Rounding and Estimation

What Is Rounding?#

Rounding means finding the nearest 10, 100, or other round number that is closest to your original number. Rounded numbers are easier to work with and very useful in everyday life.

Why Do We Round?#

Rounding is used constantly in real life:

• A movie lasts about 2 hours (not 1 hour, 58 minutes, 43 seconds)
• A school has about 300 students (not exactly 287)
• You need roughly **$10** for snacks (not exactly$10∗∗forsnacks(notexactly9.47)

Rounding makes numbers more manageable for mental math and quick estimates.

Rounding to the Nearest 10#

To round a number to the nearest 10:

1. Look at the ones digit (the digit in the ones place)
2. If it is 0, 1, 2, 3, or 4 → round down (keep the tens digit the same, change ones to 0)
3. If it is 5, 6, 7, 8, or 9 → round up (increase the tens digit by 1, change ones to 0)

Examples:

• 43 → ones digit is 3 (< 5) → round down → 40
• 67 → ones digit is 7 (≥ 5) → round up → 70
• 85 → ones digit is 5 (≥ 5) → round up → 90
• 31 → ones digit is 1 (< 5) → round down → 30

Rounding to the Nearest 100#

To round to the nearest 100:

1. Look at the tens digit
2. If it is 0, 1, 2, 3, or 4 → round down (keep hundreds digit, change tens and ones to 00)
3. If it is 5, 6, 7, 8, or 9 → round up (increase hundreds digit by 1, change tens and ones to 00)

Examples:

• 230 → tens digit is 3 (< 5) → round down → 200
• 780 → tens digit is 8 (≥ 5) → round up → 800
• 450 → tens digit is 5 (≥ 5) → round up → 500
• 124 → tens digit is 2 (< 5) → round down → 100

The Rounding Rhyme#

Here is a helpful rhyme to remember the rule: "Find your place, look next door. 5 or more — add one more. 4 or less — just rest."