How many centimeters are in an inch? How many liters are in an American gallon? Converting between units of measurement is one of the most common operations in daily life, from cooking to DIY, from traveling abroad to scientific study. In this complete guide, you will find conversion tables for all major quantities, the history of the metric system, and the SI prefixes that make navigating different scales easy.
Metric system vs. imperial system
Two main measurement systems coexist in the world: the International System (SI), based on the metric decimal system and used in most countries (including Italy), and the imperial system (or customary system), used primarily in the United States, the United Kingdom (partially), and a few other countries.
Fundamental differences
| Feature | Metric system (SI) | Imperial system |
|---|---|---|
| Base | Decimal (multiples of 10) | Variable fractions (12, 16, etc.) |
| Base length | Meter (m) | Foot, inch |
| Base weight | Kilogram (kg) | Pound |
| Base volume | Liter (l) | Gallon |
| Temperature | Celsius (°C) | Fahrenheit (°F) |
| Countries using it | Almost the entire world | USA, Liberia, Myanmar |
Length conversions
Length is probably the quantity that requires the most conversions in everyday life, especially when shopping online from American websites or following instructions in imperial format.
Length conversion table
| From | To | Conversion factor |
|---|---|---|
| 1 inch | centimeters | 2.54 cm |
| 1 foot | centimeters | 30.48 cm |
| 1 yard | meters | 0.9144 m |
| 1 mile | kilometers | 1.60934 km |
| 1 nautical mile | kilometers | 1.852 km |
| 1 meter | feet | 3.28084 ft |
| 1 kilometer | miles | 0.62137 mi |
Quick height conversions for people
In the USA, height is expressed in feet and inches (e.g., 5'10"). Here is a quick conversion table:
| Feet and inches | Centimeters |
|---|---|
| 5'0" | 152 cm |
| 5'4" | 163 cm |
| 5'6" | 168 cm |
| 5'8" | 173 cm |
| 5'10" | 178 cm |
| 6'0" | 183 cm |
| 6'2" | 188 cm |
| 6'4" | 193 cm |
Weight and mass conversions
The kilogram is the base unit of mass in the SI system. The imperial system uses ounces and pounds.
Weight conversion table
| From | To | Conversion factor |
|---|---|---|
| 1 ounce (oz) | grams | 28.3495 g |
| 1 pound (lb) | kilograms | 0.45359 kg |
| 1 stone | kilograms | 6.35029 kg |
| 1 metric ton (t) | kilograms | 1,000 kg |
| 1 kilogram | pounds | 2.20462 lb |
| 1 gram | ounces | 0.03527 oz |
| 1 quintal | kilograms | 100 kg |
Body weight: kg vs. pounds
To quickly convert body weight: multiply kg by 2.2 to get pounds, or divide pounds by 2.2 for kilograms. Example: 70 kg = 154 lb; 180 lb = 81.8 kg.
Volume conversions
Volume is a quantity that varies considerably between systems. Note: the American gallon is different from the British gallon!
Volume conversion table
| From | To | Conversion factor |
|---|---|---|
| 1 US gallon | liters | 3.78541 l |
| 1 UK gallon | liters | 4.54609 l |
| 1 US fluid ounce | milliliters | 29.5735 ml |
| 1 UK fluid ounce | milliliters | 28.4131 ml |
| 1 US pint | liters | 0.47318 l |
| 1 US cup | milliliters | 236.588 ml |
| 1 tablespoon | milliliters | 14.787 ml |
| 1 teaspoon | milliliters | 4.929 ml |
| 1 liter | US gallons | 0.26417 gal |
Kitchen measurements
American recipes use cups, tablespoons, and teaspoons. Here are the most useful equivalents for those following international recipes:
- 1 cup of flour = approximately 125 g
- 1 cup of sugar = approximately 200 g
- 1 cup of butter = approximately 225 g
- 1 cup of liquid = 236 ml (approximately 240 ml rounded)
- 1 stick of butter = 113 g = 1/2 cup
Temperature conversions
Temperature uses three main scales: Celsius (used in Italy and almost everywhere), Fahrenheit (USA), and Kelvin (science).
Conversion formulas
- Celsius to Fahrenheit: °F = (°C x 9/5) + 32
- Fahrenheit to Celsius: °C = (°F - 32) x 5/9
- Celsius to Kelvin: K = °C + 273.15
- Kelvin to Celsius: °C = K - 273.15
Reference temperatures
| Description | Celsius | Fahrenheit | Kelvin |
|---|---|---|---|
| Absolute zero | -273.15 | -459.67 | 0 |
| Water freezes | 0 | 32 | 273.15 |
| Body temperature | 37 | 98.6 | 310.15 |
| Water boils | 100 | 212 | 373.15 |
| Medium oven | 180 | 356 | 453.15 |
Quick trick: for an approximate Celsius to Fahrenheit conversion, double the Celsius degrees and add 30. Example: 20°C -> 40 + 30 = 70°F (the exact value is 68°F, so very close).
Area and surface conversions
| From | To | Conversion factor |
|---|---|---|
| 1 square meter (m²) | square feet | 10.7639 ft² |
| 1 hectare (ha) | square meters | 10,000 m² |
| 1 acre | square meters | 4,046.86 m² |
| 1 acre | hectares | 0.40469 ha |
| 1 square kilometer | hectares | 100 ha |
| 1 square foot | square meters | 0.09290 m² |
International System (SI) prefixes
SI prefixes allow expressing very large or very small quantities in a compact way. Knowing them makes any conversion within the metric system immediate.
