Adjusted net income is the total taxable income, before any personal allowances and less certain tax reliefs.
Guidance on how to calculate this can be found here:
Personal Allowances: adjusted net income
It is not necessary to calculate your adjusted net income when completing your Self Assessment tax return. Self Assessment will do this for you, as it is based on the entries on the tax return.
On SA102 or the employment section of the online form, you enter the pay figure stated on your P60/P45, not forgetting tax deducted. The P60/P45 pay figure is your earnings after payments to your pension scheme have been removed. You should also include benefits provided by your employer on a P11d.
To claim higher rate tax relief on your pension payments, you enter the amount of payments made on page 4 of SA100, box TR4.1. You also enter gifts made to charity.
In the other sections of the tax return, you report any other sources of taxable income.
Some entries on your tax return will impact the tax code for the next year.