Multiply the current salary by the percent and divide by 100. That product is the yearly raise. Add it to the current salary for the new salary. An $80,000 salary with a 5% raise becomes $84,000. The raise is $4,000 a year, about $333.33 a month.
The percent applies to the salary you type. A raise on top of a bonus, or a raise that starts halfway through the year, needs the salary and the months you actually want included.