Unit Conversion Calculator
Select a category, choose your units, enter a value — see instant results plus step-by-step working
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What Is Unit Conversion? A Complete Guide
Understanding measurement systems, why unit conversion matters, and how it works in everyday life
Definition
Unit Conversion: Expressing the Same Quantity Differently
Unit conversion is the process of expressing a measurement in one unit as an equivalent measurement in another unit. The quantity being measured stays exactly the same — only the unit (and therefore the number) changes. Knowing how to convert units is essential in science, engineering, cooking, travel, medicine, and everyday life.
For example, 1 meter and 100 centimeters represent the same physical length — just expressed with different units. Similarly, 0°C and 32°F represent the exact same temperature. The conversion process applies a mathematical formula to shift from one scale to another without changing the underlying physical quantity.
📐 Core Principle: A unit conversion never changes what you are measuring — it only changes how you express it. 1 kg = 2.20462 lbs — both numbers describe the same mass, just using different yardsticks.
There are two dominant systems in use today: the metric system (SI), used by nearly every country and all scientific disciplines, and the imperial/US customary system, still used in the United States and partially in the United Kingdom. Converting between these systems is one of the most common everyday tasks this calculator handles.
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Metric System (SI)
Decimal-based system used by 195+ countries. Units scale by powers of 10 using standard prefixes (kilo, centi, milli). Founded in France in 1795. Adopted globally for all science and engineering.
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Imperial / US Customary
Used in the USA for everyday life. Based on historical British units with non-decimal conversions (12 in = 1 ft, 5280 ft = 1 mi). Still used in UK for road distances and body weight.
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SI Base Units
The SI system has 7 base units: meter (length), kilogram (mass), second (time), ampere (current), kelvin (temperature), mole (amount), candela (luminosity). All other units derive from these.
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Why It Matters
In 1999, NASA's Mars Climate Orbiter was lost because one team used metric and another used imperial units. Unit conversion errors cost $327.6 million. Accurate conversion is critical everywhere.
SI Metric Prefixes — Complete Reference
Every standard SI prefix from yotta (10²⁴) to yocto (10⁻²⁴) with symbols and examples
Metric System
The Power of Decimal Prefixes
The metric system's genius is its use of standard prefixes that multiply or divide the base unit by powers of 10. Once you know these prefixes, you can immediately understand any metric measurement without a calculator.
| Prefix | Symbol | Power | Value | Example |
|---|---|---|---|---|
| Tera | T | 10¹² | 1,000,000,000,000 | 1 TB = 1 trillion bytes |
| Giga | G | 10⁹ | 1,000,000,000 | 1 GB = 1 billion bytes |
| Mega | M | 10⁶ | 1,000,000 | 1 MHz = 1 million hertz |
| Kilo | k | 10³ | 1,000 | 1 km = 1,000 m |
| Hecto | h | 10² | 100 | 1 hPa = 100 Pa (weather) |
| Deca | da | 10¹ | 10 | 1 dam = 10 m |
| (Base) | — | 10⁰ | 1 | meter, gram, second |
| Deci | d | 10⁻¹ | 0.1 | 1 dm = 0.1 m (decimeter) |
| Centi | c | 10⁻² | 0.01 | 1 cm = 0.01 m |
| Milli | m | 10⁻³ | 0.001 | 1 mm = 0.001 m |
| Micro | μ | 10⁻⁶ | 0.000001 | 1 μm = 1 micrometer |
