Unit Conversion Dictionary: Common Units Defined and Cross-Referenced
This dictionary provides concise definitions of the most common measurement units encountered in everyday conversion tasks — organized by category for easy reference. Each entry gives the unit's full name, symbol, category, definition, and where applicable, a practical reference value.
This page is a companion to the Measurement Systems Handbook and the Metric Prefixes Guide. For calculation tools and full conversion tables, use the linked converters. For the formulas behind each conversion, see the Conversion Formulas reference.
Length Units
Millimeter (mm): SI/metric. 0.001 m. Used for precision manufacturing, rainfall depth, screw dimensions. 1 mm = 0.0394 inches. Centimeter (cm): SI/metric. 0.01 m. Used for clothing measurements, body dimensions, screen sizes. 1 cm = 0.394 inches. Meter (m): SI base unit of length. Used for room dimensions, athletics, scientific measurements. 1 m = 3.281 feet. Kilometer (km): SI/metric. 1,000 m. Used for road distances, geography. 1 km = 0.621 miles.
Inch (in): US customary / imperial. Defined as exactly 25.4 mm. Used for screen sizes, plumbing, US body measurements. 12 inches = 1 foot. Foot (ft): US customary / imperial. 0.3048 m. Used for room height, elevation, altitude. 3 feet = 1 yard. Yard (yd): US customary / imperial. 0.9144 m. Used for American football, fabric. Mile (mi): US customary / imperial. 1,609.344 m = 1.609 km. Used for road distances, running. Nautical mile (nmi): 1,852 m. Used in aviation and maritime navigation. 1 nmi = 1 arc-minute of Earth's latitude.
Mass and Weight Units
Milligram (mg): SI/metric. 0.001 g. Used for drug dosages, supplement labels. 1,000 mg = 1 g. Gram (g): SI/metric. 0.001 kg. Used for food portion weights, cooking ingredients. 1 g = 0.0353 oz. Kilogram (kg): SI base unit of mass. Used for body weight in most countries, produce, packages. 1 kg = 2.205 lb.
Ounce (oz): US customary / imperial. 28.3495 g. Used for food packaging, postal weights. 16 oz = 1 lb. Pound (lb): US customary / imperial. 453.592 g. Used for body weight in the US, construction materials. Stone (st): UK imperial. 6.35 kg = 14 lb. Used for body weight in the UK. Metric ton / tonne (t): SI. 1,000 kg. Used for industrial and shipping loads. US short ton = 2,000 lb = 907.185 kg.
Volume Units
Milliliter (mL): SI/metric. 0.001 L. Numerically equal to 1 cubic centimeter (cc). Used for liquid medication, beverages. 1 mL ≈ 0.034 US fl oz. Liter (L): SI. 0.001 m³. Used for bottled beverages, fuel in metric countries. 1 L = 33.81 US fl oz = 4.227 US cups. Fluid ounce (fl oz): US customary. 29.574 mL (US); 28.413 mL (UK). Volume measure only — distinct from the weight ounce.
Cup (c): US customary. 236.588 mL = 8 fl oz. Used in US cooking recipes. Australian cup = 250 mL. Pint (pt): US = 473.176 mL = 2 cups; UK = 568.261 mL. US and UK pints are not equal. Quart (qt): US = 946.353 mL = 4 cups = 2 pints. Gallon (gal): US = 3,785.41 mL = 4 quarts; UK = 4,546.09 mL. Tablespoon (tbsp): US = 14.787 mL = 3 tsp. Australian = 20 mL. Teaspoon (tsp): US = 4.929 mL.
Temperature Units
Celsius (°C): SI/metric. The standard temperature scale in most countries. Defined with 0°C = freezing point of water, 100°C = boiling point at 1 atm. Formula to Fahrenheit: °F = (°C × 9/5) + 32. Fahrenheit (°F): US customary. Used in the US for weather, cooking, and body temperature. 32°F = 0°C (freezing); 212°F = 100°C (boiling); 98.6°F = 37°C (normal body temperature). Formula to Celsius: °C = (°F − 32) × 5/9.
Kelvin (K): SI base unit of thermodynamic temperature. No degree symbol — just K. 0 K = absolute zero (−273.15°C), the coldest possible temperature. Formula: K = °C + 273.15. Used in science, physics, and astronomy. Rankine (°R): US engineering absolute temperature scale. 0°R = absolute zero. °R = °F + 459.67. Used in some US thermodynamic engineering applications.
