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U.S. customary units: distance

Sal discusses US customary units of length or distance such as inch, foot, and yard. Created by Sal Khan.

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  • male robot hal style avatar for user Michael Le Clech
    Why do we still use the English system of measurement? The rest of the world uses Metric.
    (118 votes)
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    • leaf blue style avatar for user Pi is the best
      A lot of the Imperial (U.S.) measurements are still used as a standard in many industries. The most prominent one that comes to mind is the construction industry. Many of the supplies are in Imperial measurements (2x4’s) and industry standards are done in Imperial, such as the spacing between joists and rafters (12, 16, and 24 inches are fairly common). If you were to convert 16 inches into mm you would have roughly 406 mm. This isn’t the most easy to work with number nor is it necessarily easy to remember. One option would be to round up or down to 400 or 410 mm but then you are changing the whole engineering of the building to make it easier for tradespeople to work with. Many suppliers will have generic sizes for windows and doors that would also need to be changed or your standard sizes would involve decimal places or fractions of metric measurements.

      I know when I used to make cylinder heads for Caterpillar everything was done in metric, except the threads. I can only guess why Caterpillar would use US threads instead of metric ones. Maybe it was for compatibility in parts with older equipment?

      If you think about it, there would need to be a major overhaul in a lot of industries to switch strictly to a metric system. Tooling, machines, scales, road signs, educational content, industry and government standards would all need to be changed (just naming a few). I believe in the long run it would be much easier and ultimately cheaper if there was one standard in place. Inventories could be reduced if you weren’t having to carry both metric and imperial nuts and bolts. The same could also be said about tooling, imagine only needing metric sockets and wrenches. I think we will eventually end up with just the metric system, but it is going to take a long time to wean off the old Imperial system. I hope this might give you some insight as to why the Imperial system is still kicking around.
      (8 votes)
  • ohnoes default style avatar for user Cyan Wind
    I am living in a country using metric system. Can anybody write down the abbreviations of these units for me?
    (2 votes)
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  • aqualine ultimate style avatar for user programmerevan
    in the metric system they just add a prefix to a word but in the U.S customary system they have all different words. Why is this?
    (6 votes)
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  • hopper jumping style avatar for user ShadowNahila4444
    When specificly when these units were made and who made them and how?
    (5 votes)
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    • winston default style avatar for user WinstonTRON
      The first units of measurement were made in like, 200 BC because Egyptians made the first unit of measurement the cubit, and then later the early English of New England made rocks units of measurement. Then they made the customary and metric system.


      Great question ( :
      (6 votes)
  • blobby green style avatar for user ballardr2
    Why do we still use the English system of measurement? The rest of the world uses Metric.
    (6 votes)
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    • leaf green style avatar for user binduraol
      I think it's because people are resistant to change, even though the metric system is easier. People in the US have used the US system for awhile, and it's what they are used to. The metric system is used in health care and lab settings in the US, though.
      (2 votes)
  • starky ultimate style avatar for user Noah Rendall (Relentless)
    How many meters is a mile.
    (3 votes)
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  • leafers ultimate style avatar for user Nicholas Clancy
    if romans had the mile, why does Europe use the metric? I mean, romans are very appreciated in Europe, in fact romans are some of the bases of Europe today.
    (3 votes)
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    • male robot johnny style avatar for user Creston Hulett
      What a great question! The way I understand this, the different countries in Europe each understood the various units of measure to actually be different lengths. For example, the Roman mile was only 5000 feet when our mile is 5280 feet. Then we might consider the foot. Some countries divided the foot into 12 inches while others divided the foot into 16 digits. The metric system was developed to be a unique system that would bring unity to all the countries without showing favoritism to one country's measuring system above another. It seems to have worked since Europe uses the Metric System in common.

      The USA is so large and there are no measurement disputes, so the mile, foot and inch have remained as the base unit of measure.
      (6 votes)
  • duskpin seedling style avatar for user ashton
    how many is 1 light year
    (3 votes)
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    • leaf green style avatar for user binduraol
      A light-year is the distance light travels in one year. How far is that? Multiply the number of seconds in one year by the number of miles or kilometers that light travels in one second, and there you have it: one light-year. It's about 5.88 trillion miles (9.5 trillion km). You can also view this website for more info - en.wikipedia.org/wiki/Light-year
      (3 votes)
  • marcimus pink style avatar for user erinm2446
    how many yards are in 156 inches
    (3 votes)
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  • starky ultimate style avatar for user someone
    technoblade🐷🗡
    (3 votes)
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Video transcript

Let's talk a little bit about the US customary units for distance. If we go to a human scale, the most typical used unit in the US customary system would be the foot. And as you could imagine, the term foot, and if you have many of them, you are now talking about feet, comes from the same word on the human body. We have a foot or we have two feet. Now, not everyone's feet are the same length. But the length of a foot is roughly a little bit larger than most people's feet. And there are definitely people whose feet are the size of a foot. And if you're in the US, you've probably heard of people measuring their height in terms of feet. So for example, a slightly above average man of slightly above average height would have a height of 6 feet. So for example, right over here, this gentleman right over here was slightly above average height. His height might be 6 feet. And that might also be his wing-span. So it would be a distance something like that. If you wanted to measure things that are a little bit smaller, say smaller than a foot, or if we wanted to measure parts of a foot, then you would go to the inch. And the relationship between the foot and the inch is that 12 inches are equal to one foot. So for example, this little mouse right over here, it might be reasonable to measure it in inches. On this scale, an inch might be, depending on the size of the mouse, an inch might be something like that. And it's also useful for measuring things like human height, because very few people are a whole number of feet. They're usually some fraction of feet. So for example, I am 5 feet and 9 inches. And sometimes this is denoted as 5, and you have this apostrophe here, one apostrophe, and then 5 foot 9 inches, and this is two apostrophes, kind of saying hey, you're going to the next unit down. These are the inches right over here. Now if you want to go to things that are larger than maybe human scale, we have units for that as well, including the yard. The yard is equal to 3 feet. And 3 feet, well, each foot is 12 inches, so this is the same thing as 36 inches. And this is most typically used for measuring distances of a lot of some kind. And in the United States, most famously for in football. So your regulation-size football field, the playing field, the distance between the end zones, is exactly 100-- I know you can't see my writing there-- is exactly 100 yards. This distance right over here. Right between the end zone. Now, if you want to measure even longer distances, that's where we go to the mile. And all of these units are ancient units, going back, actually, thousands of years. And the mile, based on the reading I've found, was even used in Roman times. And actually, that's some of the first examples of this term being used. And it really comes from the same root word as 1,000, of mil, of 1,000 footsteps of a Roman soldier. And now, a standard mile is exactly 5,280 feet. And if you wanted to figure out this in inches, you could just multiply by 12. But just to get a sense of a mile, here is a mile on this map of New York. So if you wanted to put it right on Manhattan, this distance right over here is about a mile. So you want to measure really far distances, either from one point in a city to another, or maybe even the distance between cities, or maybe even the size of the planet, or even the distance from the Earth to the moon, or something like that, you will hear that referenced in miles, even the distance between the Earth and the sun. People say it's 93 million miles. If you're talking about maybe, a plot of land, you want to measure its dimensions, or you're talking about a football field, then yards feel appropriate. If you're kind of getting to human scale, well, human scale on one level, but you could get a little bit larger if you're maybe talking about the height of a building or something like that, or you're still talking about measurement of say, a plot of land, you will still hear people talk in terms of feet. And if you're measuring either parts of a foot, or things that are smaller than a foot, that's when inches come into play.