How the AcciVell UK Tax Calculator Works
Learn how the AcciVell UK self-employed tax calculator estimates Income Tax and Class 4 National Insurance. See how tax bands, Personal Allowance and the £100,000 allowance taper affect your estimated tax and Net Income.
Key takeaway: The calculator uses official UK tax thresholds and rates to estimate Income Tax, Class 4 National Insurance, and your estimated Net Income.
How allowable business expenses affect your tax
If you are self-employed, your tax is generally calculated based on your business profit rather than your total business income.
The calculator estimates your profit by subtracting your allowable business expenses from your business income:
Business income − Allowable business expenses = Business profit
Your estimated business profit is then used to calculate Income Tax and Class 4 National Insurance.
Only expenses that qualify as allowable business expenses should be entered into the calculator. The treatment of expenses can depend on your individual circumstances.
Official guidance: HMRC: Expenses if you're self-employed
How your tax is calculated
AcciVell converts your business numbers into clear tax liabilities in simple steps.
Business Income
Total revenue from self-employment
Allowable Expenses
Deducted from gross income
Taxable Profit
Basis for tax calculation
Income Tax
Progressive bands (20%, 40%, 45%) with Personal Allowance.
Class 4 NI
Calculated on profit bands above threshold (6% & 2%).
Income Tax — how it is calculated
Income Tax in the UK is calculated using a progressive system. This means that different portions of your profit are taxed at different rates, depending on the official tax bands.
- Personal Allowance: up to £12,570 of income may be tax-free, depending on your circumstances.
- Basic Rate (20%): applies to taxable income within the basic rate band.
- Higher Rate (40%): applies to taxable income above the basic rate band.
- Additional Rate (45%): applies to taxable income above £125,140.
If your adjusted net income exceeds £100,000, your Personal Allowance is reduced. For every £2 over £100,000, your allowance decreases by £1. This creates a higher effective tax rate between £100,000 and £125,140.
Official source: HMRC Income Tax rates and allowances
Class 4 National Insurance - how it is calculated
If you are self-employed, you also pay Class 4 National Insurance contributions. These are calculated on your annual profit and are not affected by the Personal Allowance.
- 0% on profits up to £12,570
- 6% on profits from £12,571 to £50,270
- 2% on profits above £50,270
Official source: HMRC Rates and allowances: National Insurance contributions HMRC National Insurance rates and categories
Understanding your results
After entering your income and expenses, the calculator displays your results using charts and a detailed table.
- Pie Chart - shows how your total profit is split between Income Tax, National Insurance, and Net Income.
- Bar Chart - compares your Profit, total taxes, and Net Income side by side.
- Detailed table - breaks down each tax band, the taxable amount, and how much tax is due at each rate.
Why this matters for self-employed professionals
Understanding how tax and National Insurance are calculated helps you plan your finances, avoid surprises, and make informed decisions throughout the tax year.
This calculator is designed to support self-employed individuals (Class 4) by turning complex tax rules into clear numbers and visual insights.
These calculations are estimates based on current UK tax thresholds and rates. This tool does not replace an official HMRC calculation or professional tax advice.
Ready to estimate your UK self-employed tax?
Enter your income and allowable business expenses to see your estimated Income Tax, Class 4 National Insurance, Net Income, and tax breakdown.
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