SI prefix table
| Prefix | Symbol | Factor | Example |
|---|---|---|---|
| tera | T | 1012 | 1 TB = 1,000,000,000,000 bytes |
| giga | G | 109 | 1 GHz = 1,000,000,000 Hz |
| mega | M | 106 | 1 MW = 1,000,000 watts |
| kilo | k | 103 | 1 km = 1,000 meters |
| hecto | h | 102 | 1 hg = 100 grams |
| deca | da | 101 | 1 dam = 10 meters |
| (base) | — | 100 | meter, gram, liter |
| deci | d | 10-1 | 1 dl = 0.1 liters |
| centi | c | 10-2 | 1 cm = 0.01 meters |
| milli | m | 10-3 | 1 mm = 0.001 meters |
| micro | μ | 10-6 | 1 μm = 0.000001 meters |
| nano | n | 10-9 | 1 nm = 0.000000001 meters |
In Italy, the term "etto" (hectogram, 100 g) is commonly used for weight, especially at the deli counter and butcher shop. It is a peculiarity of everyday Italian language.
A brief history of the metric system
The metric decimal system was introduced in France during the French Revolution in 1795. The idea was to create a universal, rational system based on natural constants. The meter was originally defined as one ten-millionth of the distance from the North Pole to the Equator along the Paris meridian.
In 1960, the General Conference on Weights and Measures established the International System of Units (SI), which today comprises seven fundamental units: meter (length), kilogram (mass), second (time), ampere (electric current), kelvin (temperature), mole (amount of substance), and candela (luminous intensity).
Italy adopted the metric system in 1861, immediately after national unification. In the United States, despite the Metric Conversion Act of 1975, the imperial system remains the standard in everyday life, although scientific institutions use the SI.
Speed conversions
Speed is measured in km/h in Italy and most of the world, but in miles per hour (mph) in the USA and United Kingdom. Here are the main conversions:
| From | To | Factor |
|---|---|---|
| 1 km/h | mph | 0.62137 |
| 1 mph | km/h | 1.60934 |
| 1 m/s | km/h | 3.6 |
| 1 knot | km/h | 1.852 |
Speed limits: Italy vs. USA comparison
| Road type | Italy (km/h) | USA (mph) | USA (km/h equiv.) |
|---|---|---|---|
| City/Residential zones | 50 | 25-35 | 40-56 |
| Non-urban roads | 90 | 55 | 89 |
| Highway | 130 | 65-75 | 105-121 |
Pressure and energy conversions
These conversions are useful for tires, meteorology, energy bills, and much more.
Pressure
| From | To | Factor |
|---|---|---|
| 1 bar | PSI (lb/in²) | 14.5038 |
| 1 atm | bar | 1.01325 |
| 1 atm | mmHg | 760 |
| 1 PSI | bar | 0.06895 |
For tires: in Italy, pressure is expressed in bar, while in the USA it is in PSI. A typical pressure of 2.2 bar equals approximately 32 PSI.
Energy
| From | To | Factor |
|---|---|---|
| 1 kWh | joules | 3,600,000 |
| 1 calorie (cal) | joules | 4.186 |
| 1 kcal | kJ | 4.186 |
| 1 BTU | kJ | 1.055 |
Conversions in everyday life
Here are some common situations where conversions are needed:
- Online shopping: American clothing and shoe sizes use different units (inches for shoes, different numerical sizes for clothing)
- Cooking: Anglo-Saxon recipes use cups, ounces, and tablespoons
- Travel: in the USA, speed limits are in miles per hour (mph), distances in miles
- DIY: many screws, bolts, and pipes follow imperial standards (inches)
- Technology: monitors are measured in inches (diagonal), disks in GB/TB
- Sports: in American races, distances are expressed in yards and miles
- Medicine: blood pressure is measured in mmHg, temperature in Celsius (in Italy) or Fahrenheit (in the USA)
Common conversion mistakes
Here are the most frequent errors to avoid when converting between units of measurement:
- Confusing US gallon and imperial gallon: the American gallon (3.785 l) is smaller than the British gallon (4.546 l). If you are converting American data, use the US gallon.
- Forgetting the difference between weight ounce and fluid ounce: the avoirdupois ounce (weight) equals 28.35 g, while the fluid ounce (volume) equals approximately 29.57 ml (USA) or 28.41 ml (UK).
- Converting square units as if they were linear: 1 meter = 100 cm, but 1 m² = 10,000 cm² (not 100 cm²). Area units are converted using the square of the linear factor.
- Confusing metric ton and short ton: the metric ton is 1,000 kg, the American short ton is 907.18 kg.
Frequently asked questions (FAQ)
How many centimeters are in an inch?
One inch equals exactly 2.54 centimeters. This is a precisely defined conversion, not an approximation.
How do you convert Fahrenheit to Celsius?
Subtract 32 and multiply by 5/9. For example: 77°F -> (77 - 32) x 5/9 = 45 x 5/9 = 25°C.
How many liters are in a gallon?
One American gallon equals 3.785 liters. One British (imperial) gallon is larger: 4.546 liters.
What is the difference between mass and weight?
Mass (in kg) is the amount of matter and does not change. Weight (in newtons) is the gravitational force and varies by location. In everyday life, the two terms are used interchangeably.
Why doesn't the USA use the metric system?
For historical and cultural reasons. The cost of converting all infrastructure (signage, tools, training) has always been considered too high relative to the perceived benefits.
How do SI prefixes work?
Each prefix represents a power of 10. To convert, simply multiply or divide by the corresponding power. For example, 1 kilometer = 10³ meters = 1,000 meters.
Conclusion
Converting between units of measurement is a fundamental skill in a globalized world. Whether you are cooking an American recipe, buying a 27-inch monitor, planning a trip to the United States, or studying physics, having the right tables and formulas at hand saves you errors and time. Our unit converter allows you to perform any conversion instantly and accurately: select the quantity, enter the value, and get the result in all available units.
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