To calculate a percentage of a value: `=A1*10%`
To find what percentage one value is of another: `=A1/B1`
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 format a cell as percentage: select the cell, then press `Ctrl+Shift+%`
To show a result as a percentage, multiply the formula by 100 only if the cell is not formatted as percentage: `=(A1/B1)*100`
To copy a percentage formula down a column: drag the fill handle or double-click it
To keep a reference fixed in a percentage formula: use absolute references like `=$A$1/B1`