National Minimum Wage Entitlement
First you need to check that you are eligible to receive the National Minimum. If you are over the compulsory school age, but under 25 and you are a worker/employee/apprentice it is likely that you will be entitled to receive the National Minimum Wage. There are 3 different hourly rates of pay you can receive according to your age and a set minimum wage if you are an apprentice.
If you are 25+ you are entitled to receive the National Living Wage, which is paid at a slightly higher rate. This should not be confused with the Living Wage, as set by the Living Wage Foundation and not legally binding.
Second you need to calculate your hourly rate of pay. This is done by dividing the total sum you are paid in your relevant pay period by the total number of hours worked or deemed to have been worked over the pay reference period. The detail of the calculation depends partly on the type of worker you are.
There are four types of work: Salaried hours, time work, output work and unmeasured work. If you are a salaried worker, the calculation is straight forward as you would divide your contracted working hours by the amount of pay you receive. For the other types of work the calculation can be more difficult.
If your hourly rate is below the NMW/NLW, you should raise the matter with your employer. If you remain unsatisfied after exhausting your internal processes, you can either raise the issue through HMRC or alternatively through an Employment Tribunal.