Significant Digits
The last digit in a measurement is always the uncertain digit(it could be one digit higher or lower, we don't know). The significant figure in the measurement includes all of the certain digits and the uncertain digits.

Significant Figure Rules
1. Leading zeros are not counted
Example: 0.01 = 1 SF
2. Trailing zeros after the decimal point are counted
Example 10.050 = 5 SF
3. Trailing zeros in front of the decimal point are not counted
Example  1200000000000 = 2 SF
4. Zeros between significant figures are counted
Example  123000.000123 = 12 SF

Rounding Rules
To start off, rounding just almost like we do it in math
1.Look at the digit to the right the the digit you are rounding (The uncertain digit)
2. If that digit > 5 then you round up
3. If that digit < 5 then you keep it the same
4. If that digit = 5 and it has more non-zero digits following behind it, round up
5. If that digit = 5 and ends at that 5, round to make digit your rounding even (2,4,6,8)

Math "+" & "-" Rules
When you add or subtract, round to the number with the fewest DECIMAL places
For Example
222.123123
+    1.1
223.223123
Which becomes 223.2 because the 1.1 had the lowest decimal point

Math "X" & "÷" Rules
When you multiply or divide, round to the number with the fewest number of significant figures
For Example
(123)(1)  =  123
Which becomes 100 because the number with the lowest significant figures (that 1) only had 1 SF.