else. Knots are used in aviation and maritime navigation, 1 knot = 1 nautical mile per hour = 1.852 km/h. Mach numbers express speed relative to the speed of sound (≈ 343 m/s at sea level, 20°C). Mach 1 ≈ 1,235 km/h ≈ 767 mph.
Data units use binary (base-2) prefixes in computing contexts: 1 KB = 1,024 bytes, 1 MB = 1,024 KB, 1 GB = 1,024 MB. This calculator uses binary prefixes (powers of 1,024), which is how operating systems report file sizes. Network speeds are measured in bits (not bytes), divide by 8 to convert: 100 Mbps ÷ 8 = 12.5 MB/s theoretical maximum download speed.
The Pascal (Pa) is the SI unit of pressure, equal to one newton per square meter. Atmospheric pressure at sea level is approximately 101,325 Pa = 1 atm = 1.01325 bar = 14.696 psi. Blood pressure is reported in mmHg (millimeters of mercury). Tire pressure in the US uses PSI; in most of the world, bar or kPa. Weather maps use hectopascals (hPa), where 1 hPa = 1 millibar.
The Joule is the SI unit of energy. A calorie (cal) is the energy needed to raise 1 gram of water by 1°C. A food Calorie (kcal) is 1,000 cal, always written with a capital C on nutrition labels. 1 kWh (kilowatt-hour) = 3,600,000 joules = 3,600 kJ. The BTU (British Thermal Unit) is used in HVAC and power generation: 1 BTU = 1,055.06 J, the energy needed to heat 1 pound of water by 1°F.
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