Complete Unit Conversion Handbook

Use the guide for your specific conversion need. Each category guide covers that unit family in full depth — definitions, formulas, examples, common mistakes, and 10+ FAQs.

Length: meters, feet, inches, miles, kilometers, yards, nautical miles — see the Length Conversion Guide (/articles/length-conversion-guide). Weight: kilograms, pounds, grams, ounces, stones, metric tons — see the Weight Conversion Guide (/articles/weight-conversion-guide). Volume: liters, gallons, cups, fluid ounces, milliliters, tablespoons — see the Volume Conversion Guide (/articles/volume-conversion-guide). Temperature: Celsius, Fahrenheit, Kelvin — see the Temperature Conversion Guide (/articles/temperature-conversion-guide). Time: seconds, minutes, hours, days, weeks, months, years — see the Time Conversion Guide (/articles/time-conversion-guide). Area: square feet, square meters, acres, hectares, square miles — see the Area Conversion Guide (/articles/area-conversion-guide). Speed: mph, km/h, knots, m/s — see the Speed Conversion Guide (/articles/speed-conversion-guide). Data Storage: bits, bytes, KB, MB, GB, TB and binary units — see the Data Storage Conversion Guide (/articles/data-storage-conversion-guide).

Length is measured in two dominant systems: the metric system (millimeters, centimeters, meters, kilometers) used by most of the world, and the US customary system (inches, feet, yards, miles) used primarily in the United States, Liberia, and Myanmar. The two systems are bridged by a single exact definition: 1 inch = 2.54 centimeters, established by international agreement in 1959.

Common uses by unit — millimeters (mm): precision manufacturing, rainfall totals, bolt diameters. Centimeters (cm): body measurements, clothing dimensions. Meters (m): room dimensions, athletic distances, building heights. Kilometers (km): road distances, geographic scales. Inches (in): screen sizes, hardware, US body measurements. Feet (ft): US room heights, altitude. Yards (yd): US football fields, fabric. Miles (mi): US road signs, marathon distances.

Key conversion relationships: 1 inch = 2.54 cm | 1 foot = 30.48 cm = 0.3048 m | 1 yard = 0.9144 m | 1 mile = 1.60934 km | 1 meter = 3.28084 ft = 39.3701 in | 1 kilometer = 0.62137 miles. For quick mental math: multiply miles by 1.6 to get kilometers, multiply kilometers by 0.6 to get miles.

Common mistakes: confusing meters and yards (a yard is about 8% shorter than a meter); assuming miles and kilometers are close (a mile is 61% longer than a kilometer); forgetting that 1 foot = 12 inches, not 10.

Weight (technically mass) is measured in the metric system using milligrams, grams, kilograms, and metric tons, and in the US customary system using ounces, pounds, and tons. The metric system is decimal-based and straightforward: each unit is a factor of 1,000 from the next. The US system uses non-decimal ratios: 16 ounces per pound, 2,000 pounds per short ton.

Common uses by unit — milligrams (mg): medications, nutritional micro-measurements. Grams (g): food ingredients, postal weights, jewelry. Kilograms (kg): body weight outside the US, luggage, produce. Metric ton (t): industrial freight, large commodities. Ounces (oz): US cooking and nutrition labels. Pounds (lb): US body weight, luggage limits, grocery items. Short ton (US ton): US construction, vehicle loads.

Key conversion relationships: 1 pound = 453.592 g = 0.453592 kg | 1 kilogram = 2.20462 lb | 1 ounce = 28.3495 g | 1 gram = 0.03527 oz | 1 stone (UK) = 14 lb = 6.3503 kg | 1 metric ton = 1,000 kg = 2,204.62 lb | 1 US ton = 2,000 lb = 907.185 kg.

Practical anchors: 100 lb ≈ 45.4 kg; 70 kg ≈ 154 lb; 23 kg = 50.7 lb (standard international airline baggage limit); 1 cup of water weighs approximately 236 g (8.3 oz). Common mistake: confusing metric tons (1,000 kg) with US short tons (≈ 907 kg) — they differ by about 10%.

Volume measures the three-dimensional space an amount of liquid or solid occupies. The metric system uses liters (L) and milliliters (mL) for everyday volumes. The US customary system uses fluid ounces, cups, pints, quarts, and gallons. Additionally, cooking-specific units — teaspoons, tablespoons — appear in both systems but with different exact definitions.

Common uses by unit — milliliters (mL): beverages, medicine doses, lab measurements. Liters (L): fuel, bottled water, large beverage containers. Fluid ounces (fl oz): US beverage cans (12 fl oz = 355 mL), US nutrition labels. Cups (US cup = 237 mL): US recipes. Pints, quarts: US dairy, produce. Gallons (gal): US fuel, paint, large containers. Teaspoons/tablespoons: cooking and baking.

Key conversion relationships: 1 US gallon = 3.78541 L = 128 fl oz = 16 cups | 1 liter = 0.26417 gal = 33.814 fl oz | 1 US fl oz = 29.5735 mL | 1 US cup = 236.588 mL = 8 fl oz | 1 pint = 473.176 mL = 2 cups | 1 quart = 946.353 mL = 4 cups | 1 tablespoon = 14.787 mL | 1 teaspoon = 4.929 mL.

Important distinction: US cups and metric cups differ. A US cup is 236.588 mL; an Australian metric cup is 250 mL; a UK cup is 284 mL. For international recipes, always verify which cup definition is intended. Also note that US fluid ounces and UK fluid ounces are different: 1 US fl oz = 29.57 mL, 1 UK fl oz = 28.41 mL.

