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How can I work out how much a side-job contracting will be taxed for?
Being self-employed, your "profit" is calculated as all the bills you send out, minus all business-related cost that you have (you will need a receipt for everything, and there are different rules for things that last for long time, long tools, machinery).  You can file your taxes yourself - the HRS website will tell you how to, and you can do it online. It's close to the same as your normal online tax return. Only thing is that you must keep receipts for all the cost that you claim.  Your tax: Assuming your gross salary is £25,000 and your profits are about £10,000, you will be paying 8% for national insurance, and 20% income tax. If you go above £43,000 or thereabouts, you pay 40% income tax on any income above that threshold, instead of 20%, but your national insurance payments stop.