Unit Converter
Convert between any units of measurement. Length, weight, temperature, volume, area, speed, data, and more.
How Unit Conversion Works
Unit conversion uses conversion factors that relate one unit to another. To convert 5 miles to kilometers, multiply by the conversion factor 1.60934: 5 x 1.60934 = 8.047 km. Conversions can be chained: to convert square feet to square meters, convert feet to meters (divide by 3.281), then square the result. This calculator handles the math for hundreds of unit pairs.
For tire measurements, the Tire Size Calculator converts between metric width, aspect ratio, and overall diameter in inches.
How to Use This Calculator
Select a category (length, weight, volume, temperature, area, speed, time, digital storage, or energy), enter a value, and select the input and output units. The calculator instantly shows the converted value with full precision. You can also see a table of common conversions for the selected unit.
Most Common Conversions
Length: 1 mile = 1.609 km, 1 inch = 2.54 cm, 1 foot = 0.3048 m. Weight: 1 pound = 0.4536 kg, 1 kg = 2.205 lbs, 1 ounce = 28.35 g. Volume: 1 gallon = 3.785 liters, 1 liter = 33.814 fl oz, 1 cup = 236.6 mL. Temperature: F to C: (F - 32) x 5/9. C to F: (C x 9/5) + 32. For cooking-specific conversions, use the Cooking Converter.
Unit Converter FAQ
Why does the U.S. use different units?
The U.S. customary system evolved from the British Imperial system and was never officially replaced by metric, though metric is used in science, medicine, and the military. The U.S. is one of only three countries (along with Myanmar and Liberia) that has not officially adopted the metric system for everyday use.
Common Unit Conversion Reference
Frequently needed conversions include: 1 mile = 1.609 kilometers, 1 kilogram = 2.205 pounds, 1 gallon = 3.785 liters, 1 inch = 2.54 centimeters, 1 acre = 43,560 square feet, and 1 ounce = 28.35 grams. Temperature conversions follow distinct formulas: Celsius to Fahrenheit multiply by 9/5 then add 32, Fahrenheit to Celsius subtract 32 then multiply by 5/9. The metric system uses base-10 prefixes (kilo = 1,000, centi = 1/100, milli = 1/1,000) which makes internal conversions straightforward. The US customary system uses irregular relationships (12 inches to a foot, 3 feet to a yard, 5,280 feet to a mile) that require memorization or a calculator.
The Metric and Imperial Systems
The metric system (International System of Units, SI) is used by virtually every country except the United States, Liberia, and Myanmar. It is built on base-10 relationships: 1 kilometer = 1,000 meters = 100,000 centimeters. The imperial/U.S. customary system uses arbitrary ratios: 1 mile = 5,280 feet = 63,360 inches. This inconsistency makes imperial calculations more error-prone, most famously causing the $125 million loss of NASA's Mars Climate Orbiter in 1999 when one team used metric units while another used imperial.
Common conversion factors worth memorizing: 1 inch = 2.54 cm (exact), 1 mile = 1.609 km, 1 pound = 0.4536 kg, 1 gallon = 3.785 liters, 1 fluid ounce = 29.57 mL. Temperature conversion between Fahrenheit and Celsius uses the formula: F = (C x 9/5) + 32. Key reference points: water freezes at 0C / 32F, boils at 100C / 212F, human body temperature is 37C / 98.6F, and room temperature is approximately 20-22C / 68-72F.
When Precision Matters
In cooking, approximate conversions are usually sufficient (1 cup is roughly 240 mL). In engineering and science, precision matters: material specifications, machining tolerances, and pharmaceutical dosing require exact conversions carried to appropriate significant figures. The Cooking Converter handles kitchen-specific measurements, while this tool provides general-purpose unit conversion across length, weight, volume, temperature, area, speed, and data storage categories.
The metric system's coherence (all units relate by powers of 10) creates enormous practical advantages for scientific and engineering work. Converting 3.5 kilometers to meters requires only moving the decimal point (3,500 m), while converting 3.5 miles to feet requires multiplying by 5,280 (18,480 ft). This simplicity reduces calculation errors and is the primary reason the metric system is used universally in science, medicine, and international commerce. The U.S. metric conversion effort, officially begun with the Metric Conversion Act of 1975, remains incomplete largely due to the cost of replacing existing infrastructure, signage, and consumer familiarity with imperial units. In practice, many U.S. industries (automotive, pharmaceutical, military, science) already use metric internally.
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