I have a birthday present for my best friend- a gumball machine bank I have a birthday present for my best friend- a gumball machine bank. I know he’ll like it, and his party is tomorrow. How can I estimate to figure out how the box should be?

In this lesson, you will learn what it means to estimate the length of an object by comparing it to other objects.

How to compare parts of your body to standard units: Thumb- inch Arm- foot Arm span- yard or meter Pinky nail- centimeter You already know how to compare parts of your body to standard units. The first knuckle to the end of your thumb is about an inch, your arm from elbow to wrist is about a foot, your arm span is about a yard or a meter, and the nail on your pinky finger is about a centimeter. Remember, these aren’t the exact measurements you might get if you used a ruler, but these body parts will give you a good idea of how long something is.

A close guess- not exact An educated guess- based on information Reasonable! So, what is an estimate, anyway? An estimate is a close guess, when your measurement doesn’t have to be exact. It’s also an educated guess, which means it’s smart, based on what you know, and it makes sense. It’s reasonable. For example, tThe lines on the baseball field are too long to measure in a small unit like inches. Instead, we need a bigger unit, like yards.

Some people think that their first estimate has to be exactly right Some people think that their first estimate has to be exactly right. Remember, when we estimate, we’re trying to make a reasonable guess, and it might turn out that our guess isn’t exactly or that we learn more and we need to revise, or change, our thinking. That’s okay. Good estimators know how to make an estimate, then think carefully, perhaps double check, then make a new estimate. Wait! I didn’t say the crayon is 3 feet, I meant 3 inches!

When do we estimate? How long? How much? How far? How many? Really? When do we estimate? All the time! Each afternoon after school you might ask yourself, “About how long will it take to do my homework?” If you are going shopping and need to decide how much money to bring you might say, “About how much money do I need to buy 3 pair of socks?” We also estimate length. For example, if you’re walking to your neighbor’s house, you might ask, “About how far is it from my house to the one next door?” Or if you’re trying to hang up pictures, you might say, “About how many pictures will fit in the frame?” If you’re at a restaurant that serves subs, you might wonder, “Is the foot-long sandwich really one foot long? What do you notice about all of these questions? They all have the word “about” in them, meaning that we’re not looking for the precise, or exact answer, we’re looking for an estimate, or a reasonable guess.

Estimation is something we do in our heads Estimation is something we do in our heads. To estimate, you start by looking at the object. Then picture another object you have already measured that is close to the same size. It’s okay to change your estimate at any point.

We can use estimates of length in all kinds of situations We can use estimates of length in all kinds of situations. About how long is this clipboard? Close to the length of my forearm… They are a good guide or starting point, but sometimes you need an exact measurement. For an exact measurement, you will need a tool- like a ruler.

So back to my gumball machine So back to my gumball machine. I have this gumball machine and this box to put it in. I want to know if the box is tall enough so that the machine can fit inside of it standing up. I can estimate the length of the machine and the box to get a pretty good idea if it will fit inside of it. And I’m going to do this all in my head. First, I’m going to make a mental picture to compare the box to something I know, like a piece of paper. And I can see in my mental picture that the paper is roughly the same height as the machine. Then I’m going to try to imagine if the paper would fit inside the box by making another mental picture. Yes, in my mental picture, the paper fits inside the box…so this box will likely work. You can also make mental rulers to estimate lengths in inches, centimeters, and other units

In this lesson you have learned what it means to estimate the length of an object by comparing it to other objects.