Rounding

Rounding to the Nearest Ten

Imagine you have 29 marbles. To quickly tell a friend, you might say, "I have about 30 marbles." This is rounding!
We round numbers to:

Definition Rounding

Rounding means finding the closest ten or hundred to a number.

Method Rounding to the Nearest Ten

If the ones digit is 0, 1, 2, 3, or 4, change it to 0. This is called rounding down.
If the ones digit is 5, 6, 7, 8, or 9, change it to 0 and increase the tens digit by 1. This is called rounding up.

Example

Round 324 to the nearest ten.

The ones digit is \(\textcolor{colordef}{4}\lt 5\), so change it to 0. After rounding, \(324\) becomes \(320\).
\(324\) is between \(320\) and \(330\) but closer to \(320\).

Sometimes, we round numbers to make them easier to work with. For example:

Instead of saying "I have 372 marbles," you could say, "I have about 400 marbles."
Instead of saying "I have 324 marbles," you could say, "I have about 300 marbles."

This is rounding to the nearest hundred.

Method Rounding to the Nearest Hundred

If the digit in the tens column is 0, 1, 2, 3, or 4, replace the tens and ones digits with 0. This is called rounding down.
If the digit in the tens column is 5, 6, 7, 8, or 9, replace the tens and ones digits with 0 and add 1 to the hundreds digit. This is called rounding up.

Example

Round 324 to the nearest hundred.

Hundreds Tens Ones

\(3\) \(\textcolor{colordef}{2}\) \(4\)
The tens digit is \(\textcolor{colordef}{2}\lt 5\). Replace the tens and ones digits with 0:

Hundreds Tens Ones

\(3\) \(\textcolor{colordef}{0}\) \(\textcolor{colordef}{0}\)
After rounding, \(324\) becomes \(300\).
\(324\) is closer to \(300\) than \(400\).

Hundreds	Tens	Ones
\(3\)	\(\textcolor{colordef}{0}\)	\(\textcolor{colordef}{0}\)