The metric system (formally the International System of Units, SI) is used by the majority of the world and by all of science and medicine. The US, Liberia, and Myanmar are the only countries that have not officially adopted the metric system as their primary measurement standard — though even in the US, metric is used in science, medicine, pharmaceuticals, and international trade.
The core advantage of metric is its base-10 structure. There are exactly 100 centimeters in a meter, 1,000 meters in a kilometer, 1,000 grams in a kilogram, and 1,000 milliliters in a liter. Converting between units requires only moving the decimal point. Imperial conversions are arbitrary: 12 inches in a foot, 3 feet in a yard, 1,760 yards in a mile, 16 ounces in a pound, 2,000 pounds in a short ton.
Critical conversion facts for everyday use
Length: 1 inch = 2.54 cm (exact, by definition). 1 foot = 30.48 cm. 1 mile = 1.60934 km. A quick rough conversion: multiply miles by 1.6 to get kilometers. Speed: 60 mph ≈ 97 km/h. Temperature: °F = (°C × 9/5) + 32. A body temperature of 37°C = 98.6°F. A useful reference: 16°C ≈ 61°F, 28°C ≈ 82°F.
Weight and volume: 1 kg = 2.20462 lbs (roughly 2.2). 1 US gallon = 3.785 liters. 1 fl oz = 29.57 ml. UK and US use different gallons — a UK (imperial) gallon is 4.546 liters, about 20% larger than a US gallon. This causes genuine confusion in recipes and fuel economy comparisons between American and British sources.
The Mars Climate Orbiter and why unit confusion is dangerous
In 1999, NASA's Mars Climate Orbiter spacecraft was lost because one engineering team delivered thruster data in pound-force seconds while the receiving software expected newton-seconds. The resulting navigation error caused the spacecraft to enter the Martian atmosphere at the wrong angle and disintegrate. The mission cost $327.6 million. The failure was a direct result of unit system confusion between two teams working on the same project.
In medical contexts, unit confusion is a documented cause of medication errors. Doses may be written in mcg (micrograms) or mg (milligrams) — a 1,000-fold difference. Insulin is measured in units, not milligrams. Pediatric dosing is often based on mg/kg body weight, requiring the patient's weight in the right unit system. Healthcare systems that mix metric and imperial face systematic error risks that purely metric systems avoid.