How many significant digits in 1001?

Answer 1

4.

The idea of "SigFigs" is that "is the number rounded off?" If someone says that there are 1000 ounces in a bucket, you can figure that they could be rounding off to the nearest hundred, or even to the nearest thousand (it could be 700 and just look like 1000.) If they say there are 1001, you have more confidence that they are exact to all 4 digits.

1001 is not going to be a round-off of anything short of being rounded to the nearest unit, so all digits from the unit going left are significant.

Follow this rule. Going from the decimal point to the left, any zeros are NOT significant until you hit a non-zero. In this case, the first number you hit going left is the 1, so all digits are significant.

Note, any zeroes to the right of a decimal are all significant, since 25.00 gives more information than just 25. ■