Pressure Units
Pascal (Pa): SI derived unit of pressure. 1 Pa = 1 N/m². Very small — standard atmosphere is 101,325 Pa. Kilopascal (kPa): 1,000 Pa. Used for tire pressure in metric countries, engineering. Standard atmosphere ≈ 101.325 kPa. Bar: 100,000 Pa = 100 kPa. Used in European industry, HVAC, scuba. 1 bar ≈ 0.987 atm ≈ 14.504 PSI.
PSI (pound-force per square inch): US customary / imperial pressure unit. 1 PSI = 6.895 kPa. Used for tire pressure, hydraulics, water pressure in North America. Standard atmosphere (atm): exactly 101,325 Pa. Reference pressure for gas law calculations, diving, altitude. 1 atm = 14.696 PSI = 760 mmHg. mmHg (millimeters of mercury): 133.322 Pa. Used for blood pressure and medical measurement. Torr: nearly equal to 1 mmHg (1 Torr = 1/760 atm).
Area Units
Square meter (m²): SI unit of area. 1 m² = 10.764 sq ft. Used globally for floor area, land measurement. Square foot (ft²): US customary. 0.0929 m². Used for real estate, flooring, room sizes in the US. Square kilometer (km²): 1,000,000 m². Used for land area, country size. Hectare (ha): 10,000 m² = 0.01 km². Standard metric land area unit — used for agricultural and rural land measurement worldwide.
Acre (ac): US customary / imperial. 4,046.86 m² = 0.4047 ha. Used for land measurement in the US and UK. 1 square mile = 640 acres. Square mile (mi²): US. 2.590 km². Used for large area descriptions (city size, national parks) in the US.
Speed Units
Meter per second (m/s): SI unit of speed. 1 m/s = 3.281 ft/s = 3.6 km/h. Used in scientific and physics contexts. Miles per hour (mph): US customary. 1 mph = 1.609 km/h = 0.447 m/s. Used for road speed limits and vehicle speed in the US and UK. Kilometer per hour (km/h): metric. 1 km/h = 0.621 mph. Standard speed unit for road use in most countries.
Knot (kn): 1 nautical mile per hour = 1.852 km/h = 1.151 mph. Used exclusively in aviation and maritime navigation. Mach: ratio of an object's speed to the local speed of sound. Mach 1 ≈ 343 m/s (at sea level, 20°C). Used for aircraft and projectile speeds. Feet per second (ft/s): US engineering. 1 ft/s = 0.305 m/s = 0.682 mph.
Data Storage Units
Bit (b): the fundamental unit of digital data — a single binary 0 or 1. Network speeds are measured in bits per second (bps, Kbps, Mbps, Gbps). Byte (B): 8 bits. The basic addressable unit of data storage. File sizes are measured in bytes. Kilobyte (KB): 1,000 bytes (SI decimal). Megabyte (MB): 1,000 KB = 1,000,000 bytes. Used for file sizes, RAM descriptions.
Gigabyte (GB): 1,000 MB = 10⁹ bytes. Standard for hard drive capacity, smartphone storage. Terabyte (TB): 1,000 GB = 10¹² bytes. Used for large drives and cloud storage. Binary variants: kibibyte (KiB) = 1,024 B; mebibyte (MiB) = 1,024 KiB; gibibyte (GiB) = 1,024 MiB. Operating systems historically reported storage in GiB but labeled it GB, causing the apparent 'missing' space on drives.
Frequently Asked Questions
What is the difference between a unit and a measurement system?
A unit is a defined quantity used as a standard reference (e.g., the meter, the pound, the degree Celsius). A measurement system is a coordinated collection of units designed to work together — such as SI, the US customary system, or the imperial system. Units can often be used across systems once conversion factors are established.
Why are there so many different units for the same quantity?
Different units emerged historically in different cultures and contexts before standardization. The US gallon and UK gallon both measure volume but were standardized separately. Scientific precision created additional units (the pascal for engineering, mmHg for medicine). Trade, industry, and tradition have kept older units alive even after SI became international standard.
What is the most important conversion factor to memorize?
It depends on your context. For general US-to-metric: 1 inch = 2.54 cm (exact), 1 pound = 0.454 kg, 1 mile = 1.609 km, and 1 gallon = 3.785 L are the four most commonly needed. For pressure: 1 PSI ≈ 6.895 kPa.
What is a derived unit?
A derived unit is formed by combining base units. Speed (m/s) is length per time. Pressure (Pa = N/m²) is force per area, where force (N = kg·m/s²) itself is mass times acceleration. The SI system has 7 base units and defines all other units as combinations of those 7, with no additional conversion factors needed.
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Sources
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