Temperature is measured in three scales: Celsius (°C), Fahrenheit (°F), and Kelvin (K). Celsius is the global standard for everyday temperature — used in weather forecasts, cooking, and science in most countries. Fahrenheit is used for everyday weather in the United States and a few other places. Kelvin is the SI absolute temperature scale used in science and engineering, where 0 K represents absolute zero (−273.15 °C).

The conversion formulas: Celsius to Fahrenheit: °F = (°C × 9/5) + 32. Fahrenheit to Celsius: °C = (°F − 32) × 5/9. Celsius to Kelvin: K = °C + 273.15. Kelvin to Celsius: °C = K − 273.15. Fahrenheit to Kelvin: K = (°F + 459.67) × 5/9.

Reference points to memorize: Water freezes at 0°C / 32°F / 273.15 K. Water boils at 100°C / 212°F / 373.15 K. Normal body temperature is 37°C / 98.6°F. Room temperature is roughly 20–22°C / 68–72°F. A hot summer day is 35°C / 95°F. Standard oven baking temperature is 175°C / 350°F.

Common mistakes: forgetting to subtract 32 before multiplying when converting °F to °C (a frequent calculator error); confusing Kelvin with Celsius for absolute temperature calculations in science; assuming 0°C and 0°F are the same (they are not — 0°F ≈ −17.8°C).

Time is universally standardized: seconds, minutes, hours, days, weeks, months, and years are used globally. The metric system defines the second as the SI base unit, but everyday time-keeping is non-decimal. This means there is no shortcut multiplication factor for all time conversions — you need the specific ratios.

Exact conversion relationships: 1 minute = 60 seconds | 1 hour = 60 minutes = 3,600 seconds | 1 day = 24 hours = 86,400 seconds | 1 week = 7 days = 604,800 seconds | 1 Julian month (average) = 30.4375 days = 2,629,800 seconds | 1 Julian year = 365.25 days = 31,557,600 seconds.

Practical uses: seconds are used in sports timing, cooking timers, and network latency. Minutes and hours dominate scheduling and duration. Days are used in project planning. Weeks appear in pregnancy tracking and product development cycles. Months and years are used in finance, contracts, and age calculations.

Common mistakes: assuming all months have 30 days (February has 28 or 29; months have 28–31 days); forgetting leap years in annual calculations; confusing elapsed time (duration) with a point in time when using date calculators.

Area measures two-dimensional surface size. The metric system uses square meters (m²) as the base unit, with square centimeters, square kilometers, and hectares as practical sub- and super-units. The US system uses square inches, square feet, square yards, acres, and square miles. Agriculture and land use rely on acres (US/UK) and hectares (metric).

Key conversion relationships: 1 sq foot = 0.092903 m² | 1 sq meter = 10.7639 sq ft | 1 acre = 43,560 sq ft = 4,046.86 m² | 1 hectare = 10,000 m² = 2.47105 acres | 1 sq mile = 640 acres = 2.58999 km² | 1 sq kilometer = 0.386102 sq miles = 247.105 acres.

Common uses: square feet are used for US real estate (apartment sizes, house footprints). Square meters are used in most countries for the same. Acres and hectares measure land parcels and agricultural plots. Square miles and square kilometers measure geographic regions, countries, and national parks.

Speed measures distance traveled per unit of time. The most common everyday units are miles per hour (mph) used on US and UK roads, and kilometers per hour (km/h or kph) used on roads in metric countries. Aviation and maritime navigation use knots (nautical miles per hour). Science uses meters per second (m/s).

Key conversion relationships: 1 mph = 1.60934 km/h | 1 km/h = 0.62137 mph | 1 knot = 1.15078 mph = 1.852 km/h | 1 m/s = 3.6 km/h = 2.23694 mph | Speed of sound (at sea level, 20°C) ≈ 343 m/s ≈ 767 mph ≈ 1,235 km/h.

Common uses: mph for US highway speeds (65–75 mph = 105–121 km/h). km/h for road speeds in Europe, Canada, Australia (100–130 km/h = 62–81 mph). Knots for aviation and maritime contexts. m/s for physics and engineering calculations. Common mistake: a 100 km/h speed limit is not 100 mph — it is about 62 mph.

Digital data storage uses a hierarchy of units based on bytes. Two parallel systems exist: the decimal (SI) system where 1 kilobyte (KB) = 1,000 bytes, and the binary (IEC) system where 1 kibibyte (KiB) = 1,024 bytes. Storage manufacturers and operating systems have historically mixed these, causing confusion about drive sizes.

Decimal (SI) hierarchy: 1 KB = 1,000 B | 1 MB = 1,000 KB = 1,000,000 B | 1 GB = 1,000 MB | 1 TB = 1,000 GB | 1 PB = 1,000 TB. Binary (IEC) hierarchy: 1 KiB = 1,024 B | 1 MiB = 1,024 KiB | 1 GiB = 1,024 MiB | 1 TiB = 1,024 GiB.

Why this matters: a hard drive marketed as '1 TB' contains 1,000,000,000,000 bytes (decimal). Windows (historically) reports storage in binary units, making the same drive appear as ~931 GB. macOS (since 10.6) switched to decimal reporting. Cloud storage and internet speeds use decimal. RAM and CPU cache are binary. Streaming speeds are in megabits per second (Mbps), not megabytes (1 MB = 8 Mb).