Why bother with estimation?

Estimating can come into our lives in many different aspects. Estimation is the process of finding a number that is very close to the correct answer. We use it not only in math class, but we could also use it during one of my favorite activities: shopping! Using estimation can help us to determine if we have enough money for a large purchase while also helping us to create better number sense by knowing if an answer is reasonable or not.

How do we estimate?

Here’s a handy rule to follow:



How we round numbers can be different depending on what we’re trying to figure out. Let’s say we are calculating 232+431+298+501. This can be difficult to do in our heads, so we will probably use a calculator. While using a calculator, it can be easy to miss a button or hit the wrong number. We can use estimation to ensure our answer makes sense. For this problem, I would use rounding to the nearest hundred position.

This website is a great resource to use to help with rounding.

Here is a great website to use to practice rounding.


Let’s round!

            232 is closer to 200~431 is closer to 400~298 is closer to 300~501 is closer to 500

What does this mean?

            If compute 200+400+300+500, we get 1,400.

Now, let’s compute the numbers before we rounded.


There is only a difference of 62 between our rounded answer and our actual answer. Therefore, our answer is reasonable. If we would have made a computation error and our answer was 1,862, we would know right away that we made an error somewhere.


How does this help us with shopping?

What if we are shopping for new clothes and we only have $100 to spend? Estimating can help us to quickly decide what items we are going to buy and what items we are going to put back. Let’s say we are shopping and we find 8 pieces of clothing. The prices of these items are: $23.99, $12.99, $9.99, $14.99, $22.00, $22.99, $6.99, and $4.00. Instead of getting out a calculator, we can use estimation to quickly decide if we can afford to buy everything or if we need to put some items back.

After rounding each item, we have: $24, $13, $10, $15, $22, $23, $7 and $4. If we add these together, we get $118. This means we do not have enough money to purchase all eight items and we need to decrease our amount by about $18.



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