How many significant figures in 1.008?

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Four significant figures.

One point zero zero eight has four significant figures because every nonzero digit counts and zeros between or after nonzero digits to the right of the decimal point also count. The one is significant, the two zeros after the decimal are significant because they are between the one and the eight, and the eight is significant, so you count all four digits.

Think of significant figures like counting the important digits that show how precise a number is, like measuring with a ruler that has small marks; if you write one point zero zero eight you are saying you measured precisely enough to show those three digits after the decimal and none of them are just placeholders. Students sometimes think the zeros are never significant, but zeros after the decimal and after a nonzero digit usually are significant; another common mistake is forgetting that a trailing zero without a decimal is ambiguous, so you should use the decimal point or scientific notation to be clear. If you ever are unsure about zeros, rewrite the number in scientific notation or include the decimal so everyone knows which zeros are meaningful.

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