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Sig Fig Addition Calculator

Use this free sig fig addition calculator to add two numbers and automatically round the sum to the correct precision using the addition sig fig rule (fewest decimal places, not fewest sig figs).

Enter Values

First Number

First number to add

Second Number

Second number to add

Result

Enter values above and click Calculate to see your result.

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Formula

Round the sum to the least precise decimal place among the inputs

When adding, the result is rounded to the same decimal place as the measurement with the fewest decimal places (largest uncertainty), not the fewest significant figures.

Worked Example

Add 12.52 + 1.7 Step 1: 12.52 has 2 decimal places. 1.7 has 1 decimal place. Step 2: The limiting factor is 1 decimal place (from 1.7). Step 3: 12.52 + 1.7 = 14.22 Step 4: Round to 1 decimal place: 14.2 Result: 14.2

Sig Fig Rule for Addition

The sig fig rule for addition and subtraction is different from the rule for multiplication and division. Instead of counting significant figures, you look at the decimal place of each number. The result is rounded to the same decimal place as the measurement with the fewest decimal places.

Addition and subtraction use the "decimal places" rule, not the "sig figs" rule
150 + 2.45 = 152.45, rounded to the ones place = 152 (since 150 has no decimal places)
This can sometimes increase or decrease the number of significant figures in the result
The rule reflects the fact that you cannot know the sum more precisely than the least precise addend

Students frequently confuse the addition/subtraction rule with the multiplication/division rule. Remember: for addition and subtraction, think about decimal places. For multiplication and division, think about significant figures.

You can also calculate changes using our Sig Fig Subtraction Calculator, Sig Fig Multiplication Calculator or Sig Fig Rules Calculator.

Frequently Asked Questions

Why is the addition rule different from multiplication?

Because addition deals with absolute uncertainty (which decimal place is uncertain), while multiplication deals with relative uncertainty (what fraction of the value is uncertain). These are fundamentally different types of precision.

Can addition change the number of sig figs?

Yes. For example, 99.5 + 1.3 = 100.8, which has 4 sig figs even though both inputs had 3. The decimal place rule, not the sig fig count, determines precision in addition.

What about adding whole numbers?

Whole numbers without a decimal point are assumed to be precise to the ones place. So 150 + 2.3 rounds to the ones place: 152.

Is the subtraction rule the same?

Yes. Subtraction follows the exact same rule: round the result to the same decimal place as the input with the fewest decimal places.

How do I handle multi-step problems with both addition and multiplication?

Complete each type of operation and note the precision at each step. Apply the decimal places rule for addition/subtraction steps and the sig figs rule for multiplication/division steps. Only round the final answer.

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