| Nano | n | 10⁻⁹ | 0.000000001 | 1 nm = 1 nanometer (atoms) |
| Pico | p | 10⁻¹² | 10⁻¹² | 1 ps = 1 picosecond (lasers) |
Most Important Conversion Formulas — Quick Reference
The most searched unit conversions by category, with exact formulas and worked examples
📏 Length Conversions
| From → To | Formula | Example |
|---|---|---|
| km → miles | miles = km × 0.621371 | 10 km = 6.214 miles |
| miles → km | km = miles × 1.60934 | 26.2 mi = 42.16 km |
| meters → feet | ft = m × 3.28084 | 100 m = 328.084 ft |
| feet → meters | m = ft × 0.3048 | 6 ft = 1.8288 m |
| inches → cm | cm = in × 2.54 | 12 in = 30.48 cm |
| cm → inches | in = cm ÷ 2.54 | 100 cm = 39.37 in |
| yards → meters | m = yd × 0.9144 | 100 yd = 91.44 m |
⚖️ Weight / Mass Conversions
| From → To | Formula | Example |
|---|---|---|
| kg → lbs | lbs = kg × 2.20462 | 70 kg = 154.32 lbs |
| lbs → kg | kg = lbs ÷ 2.20462 | 154 lbs = 69.85 kg |
| kg → stone | stone = kg ÷ 6.35029 | 70 kg = 11.02 stone |
| grams → oz | oz = g ÷ 28.3495 | 100 g = 3.527 oz |
| oz → grams | g = oz × 28.3495 | 16 oz = 453.59 g |
| tonnes → lbs | lbs = t × 2204.62 | 1 t = 2,204.6 lbs |
🌡️ Temperature Conversions
| From → To | Formula | Example |
|---|---|---|
| Celsius → Fahrenheit | °F = (°C × 9/5) + 32 | 100°C = 212°F |
| Fahrenheit → Celsius | °C = (°F − 32) × 5/9 | 98.6°F = 37°C |
| Celsius → Kelvin | K = °C + 273.15 | 0°C = 273.15 K |
| Kelvin → Celsius | °C = K − 273.15 | 373.15 K = 100°C |
| Fahrenheit → Kelvin | K = (°F − 32) × 5/9 + 273.15 | 32°F = 273.15 K |
| Kelvin → Fahrenheit | °F = (K − 273.15) × 9/5 + 32 | 0 K = −459.67°F |
🧪 Volume Conversions
| From → To | Formula | Example |
|---|---|---|
| liters → US gallons | gal = L × 0.264172 | 10 L = 2.642 gal |
| US gallons → liters | L = gal × 3.78541 | 5 gal = 18.93 L |
| liters → fl oz | fl oz = L × 33.814 | 1 L = 33.814 fl oz |
| mL → cups | cups = mL ÷ 236.588 | 473 mL = 2 cups |
| cubic meters → liters | L = m³ × 1000 | 1 m³ = 1,000 L |
💡 Pro tip: For rough mental conversions — multiply km by 0.6 to get approximate miles (exact is 0.621371). Multiply lbs by 0.45 to get approximate kg (exact is 0.453592). Double Celsius and add 30 for a rough Fahrenheit estimate (exact: × 9/5 + 32).
How to Convert Units — Step-by-Step Method
The systematic approach to any unit conversion, including dimensional analysis (factor-label method)
Method
Dimensional Analysis: The Foolproof Conversion Technique
Dimensional analysis (also called the factor-label method or unit-factor method) is a systematic approach that uses conversion factors as fractions, ensuring units cancel out correctly. It is taught in every physics and chemistry course because it prevents errors.
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1Write your starting value with its unit
Always start by writing the number and unit clearly together: "60 miles/hour"
Starting value: 60 mi/hr -
2Find the conversion factor as a fraction
Write the conversion so the unit you want to eliminate is on the bottom (denominator): 1.60934 km / 1 mi
Conversion factor: (1.60934 km / 1 mi) -
3Multiply — units cancel diagonally
The "mi" in numerator and denominator cancel, leaving km/hr:
60 mi/hr × (1.60934 km / 1 mi) = 96.56 km/hr -
4Check units — they should match your target
The remaining unit after cancellation should be exactly what you want. If not, you inverted the conversion factor.
Result: 96.56 km/hr ✓ (km/hr was the target) -
5Verify with a reverse conversion
Convert your result back to the original unit to confirm. 96.56 km/hr ÷ 1.60934 = 60 mi/hr ✓
96.56 km/hr ÷ 1.60934 = 60.00 mi/hr ✓ Confirmed!
