Two Systems, One World
The world uses two main systems for measuring length, weight, and volume: the metric system and the imperial system. Most countries use metric; the United States is the most notable country still predominantly using imperial in everyday life.
The Metric System
The metric system (officially the International System of Units, or SI) is based on multiples of 10. This makes it easy to convert: just move the decimal point. Key units include the meter (length), kilogram (mass), and liter (volume). It was developed during the French Revolution and adopted internationally through the 19th and 20th centuries.
The Imperial System
The imperial system evolved from old English measurement traditions. Its conversions are less logical: 12 inches in a foot, 3 feet in a yard, 1,760 yards in a mile. Weights go from ounces to pounds (16 oz = 1 lb) to stones (14 lb = 1 stone) to tons. Volume uses fluid ounces, cups, pints, quarts, and gallons.
Key Conversion Facts
- Length: 1 inch = 2.54 cm | 1 foot = 30.48 cm | 1 mile = 1.609 km
- Weight: 1 pound = 0.4536 kg | 1 ounce = 28.35 g
- Volume: 1 US gallon = 3.785 liters | 1 fluid oz = 29.57 ml
Which Countries Use Imperial?
The United States, Liberia, and Myanmar are the only countries that have not officially adopted the metric system. The UK uses a mix: metric for most official purposes but imperial for road signs (miles), beer (pints), and body weight (stone).
Our length converter and weight converter make it easy to switch between both systems instantly.
Step-by-Step Conversion Examples
Converting a US Recipe for a European Kitchen
An American banana bread recipe calls for 2 cups of flour, 1 stick of butter (½ cup), and baking at 350°F. Translating to metric:
- 2 cups flour: 1 US cup = 236.6 mL. But for baking, use weight: 2 cups all-purpose flour ≈ 240 g
- ½ cup (1 stick) butter = 113 g (butter density: ~227 g per US cup)
- 350°F = (350 − 32) × 5/9 = 318 × 0.5556 = 176.7°C → set oven to 175–180°C
Note that US "cups" measure volume, while European baking typically weighs dry ingredients. Gram conversions are more accurate than mL conversions for flour because flour compaction varies. The recipe works identically — only the units change.
Understanding Body Measurements at a Foreign Doctor's Office
A US traveler who weighs 175 pounds and stands 5 ft 11 in needs to fill out a medical intake form in Germany asking for metric measurements:
- Weight: 175 lbs × 0.45359237 = 79.4 kg
- Height: 5 ft 11 in = 71 inches × 2.54 cm/in = 180.3 cm
BMI = kg ÷ m² = 79.4 ÷ (1.803)² = 79.4 ÷ 3.25 = 24.4 (healthy range: 18.5–24.9). Medical contexts worldwide use metric because it enables universal BMI calculations and drug dosing charts by body weight. Drug dosing (mg/kg) is universal in clinical medicine regardless of country.
Reading Road Signs While Driving Abroad
A US driver renting a car in France sees a speed limit of 130 km/h on the autoroute. Converting to mph:
130 km/h × 0.621371 = 80.8 mph
And a 50 km/h urban limit = 31.1 mph. Mental shortcut: multiply km/h by 0.6 for a rough mph estimate (130 × 0.6 = 78 mph — off by 2.8 mph, fine for practical driving). The inverse: a US 65 mph interstate limit = 104.6 km/h, which is why dual-system speedometers mark 65 mph near the 105 km/h position.
Why the US Still Uses Customary Units
The US customary system differs in several ways from the British imperial system — a US gallon (3.785 L) and a UK gallon (4.546 L) are different sizes. The US briefly attempted metrication with the Metric Conversion Act of 1975 (voluntary) and the Metric Board (abolished 1982). Science, medicine, the military, and most of manufacturing already use metric in the US. Only road signs, retail food, and everyday consumer behavior remain predominantly customary. Switching road signs alone has been estimated to cost $500M–$1B, making the transition politically difficult despite the technical benefits of base-10 conversions.
Source: NIST Handbook 44; BIPM SI Brochure 9th edition. Last verified: May 2026.
Frequently Asked Questions
Which countries use the imperial system?
Only three countries officially use the imperial/customary system as their primary measurement standard: the United States, Liberia, and Myanmar. The United Kingdom officially uses metric but retains imperial units for road distances (miles) and some traditional uses like pints for beer. Most other countries use the metric system exclusively.
Why did the US never switch to metric?
The US came close to adopting metric in the 1970s. Congress passed the Metric Conversion Act of 1975, but it was voluntary and the initiative stalled due to public resistance and the cost of converting infrastructure. Several industries — science, medicine, military, and most of manufacturing — already use metric in the US.
What is the easiest imperial to metric conversion to remember?
For everyday use: 1 kg ≈ 2.2 lbs, 1 mile ≈ 1.6 km, 0°C = 32°F and 100°C = 212°F. The inch-to-centimeter ratio (1 inch = 2.54 cm) is exact by definition. For a screen-based shortcut, just use our free Length Converter above.
Convert between metric and imperial units for length, weight, and more.
⚡ Length & Distance Converter — FreeAccuracy note: Conversion factors on SwiftConvertHub are sourced from NIST and IEC standards. Results are accurate for general use. For safety-critical or professional applications, verify results independently. Full disclaimer →
Victor A. Calvo S. is a software engineer and digital entrepreneur who builds practical, free tools for developers, students, and professionals worldwide. He is the creator of SwiftConvertHub, InstantLinkHub, and Feexio. All conversion factors are cross-referenced against NIST and IEC standards. Learn more →