Grade 3 (age 8-9)

Rounding Numbers

Rounding means showing about how much something is with a nearby number instead of the exact value. We use it all the time in daily life, like guessing roughly how much the groceries will cost. There are three ways to estimate, rounding up, rounding down, and rounding to the nearest, where a ones digit below 5 goes down and 5 or more goes up. On this page you can move the number to see which side it is closer to on the number line, then compare all three results as bars.

What is Rounding?

🎯 Estimate Roughly

①Rounding = nearby number instead of exact

②Used at the mart for rough cost

③3 methods: ceil, floor, round

Rounding on a Number Line

Number37

💡 Round Threshold is 5

①Ones digit 0,1,2,3,4 → floor

②Ones digit 5,6,7,8,9 → ceil

③5 is the middle — round up from 5

Three Methods Compared

📊 When to Use Which?

①Ceil: when "must not be short" (supplies)

②Floor: when "extra is OK" (allowance)

③Round: when "about how many" (rough count)

Summary

Rounding

≥5 → ceil, <5 → floor

Look at digit just below the round place

Ceiling

37 → 40 (always up)

Even a little → always go up

Floor

37 → 30 (always down)

Just zero out the digits below

🎯 Key Points

①Round threshold = 5 ("≥5 ceil, <5 floor")

②Ceil always up, floor always down

③Choose method based on real-life situation

④Round = whichever is closer on number line

⑤Express rounded values with "about"

Examples and Unit Test

Example 1

What is 37 rounded to the nearest ten?

Rounding looks at the ones digit. The ones digit of 37 is 7.

7 is 5 or more, so round up to 40.

▸ 40

If the ones digit is 5 or more, round up; if less than 5, round down.

Example 2

What is 42 rounded to the nearest ten?

The ones digit of 42 is 2.

2 is less than 5, so round down to 40.

▸ 40

37 and 42 both round to 40, but one rounds up and the other down.

Unit Test

What is 53 rounded UP (ceiling) to the nearest ten?

①50

②53

③60

④40

▸ ③ 60

Ceiling always goes up if there is anything in the ones place.

Since 53 has a 3 in the ones place, ceiling gives 60.

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