🔑 Multi-step conversions: To convert inches to kilometers, chain two conversions: in → cm → m → km. Each step cancels one unit and introduces the next. The calculator on this page handles all these steps automatically and shows them one by one.
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Unit Systems by Category — In-Depth Reference
Detailed context, history, and key facts for each measurement category
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Length & Distance
The meter is the SI base unit, defined as the distance light travels in 1/299,792,458 of a second. One inch = 2.54 cm exactly (defined 1959). A nautical mile = 1,852 m, based on Earth's circumference. A light year = 9.461 × 10¹⁵ m — the distance light travels in one year. The human hair is about 70 micrometers wide.
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Weight & Mass
The kilogram is the SI base unit for mass, redefined in 2019 in terms of Planck's constant. A troy ounce (used for precious metals) = 31.103 g, while a regular ounce = 28.3495 g. Note: weight and mass are technically different — mass is intrinsic (kg), weight is a force (N = kg × m/s²) and varies with gravity.
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Temperature
Celsius (0°C = freezing, 100°C = boiling at sea level) is used globally in science and most countries. Fahrenheit (32°F = freezing, 212°F = boiling) is primarily used in the USA. Kelvin is the SI unit, starting at absolute zero (−273.15°C). There is no negative temperature in Kelvin. Rankine = Kelvin × 9/5, used in thermodynamic engineering.
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Volume & Capacity
1 liter = 1 cubic decimeter = 1000 cm³. The US gallon (3.785 L) is smaller than the UK imperial gallon (4.546 L). 1 US fluid ounce = 29.57 mL; 1 UK fluid ounce = 28.41 mL. A US cup = 236.6 mL; a metric cup (AU/CA) = 250 mL. Always check whether a recipe uses US or metric cups!
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Area
Area units are the square of linear units (1 m² = 10,000 cm²). An acre = 4,047 m² (originally the area one man could plow in one day). A hectare = 10,000 m² = 2.471 acres. One square mile = 640 acres = 2.59 km². Used in land measurement, agriculture, and real estate.
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Speed & Velocity
Speed of sound at sea level ≈ 340 m/s = 1,225 km/h = 761 mph (Mach 1). Speed of light = 299,792,458 m/s ≈ 3×10⁸ m/s. A knot (nautical) = 1.852 km/h exactly. The fastest land animal (cheetah) runs ≈ 112 km/h. Formula 1 cars reach ≈ 370 km/h.
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Digital Storage
1 byte = 8 bits. In SI (decimal): 1 KB = 1,000 bytes, 1 MB = 10⁶ bytes, 1 GB = 10⁹ bytes. In binary (IEC): 1 KiB = 1,024 bytes, 1 MiB = 1,048,576 bytes, 1 GiB = 1,073,741,824 bytes. Hard drive manufacturers use SI (decimal) which is why a 500 GB drive shows as ~465 GiB in Windows.
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Energy
The joule (J) is the SI unit of energy (1 J = 1 kg·m²/s²). A food calorie (Cal, kcal) = 4,184 J. A Watt-hour = 3,600 J. A BTU (British Thermal Unit) = 1,055 J. 1 kWh = 3.6 MJ = 3,412 BTU. Electricity bills are in kWh. Nutritional energy on food labels is in kcal (marketed as "Calories").
Common Unit Conversion Mistakes — What to Avoid
The most frequent errors people make when converting units, with explanations of the correct approach
Pitfalls
Avoid These 8 Common Conversion Errors
❌ US vs UK Gallons
The US gallon (3.785 L) and UK imperial gallon (4.546 L) are different. A US recipe calling for "1 gallon of milk" means ~3.8 L, not 4.5 L. Always specify which gallon you mean.
❌ Confusing Mass & Weight
In everyday speech, "I weigh 70 kg" — but technically 70 kg is your mass. Weight is a force: 70 kg × 9.81 m/s² = 686.7 N. On the Moon, your mass is still 70 kg but you weigh about 115 N.
