The question arrives the same way for almost everyone. You do some freelance work, or start selling something, or pick up a second job, and the first payment lands. Then it occurs to you that nobody has taken any tax off it.
SideIncomeTax was built for that moment.
Who this is for
People with income outside a single PAYE job: freelancers, sole traders, people running a small side business alongside employment, and anyone with a second employment. It is aimed at the point before you have an accountant, when what you need is a realistic sense of how much to set aside.
How the calculation works
Side income does not get taxed in isolation. It stacks on top of your existing income, which means the rate you pay depends on what you already earn. Someone earning 20,000 pounds sees a very different marginal rate on an extra 5,000 than someone already earning 48,000, and the second person may cross into higher rate part way through.
The calculator therefore asks for both your existing income and your side income, applies your Personal Allowance, works out Income Tax across the bands, and adds Class 4 National Insurance on self-employed profits where it applies. It also accounts for the trading allowance, which means a small amount of casual income may not need reporting at all.
Scotland is different
Scotland has set its own Income Tax bands since 2017 and now operates six bands where the rest of the UK has three. National Insurance remains UK-wide. If you are a Scottish taxpayer, the calculator applies the Scottish bands, because using rest-of-UK rates would produce a materially wrong figure.
Who publishes this site
SideIncomeTax is built and maintained by CJ Software Ltd, a UK software company that develops a small family of free, single-purpose calculators. We are a software business, not a firm of accountants, and we say so plainly because it matters when you decide how much weight to give a number on a screen.
The site is funded by advertising. Advertisers have no influence over the calculations, the guidance, or what we choose to write about.
What it does not cover
It gives an estimate of tax due on income. It does not handle allowable business expenses in any detail, capital allowances, VAT registration once you approach the threshold, IR35 status for contractors, or the mechanics of Making Tax Digital. Expenses in particular can change the answer a great deal, and they are specific to what you actually do.
Use the figure to decide what to set aside, not as a substitute for a Self Assessment return. HMRC publishes detailed guidance, and once your side income becomes meaningful an accountant usually pays for themselves. If a figure here looks wrong, please tell us.