Information on calculating the deemed payment is included in 'Supplying Services through a limited company or partnership – A general guide to IR35'. This guide tells you what happens when the IR35 legislation applies and includes an example of a deemed payment calculation.
More detailed information on how to calculate the IR35 deemed payment is contained in the Employment Status Manual (ESM 3140 onwards).
A number of specific issues are also dealt with in the other Frequently Asked Questions on this website.
When the IR35 legislation applies, then tax and NICs must be paid on a minimum amount of salary. This may be either in the form of actual payments during the year or as a notional payment deemed to have been made to the worker by the intermediary. Guidance on how to work out the deemed payment is at Employment Status Manual (ESM 3140 ) but a spreadsheet has now been devised based on the template at ESM 3146 which will do this calculation automatically.