Unit Conversion: The Complete Reference Guide
Master unit conversions across metric, imperial, and specialized measurement systems for everyday calculations and professional applications.
Measurement Systems Overview
The world uses several measurement systems, with the metric system (SI) being the international standard and the imperial/US customary system still prevalent in the United States and a few other countries. Understanding both systems and how to convert between them is essential for international communication, cooking, travel, and countless other applications.
The Metric System (SI)
The International System of Units (SI) is built on powers of 10, making conversions straightforward. It uses standardized prefixes that apply to all units:
| Prefix | Symbol | Factor | Example |
|---|---|---|---|
| Tera | T | 10¹² | 1 TB = 1,000,000,000,000 bytes |
| Giga | G | 10⁹ | 1 GB = 1,000,000,000 bytes |
| Mega | M | 10⁶ | 1 MHz = 1,000,000 Hz |
| Kilo | k | 10³ | 1 km = 1,000 m |
| Hecto | h | 10² | 1 hL = 100 L |
| Deca | da | 10¹ | 1 dam = 10 m |
| (base) | - | 10⁰ | meter, gram, liter |
| Deci | d | 10⁻¹ | 1 dL = 0.1 L |
| Centi | c | 10⁻² | 1 cm = 0.01 m |
| Milli | m | 10⁻³ | 1 mg = 0.001 g |
| Micro | μ | 10⁻⁶ | 1 μm = 0.000001 m |
| Nano | n | 10⁻⁹ | 1 nm = 0.000000001 m |
Imperial and US Customary Systems
The imperial and US customary systems share many units but differ in some measurements, particularly for volume. These systems use non-decimal relationships that often require memorization.
Key Differences: Imperial vs US Customary
- 1 US gallon = 3.785 liters
- 1 Imperial gallon = 4.546 liters
- US fluid ounces and pints also differ from Imperial
- Dry measures (bushels, pecks) differ slightly
Length and Distance
Metric Length Units
| Unit | Symbol | Meters | Common Use |
|---|---|---|---|
| Kilometer | km | 1,000 | Road distances, geography |
| Meter | m | 1 | General measurement |
| Centimeter | cm | 0.01 | Body measurements, clothing |
| Millimeter | mm | 0.001 | Engineering, small objects |
Imperial Length Units
| Unit | Inches | Feet | Metric Equivalent |
|---|---|---|---|
| Inch | 1 | 1/12 | 2.54 cm |
| Foot | 12 | 1 | 30.48 cm |
| Yard | 36 | 3 | 0.9144 m |
| Mile | 63,360 | 5,280 | 1.609 km |
Key Length Conversion Formulas
inches → cm: multiply by 2.54
cm → inches: divide by 2.54
feet → meters: multiply by 0.3048
meters → feet: multiply by 3.281
miles → km: multiply by 1.609
km → miles: multiply by 0.621
Quick Reference Approximations
- 1 inch ≈ 2.5 cm (exact: 2.54)
- 1 foot ≈ 30 cm (exact: 30.48)
- 1 meter ≈ 39 inches or 3 feet 3 inches
- 1 mile ≈ 1.6 km
- 1 km ≈ 0.6 miles
- 5 km ≈ 3.1 miles (common race distance)
Weight and Mass
Technically, mass (measured in kilograms) and weight (a force, measured in newtons) are different. However, in everyday use, we typically use "weight" to mean mass.
Metric Weight Units
| Unit | Symbol | Grams | Common Use |
|---|---|---|---|
| Metric Ton | t | 1,000,000 | Heavy cargo, vehicles |
| Kilogram | kg | 1,000 | Body weight, groceries |
| Gram | g | 1 | Food, small items |
| Milligram | mg | 0.001 | Medication, vitamins |
Imperial Weight Units
| Unit | Ounces | Pounds | Metric Equivalent |
|---|---|---|---|
| Ounce (oz) | 1 | 1/16 | 28.35 g |
| Pound (lb) | 16 | 1 | 453.6 g |
| Stone (UK) | 224 | 14 | 6.35 kg |
| Short Ton (US) | 32,000 | 2,000 | 907.2 kg |
Key Weight Conversion Formulas
pounds → kg: multiply by 0.4536
kg → pounds: multiply by 2.205
ounces → grams: multiply by 28.35
grams → ounces: multiply by 0.0353
Cooking Weight Conversions
| US Measurement | Metric |
|---|---|
| 1/4 oz | 7 g |
| 1/2 oz | 14 g |
| 1 oz | 28 g |
| 4 oz (1/4 lb) | 113 g |
| 8 oz (1/2 lb) | 227 g |
| 16 oz (1 lb) | 454 g |
Volume and Capacity
Volume measurements are particularly tricky because the US, Imperial, and metric systems all have different standards. Always verify which system a recipe or specification uses.
