The Metric System

A centimeter? If any centimeters come crawling into this room, I'll step on 'em!

Sally, Peanuts

Before explaining why you should learn to love the metric system, we should give the traditional measurement systems a fair hearing. They do have a few advantages.

The Few Advantages of Traditional Systems Over Metric
Units usually correspond to something understandable in your environment.

A gram is about the weight of a paperclip, a litre is a half-bottle of soda, and a meter is the distance from the floor to some belly-buttons. A yard is about one step, a pint is a big glass of milk, and a pound is about how much that milk would weigh. Fahrenheit degrees are a 0-100 scale of normal temperature: 0° is quite cold, 100° is quite hot.

There are usually specialized units that work on whatever scale you need.

Inches measure paper, feet measure furniture, yards measure houses, and miles measure cities. Because metric units differ by factors of 10 or 100, there is often no convenient unit for measurement at a certain scale. For instance, there is no metric unit scaled to find the height of people. Most adults fall between 150cm and 210cm — 5 to 7 feet.

There are units tailored to every particular purpose.

For instance, miles defined in terms of feet and yards are intuitive for measuring land. But when navigating by sea or air, nautical miles are more convenient. The equator can be divided into 360 degrees. Every degree is divided into 60 minutes. One minute on the equator is one nautical mile. So you know that when you've travelled 60 nautical miles, you've gone about 1/360th of the way around the world. 1 nautical mile is about 1.15 land miles. Even though both types of measurements are very useful for their distinct purposes, in the metric system, you cannot have two units that differ by this little. One must be ten times larger than the other.

The steps between units are often small and intuitive.
  • 1000 meters is 1 kilometer.
  • A few feet is a yard.
    • A few yards is a rod.
      • A few rods is a chain.
        • 10 chains is a furlong.
          • 8 furlongs is a mile.

These gradual steps can help you picture large things intuitively. If you can imagine a foot, you can imagine a yard. If you can imagine a yard, you can imagine a rod, and so on.

Traditional units, with their intuitive definitions and scales are very well suited to imagination. When you can imagine a furlong (1/8 of a mile) and a rod (5.5 yards), an acre being a 4 rod wide field that's 1 furlong long is pictureable. Originally, an acre was how much land one man could plough with one ox in one day. A furlong was the length of one furrow.

Traditional units naturally express an estimation's margin of error.

Because there are units for every scale, my choice of units expresses my confidence in an estimate. Telling you my couch is about 10 ft. 6 in. wide expresses more confidence than saying it is about 10 ft. wide. Because metric units differ by such great amounts, there is often no way to do this. I must say my couch is about 3 meters or 300cm wide. The former implies my margin of error is 1 meter, and the latter that it's 1cm. In reality, I must depend on the roundness of 300 to imply that my margin of error is 10cm.

Large units often divide evenly into useful whole numbers of smaller units.

Because all metric units differ by multiples of ten, you can only evenly divide them into 2 or 5 smaller units. Traditional units often favor numbers like 12 and 60 which can be divided into 2, 3, 4, or 6 equal pieces. For instance, I can split a yard-long plank of wood exactly into 2, 3, 4, 6, 9, 12, 18, or 36 equal pieces with the inch marks on my ruler. For a 1 meter plank, I must divide into 2, 4, 5, 10, 20, 25, 50, or 100 pieces. Any other division requires estimating, like splitting it into three 33.3cm pieces.

The Advantages of the Metric System
It is the standard system for the whole world.

In many respects, Esperanto is a great candidate for an international language. But English actually is the de facto lingua franca. Don't be one of the few people who learn Esperanto and not English.

The units of measurement are all powers of 10.

This means you never have to look anything up since there are 1000mL in 1L instead of 16 tbsp in a cup; 2 cups in a pt; 8 pts in a gal. You may not mind not knowing how many tablespoons are in a gallon, but in science and engineering, you often compare tiny things to giant things and are happy to know there are exactly 1 trillion picolitres in a litre.

John Baez on inches.