❌ Food Calories vs Physics Calories
A food "Calorie" (capital C) = 1 kilocalorie = 4,184 joules. A physics calorie (lowercase c) = 4.184 joules. That 300-Calorie snack contains 300,000 small calories. They differ by a factor of 1,000.
❌ GB vs GiB (Hard Drive Sizes)
A "500 GB" hard drive uses decimal GB (10⁹ bytes). Windows shows GiB (2³⁰ bytes = 1,073,741,824). 500 × 10⁹ ÷ 1,073,741,824 ≈ 465.7 GiB — so the drive appears smaller than advertised.
❌ Squaring/Cubing Conversion Factors
1 m = 100 cm, but 1 m² ≠ 100 cm². You must square: 1 m² = 100² = 10,000 cm². Similarly, 1 m³ = 100³ = 1,000,000 cm³. Forgetting to square/cube the factor is a very common error.
❌ Temperature: Not Using the Offset
Celsius and Fahrenheit do not start at the same zero point — they have an offset. You cannot just multiply: 20°C × 9/5 ≠ 20°C in Fahrenheit. You must use: °F = (°C × 9/5) + 32. The +32 is critical.
❌ Miles vs Nautical Miles
A statute mile = 1,609 m. A nautical mile = 1,852 m. They are not the same. Ships and aircraft use nautical miles and knots (1 knot = 1 nautical mile/hour). Always confirm which "miles" is meant in navigation contexts.
❌ troy oz vs avoirdupois oz
Gold, silver, and gemstones are weighed in troy ounces (31.103 g). Everyday items use avoirdupois ounces (28.3495 g). A "16-oz" food item uses avoirdupois; "1 oz" of gold is a troy ounce. They differ by ~10%.
Unit Conversion FAQs
Answers to the most frequently asked questions about unit conversions
How do I convert kilometers to miles?
To convert kilometers to miles, multiply the km value by 0.621371. For example, 10 km × 0.621371 = 6.214 miles. To convert miles to kilometers, multiply by 1.60934. Quick mental trick: multiply km by 0.6 for a rough estimate (actual is 0.621). For a marathon (42.195 km): 42.195 × 0.621371 = 26.219 miles.
How do I convert Celsius to Fahrenheit?
Use the formula: °F = (°C × 9/5) + 32. Example: 25°C = (25 × 1.8) + 32 = 45 + 32 = 77°F. To go the other way: °C = (°F − 32) × 5/9. Example: 98.6°F = (98.6 − 32) × 5/9 = 66.6 × 0.5556 = 37°C (normal body temperature).
Key benchmarks to memorise: 0°C = 32°F (freezing), 20°C = 68°F (room temp), 37°C = 98.6°F (body), 100°C = 212°F (boiling).
Key benchmarks to memorise: 0°C = 32°F (freezing), 20°C = 68°F (room temp), 37°C = 98.6°F (body), 100°C = 212°F (boiling).
What is the difference between metric and imperial units?
The metric system (SI) uses decimal scaling — every step is a power of 10 (mm → cm → dm → m → km). Used by 195+ countries and all scientific disciplines. Founded in France in 1795.
The imperial/US customary system uses arbitrary historical factors: 12 inches = 1 foot, 3 feet = 1 yard, 1,760 yards = 1 mile, 16 oz = 1 lb, 14 lbs = 1 stone, 8 pints = 1 gallon. Only the USA, Liberia, and Myanmar have not officially adopted SI as their primary system.
The imperial/US customary system uses arbitrary historical factors: 12 inches = 1 foot, 3 feet = 1 yard, 1,760 yards = 1 mile, 16 oz = 1 lb, 14 lbs = 1 stone, 8 pints = 1 gallon. Only the USA, Liberia, and Myanmar have not officially adopted SI as their primary system.
How many liters are in a gallon?