Metric Volume Units
| Unit | Symbol | Liters | Cubic cm (mL) |
|---|---|---|---|
| Kiloliter | kL | 1,000 | 1,000,000 |
| Liter | L | 1 | 1,000 |
| Deciliter | dL | 0.1 | 100 |
| Milliliter | mL | 0.001 | 1 |
US Volume Units
| Unit | Fluid Ounces | Cups | Milliliters |
|---|---|---|---|
| Teaspoon (tsp) | 1/6 | 1/48 | 4.93 |
| Tablespoon (tbsp) | 1/2 | 1/16 | 14.79 |
| Fluid Ounce (fl oz) | 1 | 1/8 | 29.57 |
| Cup | 8 | 1 | 236.6 |
| Pint | 16 | 2 | 473.2 |
| Quart | 32 | 4 | 946.4 |
| Gallon | 128 | 16 | 3,785 |
Cooking Volume Quick Reference
Essential Kitchen Conversions
- 3 teaspoons = 1 tablespoon
- 2 tablespoons = 1 fluid ounce
- 4 tablespoons = 1/4 cup
- 1 cup = 240 mL (approximate)
- 1 stick butter = 1/2 cup = 8 tablespoons = 113g
Temperature
Temperature conversion is unique because the scales don't share a common zero point. Fahrenheit and Celsius use offset conversions, while Kelvin is an absolute scale starting at absolute zero.
Temperature 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
Fahrenheit to Kelvin:
K = (°F - 32) × 5/9 + 273.15
Common Temperature Reference Points
| Description | °C | °F | K |
|---|---|---|---|
| Absolute Zero | -273.15 | -459.67 | 0 |
| Water Freezes | 0 | 32 | 273.15 |
| Room Temperature | 20-22 | 68-72 | 293-295 |
| Body Temperature | 37 | 98.6 | 310.15 |
| Water Boils | 100 | 212 | 373.15 |
Cooking Temperature Conversions
| Oven Setting | °F | °C | Gas Mark |
|---|---|---|---|
| Very Low | 250 | 120 | 1/2 |
| Low | 300 | 150 | 2 |
| Moderate | 350 | 180 | 4 |
| Moderately Hot | 375 | 190 | 5 |
| Hot | 400 | 200 | 6 |
| Very Hot | 450 | 230 | 8 |
| Extremely Hot | 500 | 260 | 10 |
Area Measurements
Area units are squared versions of length units. When converting area, remember to square the conversion factor for length.
Common Area Conversions
| Unit | Square Meters | Square Feet |
|---|---|---|
| 1 sq inch | 0.000645 | 0.00694 |
| 1 sq foot | 0.0929 | 1 |
| 1 sq meter | 1 | 10.764 |
| 1 sq yard | 0.836 | 9 |
| 1 acre | 4,047 | 43,560 |
| 1 hectare | 10,000 | 107,639 |
Area Conversion Tip
Since 1 foot = 0.3048 meters, 1 square foot = 0.3048² = 0.0929 square meters. Always square the linear conversion factor!
Speed and Velocity
Common Speed Units
| Unit | m/s | km/h | mph |
|---|---|---|---|
| 1 m/s | 1 | 3.6 | 2.237 |
| 1 km/h | 0.278 | 1 | 0.621 |
| 1 mph | 0.447 | 1.609 | 1 |
| 1 knot | 0.514 | 1.852 | 1.151 |
Quick Speed Conversions
- To convert mph to km/h: multiply by 1.6
- To convert km/h to mph: multiply by 0.6
- 100 km/h ≈ 62 mph
- 60 mph ≈ 97 km/h
- 1 knot = 1 nautical mile per hour = 1.852 km/h
Digital Storage Units
Digital storage has two competing standards: decimal (SI) and binary (IEC). This causes confusion when a "500 GB" drive shows as "465 GB" in your operating system.
Decimal (SI) vs Binary (IEC)
| SI (Decimal) | Bytes | IEC (Binary) | Bytes |
|---|---|---|---|
| 1 KB (kilobyte) | 1,000 | 1 KiB (kibibyte) | 1,024 |
| 1 MB (megabyte) | 1,000,000 | 1 MiB (mebibyte) | 1,048,576 |
| 1 GB (gigabyte) | 1,000,000,000 | 1 GiB (gibibyte) | 1,073,741,824 |
| 1 TB (terabyte) | 1,000,000,000,000 | 1 TiB (tebibyte) | 1,099,511,627,776 |
Why Drives Show Less Space
Manufacturers use SI units (1 GB = 1 billion bytes), but operating systems often use binary units (1 GiB = 1.074 billion bytes). A "1 TB" drive has 1,000,000,000,000 bytes, which equals ~931 GiB as displayed by your OS.
Data Transfer Rates
- Bits vs Bytes: 1 byte = 8 bits
- Internet speeds: Usually measured in Mbps (megabits per second)
- File sizes: Usually measured in MB (megabytes)
- Conversion: Divide Mbps by 8 to get MB/s
- Example: 100 Mbps internet = ~12.5 MB/s download speed
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