Which countries use the metric system?

195 countries officially use the metric system (SI units). Only 3 countries -- the United States, Myanmar, and Liberia -- use the imperial system as their primary system of measurement.

Why is the metric system better for science?

The metric system's decimal-based structure (units increase in multiples of 10) makes calculations simpler and more consistent. All scientific research, international trade, and medical standards worldwide use metric units for precision and reproducibility.

How do I convert metric to imperial quickly?

Common quick conversions: meters to feet (�—3.28084), kilograms to pounds (�—2.20462), liters to gallons (�—0.264172), Celsius to Fahrenheit ((C �— 9/5) + 32). Our free converters give instant, precise results.

Does the UK use metric or imperial?

The UK uses a mixed system. Metric for most commerce (food, drink, road signs show both), but miles, feet, and pounds are still commonly used informally. Officially the UK is fully metric under EU law (though post-Brexit this is softer in practice).

Why does the US still use the imperial system?

The US has never fully transitioned to metric due to the cost of conversion, cultural familiarity, and historical inertia. However, metric units are used in military, science, medicine, and some industries. A full transition would require decades and billions in costs.

Is the inch defined in metric?

Yes. Since 1959, an inch is officially defined as exactly 2.54 centimeters (25.4 mm). This international agreement made the imperial and metric systems precisely interconvertible at all scales.

What is the difference between metric and imperial measurement systems?

The metric system is a decimal-based system of measurement used worldwide, while the imperial system is a collection of units developed historically in Britain. The key difference is that metric units increase in multiples of 10 (like kilometers, meters, centimeters), whereas imperial units use irregular ratios (like 12 inches in a foot, 3 feet in a yard, 1,760 yards in a mile). The metric system is the official standard in 195 countries, while only the United States, Myanmar, and Liberia primarily use imperial units.

Which countries use the metric system and which use imperial?

One hundred and ninety-five countries officially use the metric system as their primary measurement standard. Only three countries -- the United States, Myanmar, and Liberia -- still use the imperial system as their primary system. The UK uses a hybrid system where road signs show miles, but most commercial measurements use metric. All scientific research, international trade, and medical standards worldwide are based on metric units.

Why is the metric system better for science and engineering?

The metric system's decimal structure makes calculations simpler and more consistent across all scientific disciplines. All scientific research, international trade agreements, and medical standards worldwide are based on metric units because the system uses natural constants as its foundation. The simplicity of moving decimal points instead of memorizing arbitrary conversion factors reduces calculation errors and enables precise reproducibility of experiments across different countries and laboratories.

How do I quickly convert between metric and imperial units?

The quickest mental math shortcuts are: multiply kilometers by 0.6 to get miles, multiply kilograms by 2.2 to get pounds, and for temperature, double Celsius and add 32 to get Fahrenheit. For precise conversions, use our free online converters which handle all common metric-to-imperial conversions instantly. For length, 1 meter equals 3.28084 feet; for weight, 1 kilogram equals 2.20462 pounds; and for volume, 1 liter equals 0.264172 US gallons.

Does the UK use metric or imperial measurements?

The UK operates a mixed system where metric is used for most commercial transactions, scientific contexts, and manufacturing, while imperial units persist for road distances (miles), body weight (stones and pounds), and informal speech. Officially the UK adopted metric under EU law, though post-Brexit this is applied less strictly in everyday contexts. Many older Brits still reference feet, inches, and stones in daily conversation, creating a dual-system environment that visitors often find confusing.

Why does the United States still use the imperial system instead of metric?

The United States has never fully transitioned to the metric system primarily due to the enormous cost of replacing all infrastructure, educational materials, and manufacturing tools. The Metric Conversion Act of 1975 made conversion voluntary rather than mandatory, so private industry and cultural familiarity with imperial units kept them dominant in everyday American life. A full transition would require decades and an estimated billions of dollars in conversion costs, making it a politically difficult undertaking despite the long-term efficiency benefits.

Is the inch officially defined using the metric system?

Yes, since 1959 the inch has been officially defined as exactly 2.54 centimeters, or 25.4 millimeters. This international agreement standardized the relationship between imperial and metric length measurements, making conversions between the two systems precisely interconvertible at all scales. Before 1959, different countries had slightly different definitions of an inch, which caused confusion in international trade and engineering.

What countries still use the imperial system of measurement?

Only three countries officially use the imperial system as their primary measurement system: the United States, Myanmar, and Liberia. The US uses imperial units for almost all everyday measurements including road distances, package weights, and temperature in weather reports. Interestingly, even within these countries, metric units are used in specific industries -- US science, medicine, and the military all use metric by NATO standardization.

What is the origin of the imperial measurement system?

The imperial system evolved from Roman, Anglo-Saxon, and Norman measurement traditions spanning centuries of European history. Units like the foot literally originated from the length of a human foot, while the inch was based on the width of a thumb. The British Empire formally standardized these units in the Weights and Measures Act of 1824, which is when the term 'imperial' was officially applied to the British measurement system that would later be inherited by countries in the Commonwealth.

Metric vs Imperial: What's the Difference? | Complete Side-by-Side Guide

Key Facts

•195 countries use the metric system. Only 3 countries (USA, Myanmar, Liberia) use imperial as their primary system.
•The metric system is decimal-based (multiples of 10). Imperial uses irregular ratios like 12 inches in a foot, 5280 feet in a mile.
•1 inch = exactly 2.54 cm (standardized in 1959). Before this, every country had slightly different definitions.
•A US gallon (3.785 L) is smaller than a UK gallon (4.546 L). Always check which gallon is being referenced.

