Estimation and Rounding | Second Grade
What is Estimation and Rounding?
Concept: Estimation and rounding help us find approximate answers quickly! Instead of exact numbers, we use nearby numbers that are easier to work with.
Why Do We Estimate?
✓ To make mental math easier
✓ To check if our answer is reasonable
✓ To quickly understand "about how many"
✓ To make better guesses
Understanding "About"
38
Exact number
≈
40
About/Approximately
38 is about 40!
💡 Tip: Estimation gives us a "close enough" answer, not an exact answer!
1. Estimate to the Nearest Ten
Concept: When we estimate to the nearest ten, we find the closest multiple of 10. Think: "Which ten is this number closest to?"
Number Line Method
Example: Estimate 34 to the nearest ten
30
34
40
34 is closer to 30!
34 ≈ 30
Quick Estimation Tips
Ones: 0, 1, 2, 3, 4
⬇️ Round DOWN
To the lower ten
23 → 20
44 → 40
Ones: 5, 6, 7, 8, 9
⬆️ Round UP
To the higher ten
27 → 30
48 → 50
More Examples:
12 ≈ 10
2 < 5, round down
56 ≈ 60
6 ≥ 5, round up
41 ≈ 40
1 < 5, round down
78 ≈ 80
8 ≥ 5, round up
💡 Tip: Look at the ones digit! If it's 5 or more, round up. If it's less than 5, round down!
2. Round to the Nearest Ten
Concept: Rounding is the exact process of finding the nearest ten. We follow specific rules to round numbers up or down!
The Rounding Rule
If ones digit is 0-4 → Round DOWN
If ones digit is 5-9 → Round UP
Formula: Look at the ONES digit!
📝 Step-by-Step Rounding Process
Example: Round 67 to the nearest ten
Step 1: Identify the ones digit
67 → The ones digit is 7
Step 2: Check the rule
Is 7 ≥ 5? YES!
So we round UP ⬆️
Step 3: Find the next ten
67 is between 60 and 70
Round up to the next ten = 70
67 rounds to 70
Rounding Chart
| Original Number | Ones Digit | Round to |
|---|---|---|
| 23 | 3 (< 5) | 20 |
| 35 | 5 (≥ 5) | 40 |
| 51 | 1 (< 5) | 50 |
| 89 | 9 (≥ 5) | 90 |
Special Cases:
Numbers ending in 0
30 → 30
Already a ten!
Numbers ending in 5
45 → 50
5 rounds up!
💡 Tip: Remember - 5 is the "magic number" that decides whether to round up or down!
3. Estimate Sums
Concept: To estimate a sum, we round each number first, then add the rounded numbers. This gives us an approximate answer!
Sum Estimation Process
Step 1: Round each number to the nearest ten
⬇️
Step 2: Add the rounded numbers
⬇️
Get the estimated sum!
📝 Example 1: Estimate 23 + 48
Original
23 + 48
→
Rounded
20 + 50
Step 1: Round each number
• 23 → 20 (3 < 5, round down)
• 48 → 50 (8 ≥ 5, round up)
Step 2: Add the rounded numbers
20 + 50 = 70
Estimated Sum: 70
(Actual: 23 + 48 = 71)
📝 Example 2: Estimate 36 + 52
Step 1: Round each number
• 36 → 40 (6 ≥ 5, round up)
• 52 → 50 (2 < 5, round down)
Step 2: Add the rounded numbers
40 + 50 = 90
Estimated Sum: 90
(Actual: 36 + 52 = 88)
More Sum Estimation Practice:
14 + 27
↓ Round ↓
10 + 30
≈ 40
43 + 38
↓ Round ↓
40 + 40
≈ 80
67 + 24
↓ Round ↓
70 + 20
≈ 90
Why Estimate Sums?
✓ Quick mental math - no pencil needed!
✓ Check if your exact answer is reasonable
✓ Solve real-world problems faster
✓ Understand "about how much" situations
💡 Tip: Estimating first helps you catch mistakes in your exact calculations!
Important Estimation & Rounding Formulas
Rounding Rule
If ones digit: 0, 1, 2, 3, 4
→ Round DOWN
If ones digit: 5, 6, 7, 8, 9
→ Round UP
Estimation Symbol
\( \approx \) means "approximately"
or "about"
34 ≈ 30
"34 is about 30"
Sum Estimation Formula
1. Round each number
2. Add rounded numbers
Example: 23 + 48
→ 20 + 50 = 70
🎯 Tips for Estimation & Rounding Success 🎯
- ✓ Always look at the ONES digit to decide rounding direction
- ✓ Remember: 5 is the magic number - it rounds UP!
- ✓ Use number lines to visualize which ten is closer
- ✓ Estimate first, then calculate exact answer to check
- ✓ Practice with real-world examples like money or ages
- ✓ Estimation is about being "close enough" - not exact!
⭐ You're an Estimation & Rounding Champion! ⭐
Outstanding work mastering estimation and rounding! You now know how to estimate to the nearest ten, round numbers using the proper rules, and estimate sums by rounding first then adding. These skills help you think about numbers flexibly and solve problems quickly in your head. Estimation is a real-world superpower - you'll use it when shopping, telling time, measuring, and in everyday situations where you need a quick answer! Understanding that 5 is the key number for rounding decisions shows excellent number sense. Keep practicing these mental math strategies - you're building tools that will help you for your entire life. You're doing absolutely phenomenal work!