This depends on which gallon:
• US liquid gallon = 3.78541 liters (most common in everyday use)
• US dry gallon = 4.40488 liters (used for dry agricultural commodities)
• UK imperial gallon = 4.54609 liters
To convert US gallons → liters: multiply by 3.78541. To convert liters → US gallons: multiply by 0.264172. Example: 20 liters = 20 × 0.264172 = 5.28 US gallons.
• US liquid gallon = 3.78541 liters (most common in everyday use)
• US dry gallon = 4.40488 liters (used for dry agricultural commodities)
• UK imperial gallon = 4.54609 liters
To convert US gallons → liters: multiply by 3.78541. To convert liters → US gallons: multiply by 0.264172. Example: 20 liters = 20 × 0.264172 = 5.28 US gallons.
How do I convert kg to pounds (lbs)?
To convert kilograms to pounds: lbs = kg × 2.20462. Example: 70 kg × 2.20462 = 154.32 lbs.
To convert pounds to kilograms: kg = lbs ÷ 2.20462 (or multiply by 0.453592). Example: 154 lbs ÷ 2.20462 = 69.85 kg.
Quick mental rule: multiply kg by 2.2 for an approximate pounds value. For stone (UK): 1 stone = 14 lbs = 6.35029 kg.
To convert pounds to kilograms: kg = lbs ÷ 2.20462 (or multiply by 0.453592). Example: 154 lbs ÷ 2.20462 = 69.85 kg.
Quick mental rule: multiply kg by 2.2 for an approximate pounds value. For stone (UK): 1 stone = 14 lbs = 6.35029 kg.
What is absolute zero and why does it matter in temperature conversion?
Absolute zero is the lowest theoretically possible temperature, where all molecular motion stops. It equals −273.15°C = −459.67°F = 0 K (Kelvin).
The Kelvin scale starts at absolute zero and uses the same degree size as Celsius — so 0°C = 273.15 K and 100°C = 373.15 K. Kelvin has no negative values. It is critical in thermodynamics, astrophysics, and engineering calculations like the ideal gas law (PV = nRT), where T must be in Kelvin.
The Kelvin scale starts at absolute zero and uses the same degree size as Celsius — so 0°C = 273.15 K and 100°C = 373.15 K. Kelvin has no negative values. It is critical in thermodynamics, astrophysics, and engineering calculations like the ideal gas law (PV = nRT), where T must be in Kelvin.
How do I convert square feet to square meters?
To convert square feet to square meters: m² = sq ft × 0.092903. Example: 1,000 sq ft × 0.092903 = 92.9 m².
To convert square meters to square feet: sq ft = m² × 10.7639. Example: 100 m² = 1,076.4 sq ft.
Important: note that 1 foot = 0.3048 m, so 1 sq ft = 0.3048² = 0.09290304 m² (you must SQUARE the linear conversion factor for area).
To convert square meters to square feet: sq ft = m² × 10.7639. Example: 100 m² = 1,076.4 sq ft.
Important: note that 1 foot = 0.3048 m, so 1 sq ft = 0.3048² = 0.09290304 m² (you must SQUARE the linear conversion factor for area).
What is the difference between a bit and a byte?
A bit (b) is the smallest unit of digital information — either a 0 or a 1. A byte (B) = 8 bits. Internet speeds are measured in bits per second (Mbps), while file sizes are in bytes (MB, GB).
This causes confusion: a "100 Mbps" internet connection can download at 100 ÷ 8 = 12.5 MB/s. The prefixes also differ: 1 KB = 1,000 bytes (SI) vs 1 KiB = 1,024 bytes (binary/IEC). Modern OS displays use binary units (GiB) while storage manufacturers use SI units (GB), making drives appear smaller than labelled.
This causes confusion: a "100 Mbps" internet connection can download at 100 ÷ 8 = 12.5 MB/s. The prefixes also differ: 1 KB = 1,000 bytes (SI) vs 1 KiB = 1,024 bytes (binary/IEC). Modern OS displays use binary units (GiB) while storage manufacturers use SI units (GB), making drives appear smaller than labelled.