To calculate a percentage of a value: `=A1*10%`
To find what percentage one value is of another: `=A1/B1`
To format the result as a percentage: select the cell and press `Ctrl+Shift+%`
To increase a value by a percentage: `=A1*(1+B1)`
To decrease a value by a percentage: `=A1*(1-B1)`
To calculate percentage change: `=(B1-A1)/A1`
To calculate percentage of total: `=A1/SUM($A$1:$A$10)`
To enter a percentage directly: type `25%`
To copy a percentage formula down a column: drag the fill handle
To keep a reference fixed in a formula: use `$`, such as `=$A$1*B1`