For employees who work the same number of hours/days each week, the formula is simple: An employee must receive a minimum amount of paid holiday which is equivalent of 5.6 weeks of work.
So an employee who works 5 days per week must have 28 (5.6 x 5) days paid holiday, including all 'enforced' holidays (when the business is closed, such as Christmas Day). Similarly, an employee who works only 2 days per week is entitled to 11.2 (5.6 x 2) days paid holiday. The only exception is that the legal entitlement can't exceed 28 days. (So someone who works 6 days per week is not entitled to any more holiday than a colleague who works 5 days per week).
Where the pattern of work is irregular, holiday entitlement has to be allocated 'pro rata'. A full-time employee, working 5 days per week for 52 weeks of the year, will actually work on 232 (52x5 - 28) days, with 28 days holiday. That means that each day (or hour) of work gains 0.1207 of a day (or hour) of holiday.
So, if an employee works irregularly, the employee must keep track of the total number of hours worked, and then multiply that by 0.1207 to calculate the amount of holiday owing.
http://www.direct.gov...dholidays/DG_10034642
Chris