Main content

### Course: 4th grade > Unit 14

Lesson 3: Estimating length# U.S. customary units: distance

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

## Want to join the conversation?

- Why do we still use the English system of measurement? The rest of the world uses Metric.(136 votes)
- 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.(18 votes)

- I am living in a country using metric system. Can anybody write down the abbreviations of these units for me?(32 votes)
- When specificly when these units were made and who made them and how?(6 votes)
- 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 ( :(12 votes)

- Why do we still use the English system of measurement? The rest of the world uses Metric.(8 votes)
- 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.(8 votes)

- 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)
- Because the customary system is older, and the different units evolved from their different purposes. For example, Sal mentioned the mile came from the distance of 1000 marching steps of a Roman soldier - neat - I didn't know that!(9 votes)

- 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.(4 votes)
- 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.(12 votes)

- how many is 1 light year(5 votes)
- 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(9 votes)

- How many meters is a mile.(4 votes)
- how many yards are in 156 inches(3 votes)
- How much larger exactly is a mile than a kilometer?(2 votes)
- There are exactly 1.60934 kilometers in a foot. So, a mile is larger. :)(1 vote)

## 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.