When students start working with significant figures, addition and subtraction often feel confusing. That’s because these operations do NOT follow the same rules as multiplication and division. Once you understand the core idea, it actually becomes very easy.
Let’s break it down clearly, step by step.
What Is the Rule for Significant Figures in Addition and Subtraction?
In addition and subtraction, the final answer is rounded based on decimal places — not the number of significant figures.
👉 The result must have the same number of decimal places as the number with the fewest decimal places.
This is the golden rule.
Why Decimal Places Matter Here
Addition and subtraction depend on place value alignment. If one number is less precise (fewer decimal places), your final answer cannot be more precise than that number.
Step-by-Step Method (Always Follow This)
- Write all numbers clearly
- Perform the calculation without rounding
- Count decimal places in each number
- Find the least number of decimal places
- Round the final answer to that decimal place
Worked Examples (With Explanation)
Example 1
12.45 + 3.2
- 12.45 → 2 decimal places
- 3.2 → 1 decimal place
Least decimal places = 1
Calculation:
12.45 + 3.2 = 15.65
Rounded answer:
15.6
Example 2
5.678 − 0.34
- 5.678 → 3 decimal places
- 0.34 → 2 decimal places
Least decimal places = 2
Calculation:
5.678 − 0.34 = 5.338
Rounded answer:
5.34
Example 3
100.0 + 23.456
- 100.0 → 1 decimal place
- 23.456 → 3 decimal places
Least decimal places = 1
Calculation:
100.0 + 23.456 = 123.456
Rounded answer:
123.5
Important Exam Tip
⚠️ Do not round individual numbers first.
Always round only the final answer. Early rounding is one of the biggest reasons students lose marks.
Common Student Mistakes
- Counting significant figures instead of decimal places
- Rounding before completing the calculation
- Forgetting that whole numbers can limit precision
- Mixing rules of multiplication/division with addition/subtraction
Quick Practice Questions
Try these yourself:
- 6.2 + 1.45
- 18.003 − 2.1
- 0.56 + 4.789
FAQs
What are the rules for sig figs when adding and subtracting?
When adding or subtracting, the result must be rounded to the same number of decimal places as the number with the fewest decimal places.
You do not count total significant figures — only decimal places matter.
What is the rule for determining significant figures when adding or subtracting?
First complete the calculation without rounding.
Then identify the number with the least number of decimal places and round the final answer to match that level of precision.
What is 363.75 to 2 significant figures?
Step 1: First two significant digits → 3 and 6
Step 2: Look at the next digit (3)
Since 3 is less than 5, the number stays the same.
Answer: 360
How do you write 57.3997 correctly to 4 significant figures?
Step 1: First four significant digits → 5, 7, 3, 9
Step 2: Look at the next digit (9)
Since 9 is greater than or equal to 5, round up.
Answer: 57.40
(The zero is included to show four significant figures.)