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Unit Conversion Reference: The Complete Guide to Length, Weight, Temperature, and Beyond
An international recipe calls for 200 grams of flour. Your kitchen scale reads in ounces. A European colleague sends dimensions in centimeters. Your lumber supplier quotes in feet and inches. A weather app shows 38 degrees and you are not sure if you need a jacket or sunscreen. Unit conversion is one of those daily frictions that seems minor until you realize how often it comes up.
This reference covers the conversions people actually need, with exact factors, mental math shortcuts, and the context behind each system. Bookmark it and stop Googling "kg to lbs" for the hundredth time. Or use the Unit Converter for instant results.
Length Conversions
Metric to Imperial
| From | To | Multiply By | Quick Mental Trick |
|---|---|---|---|
| Centimeters | Inches | 0.3937 | Divide cm by 2.54, or roughly multiply by 0.4 |
| Meters | Feet | 3.2808 | Multiply by 3.3 (1 meter is about 3 feet 3 inches) |
| Kilometers | Miles | 0.6214 | Multiply by 0.6 (or use Fibonacci: 5 km ~ 3 mi, 8 km ~ 5 mi) |
| Inches | Centimeters | 2.54 | Multiply by 2.5 |
| Feet | Meters | 0.3048 | Multiply by 0.3 |
| Miles | Kilometers | 1.6093 | Multiply by 1.6 |
The Inches to CM Converter and KM to Miles Converter handle precise conversions including fractional values.
The Fibonacci trick for km/miles: The Fibonacci sequence (1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89...) closely approximates the km-to-miles ratio. Each number in miles is approximately the next Fibonacci number in kilometers. So 5 miles is roughly 8 km, 8 miles is about 13 km, 13 miles is near 21 km. This works because the golden ratio (1.618) is very close to the km-to-miles conversion factor (1.609).
Within the Metric System
| Prefix | Symbol | Factor |
|---|---|---|
| Kilo | k | 1,000 |
| Hecto | h | 100 |
| Deca | da | 10 |
| Base unit | m, g, L | 1 |
| Deci | d | 0.1 |
| Centi | c | 0.01 |
| Milli | m | 0.001 |
| Micro | μ | 0.000001 |
| Nano | n | 0.000000001 |
Weight and Mass Conversions
| From | To | Multiply By | Quick Trick |
|---|---|---|---|
| Kilograms | Pounds | 2.2046 | Double the kg and add 10% |
| Pounds | Kilograms | 0.4536 | Halve the lbs and subtract 10% |
| Grams | Ounces | 0.0353 | Divide grams by 28 |
| Ounces | Grams | 28.3495 | Multiply oz by 28 |
| Stones | Kilograms | 6.3503 | Multiply stones by 6.35 |
| Stones | Pounds | 14 | Multiply stones by 14 |
The LBS to KG Converter handles both directions with precision.
The doubling trick: To convert 75 kg to pounds: double it (150), then add 10% of 150 (15). Result: 165 lbs. The exact answer is 165.35. Close enough for conversation, and you can do it in your head in three seconds.
Temperature Conversions
Temperature conversion trips people up because it involves both multiplication and addition, not just a simple factor.
Fahrenheit to Celsius: C = (F - 32) × 5/9
Celsius to Fahrenheit: F = (C × 9/5) + 32
The Fahrenheit to Celsius Converter handles this calculation instantly, but here are mental shortcuts for common ranges:
| Fahrenheit | Celsius | Context |
|---|---|---|
| 32 | 0 | Water freezes |
| 50 | 10 | Cool jacket weather |
| 68 | 20 | Room temperature |
| 77 | 25 | Comfortable warm day |
| 86 | 30 | Hot summer day |
| 98.6 | 37 | Body temperature |
| 100 | 37.8 | Fever territory / hot day |
| 212 | 100 | Water boils |
Quick estimation trick: For rough conversion from Celsius to Fahrenheit, double the Celsius value and add 30. Example: 25C becomes 50 + 30 = 80F. The exact answer is 77F. Not perfect, but it tells you immediately whether to pack shorts or a coat.
Volume Conversions
| From | To | Factor |
|---|---|---|
| Liters | US Gallons | 0.2642 |
| US Gallons | Liters | 3.7854 |
| Milliliters | US Fluid Ounces | 0.0338 |
| US Fluid Ounces | Milliliters | 29.5735 |
| US Cups | Milliliters | 236.588 |
| Tablespoons | Milliliters | 14.787 |
| Teaspoons | Milliliters | 4.929 |
Cooking approximations: 1 cup is about 240 mL. 1 tablespoon is about 15 mL. 1 teaspoon is about 5 mL. For baking, these approximations are usually close enough. For chemistry, use exact values.
US vs. Imperial gallons: A US gallon is 3.785 liters. A UK (Imperial) gallon is 4.546 liters. They are different measures with the same name, which causes confusion in international contexts. Always clarify which gallon you mean.
Speed Conversions
| From | To | Factor |
|---|---|---|
| mph | km/h | 1.6093 |
| km/h | mph | 0.6214 |
| Knots | mph | 1.1508 |
| Knots | km/h | 1.852 |
| m/s | km/h | 3.6 |
| m/s | mph | 2.237 |
The same km/miles Fibonacci trick works for speed. 60 mph is roughly 97 km/h (100 km/h is a common speed limit in many countries, equivalent to about 62 mph). A highway speed limit of 120 km/h is about 75 mph.
Digital Storage Conversions
This one confuses even technical people because storage manufacturers and operating systems measure differently.
| Unit | Bytes (Decimal/SI) | Bytes (Binary/IEC) |
|---|---|---|
| Kilobyte / Kibibyte | 1 KB = 1,000 B | 1 KiB = 1,024 B |
| Megabyte / Mebibyte | 1 MB = 1,000,000 B | 1 MiB = 1,048,576 B |
| Gigabyte / Gibibyte | 1 GB = 10^9 B | 1 GiB = 2^30 B |
| Terabyte / Tebibyte | 1 TB = 10^12 B | 1 TiB = 2^40 B |
When a hard drive manufacturer advertises "1 TB," they mean 1,000,000,000,000 bytes (decimal). When your operating system reports the same drive, it may show about 931 GB because it is using binary measurement (1 TB / 1.0737 = 931 GiB). You did not lose 69 GB. They are just using different definitions of "giga."
Area and Land Conversions
| From | To | Factor |
|---|---|---|
| Square feet | Square meters | 0.0929 |
| Square meters | Square feet | 10.764 |
| Acres | Hectares | 0.4047 |
| Hectares | Acres | 2.4711 |
| Square miles | Square kilometers | 2.59 |
Quick reference: 1 acre is about 43,560 square feet, or roughly the size of a football field without the end zones (which are about 57,600 sq ft including end zones). 1 hectare is 10,000 square meters, or about 2.47 acres.
Why Two Systems Exist
The metric system was developed during the French Revolution (1790s) as a rational, base-10 alternative to the chaotic mix of local measurement standards across Europe. It spread globally because it is genuinely easier to work with: everything scales by powers of 10.
The US customary system descends from English units that were standardized over centuries. The US nearly adopted metric in the 1970s (the Metric Conversion Act of 1975), but the conversion was voluntary and never completed. Today, the US, Myanmar, and Liberia are the only countries that have not officially adopted metric as their primary system, though the US uses metric extensively in science, medicine, and the military.
For anyone working across borders, in international business, science, engineering, or even cooking with recipes from different countries, fluency in both systems is not optional. The Unit Converter covers all the conversions in this guide and dozens more, running entirely in your browser with no data transmitted anywhere.