Sole trader tax is calculated on taxable profit after allowable business expenses, not on total turnover. If you are entering your revenue figure into the calculator and the result feels impossibly high, the missing step is almost always expenses.
Updated 2026/27 · SoleTraderTaxCalculator.co.uk · Editorial standards · Methodology
Turnover is the total amount your business earns from clients or sales before any costs are deducted. Taxable profit is what remains after subtracting allowable business expenses from that turnover. These two figures can differ substantially, and HMRC calculates income tax and Class 4 NI on profit, not on turnover.
Example: a freelance consultant earns £60,000 in fees but spends £14,000 on legitimate business costs — software, insurance, professional fees, mileage. Taxable profit is £46,000. The income tax and NI calculation starts from £46,000, not £60,000. The difference in tax is meaningful: roughly £5,600 less in combined income tax and Class 4 NI at basic rate.
Many people overestimate their tax exposure by entering turnover rather than profit. Others underestimate it by forgetting to account for expenses at all. Neither approach produces a useful reserve figure.
An expense is allowable if it is incurred wholly and exclusively for business purposes. The key test is whether the cost is genuinely for running and earning from your business. HMRC's detailed guidance on allowable expenses for the self-employed is published on GOV.UK.
Typical allowable costs include: accountancy and bookkeeping fees; business insurance; professional subscriptions and membership fees directly related to your trade; software, online tools and subscriptions used for work; a proportionate share of your phone bill where the phone is used for business; travel costs directly related to visiting clients or business locations (not commuting from home); equipment and tools used in the course of your trade; advertising and marketing; bank charges on a business account; and professional development directly related to maintaining existing skills.
Home costs require care. If you work from home, you can claim a proportion of household running costs based on the number of rooms used for work, the hours spent working, or by using HMRC's simplified flat-rate method. The flat-rate method avoids detailed record-keeping of actual bills and is typically the more practical route for most sole traders.
Business mileage is often under-claimed. If you use your own car for business travel — visiting clients, travelling to a temporary workplace, or making business-related journeys — you can claim HMRC's approved mileage rate: 45p per mile for the first 10,000 business miles in the tax year, then 25p per mile above that. Keep a mileage log with dates, destinations and purpose.
Professional development is another frequently missed category. Training that maintains or updates skills directly used in your existing trade is generally allowable. Books, professional journals and industry-specific courses fall into this category, provided they are for your current work rather than training for a new career.
Bad debts written off as genuinely unrecoverable can be claimed in the year they are written off. If a client has consistently failed to pay despite reasonable attempts to collect, the amount is a legitimate business loss.
Pension contributions made by sole traders work slightly differently. Personal pension contributions reduce taxable income for income tax purposes but do not reduce the profit figure used to calculate Class 4 NI. The calculator handles this correctly via the pension input field.
Non-allowable costs include anything primarily personal in nature: a holiday that happens to include a client meeting, clothing that could be worn outside work, or a laptop bought mainly for personal use. If an expense has a mixed personal and business element, only the business proportion can be claimed — and you need to apply a reasonable split.
Capital expenditure (buying equipment, vehicles or property) is not deducted as an expense in the year of purchase in the same way as running costs. Instead, capital allowances may apply, which spread the deduction over time. For many sole traders, the Annual Investment Allowance covers equipment purchases entirely in the year of acquisition, but this needs to be handled correctly in your Self Assessment return.
Fines and penalties are not allowable. Neither are entertaining costs — business entertaining that would not otherwise be allowable as a personal deduction is specifically excluded.
Before entering a figure into this calculator, deduct your realistic annual expenses from your expected annual revenue. The result is your estimated taxable profit — that is the number that belongs in the profit field.
If you are comparing sole trader and limited company structures, keep the same underlying commercial assumptions. The allowable expenses will be broadly similar; what changes is how the profit is taxed and extracted.
If you are uncertain about your expenses, make a considered estimate. A rough but realistic figure is significantly more useful than entering turnover by mistake. As your bookkeeping becomes more current, update the estimate and your monthly reserve will improve accordingly.
For formal advice on what specific costs you can and cannot claim, consult a qualified accountant or refer to GOV.UK's guidance on expenses if you are self-employed.
Profit. The calculator estimates tax on taxable profit after allowable business expenses. Entering turnover will significantly overstate your tax bill.
Yes, for genuine business travel (not commuting). HMRC's approved rate is 45p per mile for the first 10,000 business miles per tax year, then 25p per mile. Keep a mileage log.
Yes. You can claim a proportion of household running costs or use HMRC's simplified flat-rate method. The flat rate avoids detailed records and is worth considering for straightforward home-working arrangements.
No. Expenses reduce your taxable profit, which in turn reduces your tax bill. At basic rate, each additional £1 of expenses saves around 26p in combined income tax and Class 4 NI (20% tax + 6% NI). At higher rates the saving is larger.
Good practice is to keep a record of all business expenses. HMRC can request records during an enquiry. Digital receipts stored in a bookkeeping app are usually sufficient — physical receipts are not required.
The sole trader tax calculator turns this guidance into a concrete monthly take-home and tax reserve estimate, based on 2026/27 HMRC rates. Enter taxable profit — not turnover.
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