Method 1:
Start here:
https://www.gov.uk/calculate-your-holiday-entitlement
Method 2 (which is how most employment agencies work):
Simply use a multiplier, calculated as follows:
Someone who works a regular 5 day week is theoretically available for work on 52 x 5 = 260 days per year. (A pedant might argue that the figure should be 261 days, because there are 52 weeks and 1 day in a year, but 260 is normally used for the calculation). If they're given the minimum statutory leave of 28 days, that means that they actually work 232 days 'on' and 28 'off'. [i]So each hour worked gains 28 ÷ 232 hours = 7.2414 minutes of holiday.[i]