izharshah.com/tools/self-assessment-tax-calculator · The Parallel Operator · Self assessment tax calculator · Tax year 2026/27, checked against GOV.UK 20 September 2026
Self assessment: what you actually owe
Enter your takings and expenses and see the whole bill: income tax, Class 4 and Class 2 National Insurance, and the payments on account that catch most people out in their second year. It handles a job alongside the self-employment, because that is how most people start.
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Unlock the full breakdown: every band of income tax and Class 4 shown separately, the trading allowance compared against your expenses, and exactly what to pay on each of the two dates. Save or print it as a PDF to keep.
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Your full breakdown
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How this calculator works
Your taxable profit is takings minus allowable expenses. If your expenses are under £1,000 you can instead claim the £1,000 trading allowance and deduct that, which is usually better. The calculator picks whichever leaves you better off and shows the tax it actually saves, not just the reduction in profit.
The allowance has a catch worth knowing. You cannot use it against income from your own employer, your spouse or civil partner's employer, or a company or partnership you are connected to. Freelancing for the firm that already employs you is exactly the case it excludes, so there is a tick box above; tick it and the calculator uses your expenses instead.
Income tax is charged on all your income together, not on the profit in isolation. If you have a job as well, the salary uses up your personal allowance and the profit stacks on top, which is why a modest side income can be taxed at 40% even though the profit alone looks small. Enter the job income and the tax already deducted and the page works out only what is left to pay.
National Insurance comes in two pieces. Class 4 is 6% of profit between £12,570 and £50,270 and 2% above. Class 2 is treated as paid once profit reaches £7,105, which protects your National Insurance record without you paying anything. Below that you can pay it voluntarily at £3.65 a week, but only bother if the year would not otherwise count. If you have a job paying National Insurance, the year already counts and the voluntary payment buys nothing. Class 4 also stops once you reach State Pension age, hence the tick box.
Payments on account are what catch people in year two. If you owe more than £1,000 through self assessment, and less than 80% of your tax was collected at source, you pay next year's tax in two instalments on account, each half this year's bill. That makes the first January after a profitable first year one and a half times the bill you were expecting. The table above splits it by date so there is no surprise.
What it does not cover: capital allowances, losses carried forward, partnerships, limited companies, VAT, the High Income Child Benefit Charge, student loan repayments through self assessment, dividend or savings income, rental income, pension contributions, Gift Aid, or the marriage allowance. Deadline dates are worked out from the tax year rather than quoted, because GOV.UK has not yet published the 2026/27 ones.
Questions people ask about self assessment
How much tax will I pay as a sole trader?
Income tax on your profit at the usual rates once your personal allowance is used, plus Class 4 National Insurance at 6% between £12,570 and £50,270 and 2% above. On £62,000 of profit with no other income that is £12,232 of income tax and £2,496.60 of Class 4.
How much can I earn self-employed before paying tax?
With no other income, £12,570 of profit, the personal allowance. Below £1,000 of takings you need not even report it, thanks to the trading allowance. If you have a job, your salary uses the allowance first and the profit is taxed from the first pound.
What are payments on account and why is my January bill so big?
If you owe more than £1,000 through self assessment, HMRC asks for next year's tax in advance in two halves. The first is due with your balancing payment, so a first profitable year produces a January bill of one and a half times the tax you expected.
Do I have to pay Class 2 National Insurance?
Not as a payment. Once your profit reaches £7,105 it is treated as paid and your record is protected. Below that you can pay voluntarily at £3.65 a week, but it only helps if the year is not already a qualifying year. If you also have a job paying National Insurance, it is not.
When is the self assessment deadline?
For the 2026/27 tax year, 31 January 2028 for filing online and paying, with the second payment on account due 31 July 2028. Paper returns are due by 31 October 2027. Those dates follow the usual rule; GOV.UK has not published them yet.
Related tools
Guidance only, not tax advice. It assumes a sole trader with a standard personal allowance and no capital allowances, losses brought forward, or other reliefs. If your affairs are more involved than takings minus expenses, get an accountant.
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