Two measurement systems dominate the world: the metric system (used by roughly 95% of the global population) and the imperial/US customary system (used primarily in the United States for everyday life). Whether you are reading a European recipe, shopping for international products, or working with global colleagues, converting between these systems is an essential skill. This guide covers every major conversion with quick-reference tables and mental math shortcuts.
| Imperial | Metric | Conversion Factor |
|---|---|---|
| 1 inch | 2.54 cm | Exact definition |
| 1 foot (12 in) | 30.48 cm | Multiply feet by 30.48 |
| 1 yard (3 ft) | 0.9144 m | Yards ≈ meters (close to 1:1) |
| 1 mile (5,280 ft) | 1.609 km | Multiply miles by 1.6 |
| 1 cm | 0.394 in | Divide cm by 2.54 |
| 1 meter | 3.281 ft | Meters are slightly longer than yards |
| 1 kilometer | 0.621 mi | Multiply km by 0.6 |
Mental math shortcuts: Kilometers to miles: multiply by 0.6 (or use Fibonacci — consecutive Fibonacci numbers approximate the km-to-mile ratio: 8 km ≈ 5 mi, 13 km ≈ 8 mi). Inches to centimeters: multiply by 2.5 for a quick estimate. Meters to feet: multiply by 3 and add 10%.
| Imperial | Metric | Quick Estimate |
|---|---|---|
| 1 ounce (oz) | 28.35 g | ~28 grams |
| 1 pound (lb) | 453.6 g | ~450 grams or ~0.45 kg |
| 1 stone (14 lb) | 6.35 kg | Used in UK for body weight |
| 1 US ton (2,000 lb) | 907.2 kg | ~0.9 metric tonnes |
| 1 gram | 0.035 oz | A paperclip weighs ~1g |
| 1 kilogram | 2.205 lb | Multiply kg by 2.2 |
The Weight Converter handles all of these conversions instantly, including less common units like grains, carats, and troy ounces.
| US Customary | Metric | Notes |
|---|---|---|
| 1 teaspoon | 4.93 mL | ~5 mL (medical dosing uses 5 mL) |
| 1 tablespoon | 14.79 mL | ~15 mL |
| 1 fluid ounce | 29.57 mL | ~30 mL |
| 1 cup | 236.6 mL | ~240 mL (metric cup = 250 mL) |
| 1 pint | 473.2 mL | US pint ≠ imperial pint (568 mL) |
| 1 quart | 946.4 mL | ~1 liter (close but not exact) |
| 1 gallon | 3.785 L | US gallon ≠ imperial gallon (4.546 L) |
Temperature is the trickiest conversion because the scales have different zero points. Fahrenheit sets 32° as water’s freezing point and 212° as boiling. Celsius uses 0° and 100° respectively.
Formulas: °F to °C: (°F − 32) × 5/9. °C to °F: (°C × 9/5) + 32. The Temperature Converter handles this instantly, including Kelvin and Rankine scales.
| Description | °F | °C |
|---|---|---|
| Water freezes | 32 | 0 |
| Cool room | 65 | 18 |
| Comfortable room | 72 | 22 |
| Body temperature | 98.6 | 37 |
| Hot summer day | 95 | 35 |
| Water boils | 212 | 100 |
| Oven baking | 350 | 177 |
The metric system was created during the French Revolution in the 1790s with a radical idea: base all measurements on powers of ten and on physical constants. One meter was defined as one ten-millionth of the distance from the North Pole to the equator. One kilogram was the mass of one liter (1,000 cubic centimeters) of water. One liter was 1,000 milliliters. This coherent, decimal structure made the metric system far easier to learn, teach, and calculate with than the patchwork of imperial units.
By the late 20th century, every major country had adopted metric as its primary system. The United States remains the notable exception for everyday use, though American science, medicine, military, and international trade all use metric extensively. NASA famously lost the $125 million Mars Climate Orbiter in 1999 because one team used pound-force seconds while another used newton-seconds.
Convert any measurement instantly. Use the free Length Converter, Weight Converter, and Temperature Converter — no signup required.
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