How to Estimate Sums

Sometimes when we add two big numbers, we don't really want to know the exact sum. We just want to find a quick estimate.

      We can quickly estimate a sum by first rounding each of the addends and adding them together.

Example: Using Rounding to Estimate Sums

Estimate the sum of 698 and 207.

Let's skip the regrouping, 😅 and estimate a sum by rounding each of the addends first.

Rounding Review

Rounding is making a number simpler, but keeping it close to what it was.

When you round, you first pick a target digit to round to. 

      If the digit to the right of the target digit is less than 5, you round down. 👇

      If the digit to the right of the target digit is 5 or more, you round up. 👆

Tip: Unless otherwise stated, always round the numbers to the place value of the left-most digit (👈), which is the highest place value.

In the example above, let's round to the hundreds digit.

What's 698 rounded to the nearest hundred?

That's right, 700.

What's 207 rounded to the nearest hundred?

That's right, 200.

Now, we can easily add the rounded values.

Using rounding, we see that the sum of 698 and 207 is about 900.

The sum is not exactly 900, but close to that number.

Another Example

Estimate the sum of 415, 67, and 324.

Oh no, you need to add 3 numbers! 🙀

Let's round each addend.

What's 415 rounded to the nearest hundred?

That's right, 400! ✅

What's 67 rounded to the nearest ten?

That's right, 70! ✅

What's 324 rounded to the nearest hundred?

That's right, 300! ✅

Now you can estimate the sum by adding the rounded values.

So, the sum of 415, 67, and 324 is about 770. The answer isn't exact, but it's close.

You can now use rounding to estimate sums.

Good job.

Tip: Always write the word "about" before the estimated sum.

Tip: You can also use the ≈ approximate symbol, like this:

415 + 67 + 324 ≈ 770

Now, move on to the practice. It'll help you understand more and remember for longer.