What is the Difference Between Metric and Imperial?

The metric system (also called SI or International System of Units) and the imperial system are two different ways of measuring length, weight, volume, and temperature. The key difference: the metric system is decimal-based (units increase in multiples of 10), while the imperial system uses irregular, historical units (like 12 inches in a foot, 3 feet in a yard).

195 countries use the metric system officially. Only 3 countries (USA, Myanmar, Liberia) use the imperial system as their primary system.

Measurement	Metric Unit	Imperial Unit	Conversion
Length	Meter (m)	Foot (ft)	1 m = 3.28084 ft
Weight	Kilogram (kg)	Pound (lb)	1 kg = 2.20462 lb
Volume	Liter (L)	Gallon (gal)	1 L = 0.264172 gal
Temperature	Celsius (°C)	Fahrenheit (°F)	0°C = 32°F

Try These Converters

Want to convert units yourself? Here are our free tools for the measurements above:

📏 Length meters, feet, inches ⚖️ Weight kg, lbs, ounces 🧪 Volume liters, gallons 🌡️ Temperature Celsius, Fahrenheit

What Are Metric and Imperial Systems?

The metric system (International System of Units, or SI) and the imperial system represent two fundamentally different approaches to measuring the physical world. While the metric system dominates globally, used by 195 countries, the imperial system remains prevalent in the United States, Myanmar, and Liberia.

Key Differences Between Metric and Imperial

Base Units Comparison

Length: Meter (m) vs Foot (ft) - 1 meter = 3.28084 feet. See our History of the Meter and Yard.
Mass: Kilogram (kg) vs Pound (lb) - 1 kilogram = 2.20462 pounds. Learn the difference between mass and weight.
Volume: Liter (L) vs Gallon (gal) - 1 liter = 0.264172 US gallons
Temperature: Celsius (°C) vs Fahrenheit (°F) - 0°C = 32°F. Our Temperature Science guide covers all three scales.

Decimal vs Fractional

The metric system's greatest advantage lies in its decimal-based structure. Converting between units is as simple as moving a decimal point. For example, 1 kilometer = 1,000 meters = 100,000 centimeters. This logical progression eliminates the need for memorizing arbitrary conversion factors.

The imperial system, conversely, uses fractional relationships that require memorization: 1 mile = 1,760 yards = 5,280 feet = 63,360 inches. These inconsistent ratios make mental math more challenging and increase the potential for calculation errors.

Historical Context

The metric system was developed during the French Revolution (1790s) as part of a broader effort to rationalize and standardize measurements. French scientists created a system based on natural phenomena--specifically, the meter was defined as one ten-millionth of the distance from the equator to the North Pole. For more on the evolution of measurement, read our History of Measurement Systems.

The imperial system evolved from Roman, Anglo-Saxon, and Norman measurement traditions spanning centuries. Units like the "foot" literally originated from the length of a human foot, while the "inch" came from the width of a thumb. The British Empire standardized these units in the Weights and Measures Act of 1824.

Practical Applications

When to Use Metric

Scientific research: All major scientific journals and research institutions use metric units exclusively
International business: Global trade and manufacturing predominantly use metric specifications
Medical field: Medication dosages, body measurements, and medical equipment worldwide use metric units
Education: STEM curricula globally teach metric as the primary system

When to Use Imperial

United States commerce: Retail, construction, and everyday measurements in the US
Aviation: Altitude in feet and nautical miles remain standard internationally
Traditional contexts: Recipe books, historical documents, and cultural practices

Quick Conversion Tips

Mental Math Shortcuts

Kilometers to Miles: Multiply by 0.6 (more precisely 0.621371). For example, 100 km ≈ 60 miles. Convert length units instantly →

Celsius to Fahrenheit: Double the Celsius temperature, subtract 10%, then add 32. For 20°C: (20 — 2) - 4 + 32 = 68°F. Convert temperature →

Kilograms to Pounds: Multiply by 2.2. For 70 kg: 70 — 2.2 = 154 lbs. Convert weight →

Seconds to Days/Hours/Minutes: Divide by 60 for minutes, 3,600 for hours, 86,400 for days. Or use our free time converter →

Common Misconceptions

Myth: "The imperial system is more intuitive for everyday use."
Reality: Familiarity breeds comfort. Studies show people raised with the metric system find it equally intuitive. The metric system's logical structure actually makes it easier to learn and use accurately.

Myth: "Converting to metric would be too expensive for the US."
Reality: The UK, Australia, Canada, and other countries successfully transitioned over 20-40 year periods. Long-term benefits include reduced calculation errors, simplified education, and enhanced international competitiveness.

Global Adoption Trends

As of 2024, only three countries have not officially adopted the metric system as their primary measurement standard. However, even the United States uses metric units extensively in:

Military operations (NATO standardization)
Pharmaceutical industry
Scientific research and development
Automotive manufacturing (increasing use of metric fasteners)

Conclusion

Both measurement systems serve their purposes, but the metric system's decimal structure, international standardization, and scientific foundation make it superior for most applications. Whether you're converting for travel, work, or education, understanding both systems and knowing how to convert between them remains an essential skill in our interconnected world.

History

History of Measurement Systems

From Egyptian cubits to atomic clocks

Length

History of the Meter and Yard

The origins of two rival systems