Sal discusses metric units of length or distance such as millimeter, centimeter, meter, and kilometer. Created by Sal Khan.
The base unit for distance in the metric system is the meter. And to get a sense of how long a meter is, the average human being is between 1.5 and 2 meters tall. So 1.5 meters would be a not-so-tall person, but 2 meters would be quite tall. In fact, if you're 2 meters tall or above 2 meters tall, you might want to consider basketball. So, for example, in this drawing right over here by Leonardo da Vinci, if this is an average-sized male, a meter, it would be more than half way across his wingspan. So it might be about that much. And obviously, I am just roughly drawing this, just to give you a sense of how far a meter might be. And I just wrote a lowercase m here as really just a short way of writing meter. So you could imagine that the smaller things you might want to measure with the meter are things like a human being-- a human being's height or a human being's wingspan. If you wanted to say, well, what are some of the larger things that you would want to measure with a meter, you might imagine something like the height of a skyscraper. So this right over here is the Empire State Building, at one time the tallest building in the world, now just a tall building in the world. And this is 443 meters tall. So this is starting to be on the larger scale of things that you would want to measure with a meter. Now you're probably asking, well, what happens if I want to measure things that are even larger than that? Well the metric system provides us prefixes to essentially say multiples of meters. And the unit of distance that is larger than the meter that is most typically used is the kilometer. I've heard a few people say "kill-O-meter," but kilometer, I'll pronounce is "kuh-LAW-me-ter," which is equivalent to 1,000 meters. And so when you're thinking about kilometers, this is thinking about the distance within a city. This would be even the distance from one city to another city. Even the radius of the planet is often measured in kilometers. And to once again get a sense of that, this right over here is a map of New York City. And right over here they give us the scale. This distance right over here is 5 kilometers. So 5 kilometers just to give us a sense of things-- and they wrote km for short so 5 km. This lowercase k, lowercase m is just shorthand for kilometers, this would be equal to-- well if 1 kilometer is 1,000 meters, 5 kilometers is going to be 5 times as many. It's going to be 5,000 meters is this distance right over here. If you were to try to imagine 1 kilometer, it would be 1/5 of that. So 1 kilometer on this map might look something like that. Now there's other units in the metric system that allow us to describe distances less than a kilometer but more than a meter. But they're not as typically used. But I'll list them here, just so that you see that they exist. You have the "hect-O-meter," or "heck-TOM-eter"-- it's so infrequently used that I really haven't heard a lot of people say it-- which is equal to 100 meters. And you have the "DECK-a-meter" or "deck-AM-eter," and I actually think it's "DECK-a-meter." Dekameter, which is 10 meters. And I'm going to write these in orange because they're not that frequently used. For example, it's not typical to hear someone say that the Empire State Building is 4.43 hectometers or for them to say that this is 44.3 dekameters. They would typically say that this is 443 meters. Now you're probably saying, OK, well, this is fine. Using these prefixes on the meter, I'm able to describe distances that are larger than a meter, that are multiples of tens of the meter or a multiple of 100 or a multiple of 1,000 of the meter. But what if I want to go smaller? Well, the metric system has units for that. And if we go just 1/10 of a meter, this is not used as typically, but I'll write it here-- the decimeter. The decimeter is 1/10 of a meter. Or another way of saying it is 1 meter is equal to 10 decimeters, lowercase dm for short. Once again, this is not so typically used. But if we go one scale even below that, we get to the centimeter, which is a heavily used unit. And here, the prefix centi means 1/100. So this is 1/100 of a meter. Another way of thinking about it-- 1 meter is equal to 100 centimeters. And if you wanted to imagine the scale of a centimeter, you could imagine this frog right over here on this person's hand might be several centimeters long. So a centimeter on this drawing might be around that distance. Now if you want to measure things even smaller than that, we have another fairly common unit, and that is the millimeter. And the prefix milli here refers to 1/1,000. So it's 1/1,000 of a meter. Or another way of thinking about it is 1 meter is equal to 1,000 millimeter. Actually, let me just write m for short, just so we used to also seeing it that way. 1 meter is equal to 1,000 millimeters. And if you wanted to imagine how long or how far a millimeter is, you could go down the scale of things that are smaller than a centimeter. So these ants, for example, might be 3 or 4 millimeters long. So a millimeter in this drawing right over here might be something like that. The important thing to realize or just to conceptualize is you might want to use different units depending on the scale that you're measuring. And it's also useful to just realize what these typical prefixes refer to because we're seeing it used in distance here. They're all based on the base unit of distance. But when we're measuring volume or we're measuring other things in the metric system, these same prefixes also show up. So you see here when we're measuring distance, kilo refers to 1,000. A kilometer is literally 1,000 meters. Hecto refers to 100. A hectometer-- 100 meters. Deka refers to 10. Deci refers to 1/10. So decimeter, literally 1/10 of a meter. Centimeter-- 1/100, 1/100 of a meter. Milli refers to 1/1,000.