Estimating Numbers

Contributor: Rachel Lewis. Lesson ID: 12178

Scientists say there may be 100 billion galaxies in space. Who counted them? No one, of course, but they estimated, or took a reasonable guess, at the number. Learn to estimate to make math easier!


Arithmetic, Whole Numbers and Operations

learning style
personality style
Lion, Beaver
Grade Level
Intermediate (3-5)
Lesson Type
Dig Deeper

Lesson Plan - Get It!

  • How many jelly beans are in the jar?

Instead of counting each jelly bean, you can estimate!

When we estimate, we find a number that is close to the correct answer.

An estimation is not just a guess — it has to be reasonable. Would it be reasonable to estimate that there are 15 jelly beans in the jar above? No. We can tell that there are more than 15 jelly beans. So, our estimate needs to be closer to the correct answer.

Estimation is a lot like rounding numbers. If we want to estimate to the nearest tens place, then we would look to the digit in the ones place to see if we should round up or down. So, 329 would become 330.

The same is true if we wanted to estimate to the nearest hundreds place. We would look at the digit in the tens place to see if we should round the number in the hundreds place up or down. Here, 231 would become 200.

  • When do you think estimates would be useful in math? Share your ideas with a parent or teacher.

Estimates are very helpful when you want to solve math problems. Instead of adding 239 and 344, we can estimate. Finding the sum of 240 and 340 is a much easier problem to solve.

If we estimate, we see that 240 + 340 = 580. The actual sum of 239 + 344 = 583, so our estimation was close to the actual answer.

We can estimate to solve addition, subtraction, multiplication, and division problems.

Take a look at an example:

Kara made 420 cookies for her class. She sold 249 cookies. About how many cookies are left?

When we see the words “about how many” or “choose the best estimate,” we know to estimate.

Instead of subtracting 420 - 249, we can simply estimate to the nearest hundred: 400 - 200 = 200.

Kara had about 200 cookies left.

Watch How Many Jelly Beans? from PBS LearningMedia to see how Inez uses estimation to decide if she has enough jelly beans to bake cookies for her class.

  • How did Inez use estimation in the video?
  • Was her estimate reasonable?

Share your answers with a parent or teacher.

Now, move on to the Got It? section to try out estimation on your own.

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