To determine the number of significant figures in any value in science, follow this rule:
DOT LEFT, NOT RIGHT
1.
2.
If the number is written with a visible decimal point, then go to the left end of the value. Moving right, find the first non zero digit, count it and every other digit to the right of the value. (DOT LEFT).
If the number doesn’t have a decimal point, do exactly the same thing in the other direction, right to left (NOT RIGHT).
The examples that follow illustrate a complete set of number types normally encountered in scientific measurements, and show how the determination of significant figures is made in accordance with the proposed rule. (Note: marking a zero as "0" specifies its significance, and it is considered a non-zero here).
• 0.00540700 - This value has a visible decimal point. The student should go to the left end and move right to the first non-zero digit (the 5), count that digit and all the digits moving to the right end: 5, 4, 0, 7, 0, 0. There are six significant figures in this value.
• 25.0070 - This value also has a visible decimal point. The student should go to the left end and move right to the first non-zero digit (the 2), count this digit and every digit to the right end: 2,
5, 0, 0, 7, 0. There are six significant figures.
• 4500.0 - Same as above. Moving right, the first non zero digit is the 4, and counting to the right:
4, 5, 0, 0, 0. There are five significant digits.
• 350000 - This value has no visible decimal point. Therefore, the students should go to the right end and move left to the first non-zero digit (the 5), count that digit and all digits moving to the left: 5, 3. There are two significant figures.
• 350000 - This value also has no visible decimal point. The student should go to the right end and move left to the first non-zero digit (the 0), count that digit and all digits moving to the left: 0,
0, 5, 3. There are four significant figures.
• 2500. (same as 2500) - This value has a visible decimal point. So, the student goes to the left end and moves right to the first non-zero digit (the 2), counts to the end moving right: 2, 5, 0, 0.
There are four significant figures. (In the case of 2500, since there is no visible decimal point, counting is to the left starting with the first non-zero digit [the 0]: 0, 0, 5, 2; also yielding four significant figures.