Quick answer: back pay = (new rate - old rate) x hours worked since the raise took effect. A raise from 21.00 to 22.40 backdated 10 weeks at 40 hours is 1.40 x 400 = 560.
Overtime hours in the back period
If some of those hours were overtime, they were paid at 1.5 x the old rate and are now owed at 1.5 x the new one, so the difference on each overtime hour is 1.5 x (new - old). Add overtime hours x 1.5 to the hours field to include them.
Back pay for unpaid wages
The same arithmetic applies when wages were simply underpaid, for example missed overtime: hours short x the rate that should have been paid. The Department of Labor's Wage and Hour Division handles back wage claims for federal minimum wage and overtime.
Starting or leaving mid-period instead? Use the prorated salary calculator.
Formula and examples checked . Found a problem? Tell us.
Questions
Is back pay the same as retro pay?
People use both terms. Retro pay usually means a rate change applied to past hours; back pay can also mean any wages owed but not paid.
Is back pay taxed?
Yes, it is wages and is taxed when paid. This site has no tax tools; check with payroll or a tax adviser.