Tax codes that are applied on a cumulative basis means that tax calculations look at the entire tax year when performing the tax calculation. Using a tax code on a cumulative basis means that every payday, the calculation performed is to work out the tax due on an employee’s earnings for the (tax) year to date then deduct from it the tax they have already paid on their earnings that (tax) year. The remaining figure is the tax due for the pay period.
A Cumulative Tax Code allows for an individual’s weekly / monthly Tax Free Allowance to be carried forward if it is not used. As an example – if an individual were to have a break from work (for example, due to unpaid leave or sickness etc), when they resume, it is often the case that they will pay little or no Tax until they have caught up with their Tax Free Allowances.
A non-cumulative tax code would be signified by an “X” or “W1/M1″ following the code. In these cases the tax would be worked out purely on the taxable pay for each individual pay period. Each payday is treated as if it is the first week or month of the tax year. Previous pay and tax details are ignored.