Rule for sig figs in addition/subtraction?

Match the decimal place of the least precise measurement: your final answer should have the same number of digits after the decimal point as the measurement with the fewest digits after the decimal.

Think of significant figures in addition and subtraction like lining up numbers by their decimal points and then deciding how far to keep your answer based on the least precise number. If one measurement only goes to the tenths place and another goes to the hundredths place, you can only trust the tenths place in the result. So you add or subtract normally, but when you round the final number you keep digits only up to the same decimal place as the least precise measurement. This is different from multiplication and division where you count total significant digits, so be careful not to mix those rules.

A common mistake is rounding each number before you add or subtract, which can give wrong results; instead, add first and then round the final answer to the correct decimal place. Another mistake is matching the number of significant digits instead of matching the decimal place, which confuses the rule for addition and subtraction with the rule for multiplication and division; remember to look at digits after the decimal point for addition and subtraction. A short helpful tip is to write numbers in a column with decimal points aligned and underline the least precise decimal place so you know where to round the final sum.

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Significant Figures Rule for Addition/Subtraction

1. Line up decimal points
2. Use least precise decimal place
3. Add or subtract normally
4. Round final answer afterward
5. Do not mix with multiplication rules

Common Mistakes with Sig Figs in Addition

1. Rounding before adding
2. Using significant digits instead of decimals
3. Confusing with multiplication/division rules

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