You will pay Income Tax based on your Net Profit. Net Profit is the difference between your Total Income and your Total Allowable expenses. This is reflected on your Self-Assessment Summary page.
2016-2017 Tax rate
These rates assume that you have a full personal allowance, which is not always the case if you have things such as employment benefits (your personal allowance is the first bit of income that is not taxed- the standard tax free personal allowance is £11,500.
Sample Computation:Net Profit is £24,000It falls under the 20% bracket, so:(£24,000 - £11,501) * 20%£12, 499 * 20%Income Tax: £2, 499.8
Net Profit is £50,000It falls under the 40% bracket, so:(£50,000 - £45,001) * 40%£4,999 * 40%Income Tax: £1,999.6