Self Assessment Tax Return Calculator

Use this calculator to estimate how much Income Tax and Class 4 National Insurance you’ll owe on your self-employment profit. It’s designed for sole traders, freelancers, and contractors completing a Self Assessment tax return.

How This Calculator Works

Enter your self-employment profit — your income after allowable business expenses — and select the tax year. If you’re also employed, add your PAYE salary or other taxable income so the calculator applies the correct Income Tax band to your combined income. Class 4 National Insurance is calculated on your self-employment profit only, since employment income is handled separately through Class 1 NI deducted by your employer.

What’s Included

• Income Tax at the current Personal Allowance, basic, higher, and additional rates, including the Personal Allowance taper above £100,000.
• Class 4 National Insurance at 6% on profits between £12,570 and £50,270, and 2% above £50,270.
• A breakdown showing exactly how much of your income falls into each tax band.
• An estimated take-home figure and effective tax rate on your profit.

What’s Not Included

This calculator gives a straightforward estimate and doesn’t account for pension contributions, Gift Aid donations, the Marriage Allowance, student loan repayments, or Class 2 National Insurance. If any of these apply to you, your actual bill will differ from the estimate. For an accurate figure, see our Self Assessment guidance or get in touch directly.

Worked Examples

Example 1 — Self-employed only. A sole trader with £35,000 profit and no other income pays £4,486 Income Tax and £1,345.80 Class 4 NI — £5,831.80 total, leaving £29,168.20. Their effective rate on profit is 16.7%.

Example 2 — Self-employed plus PAYE salary. A contractor with £30,000 self-employment profit and a £15,000 PAYE salary from a second job has £45,000 combined taxable income. The calculator applies Income Tax across the full £45,000, but Class 4 NI only against the £30,000 self-employment profit — since Class 1 NI on the PAYE salary is already handled separately by that employer.

Example 3 — Payments on Account. If your Self Assessment bill exceeds £1,000 and less than 80% of your tax is deducted at source, HMRC usually requires two advance “payments on account” toward next year’s bill — each roughly half your previous year’s liability, due 31 January and 31 July. A £6,000 tax bill this year typically means paying £6,000 plus a £3,000 payment on account in January — £9,000 in total — with a second £3,000 payment due the following July.

Frequently Asked Questions

How accurate is this tax return calculator?

It gives a close estimate of Income Tax and Class 4 National Insurance based on current HMRC rates for England, Wales and Northern Ireland. It doesn’t account for pension contributions, Gift Aid, marriage allowance, student loan repayments, or other reliefs, so your actual bill may differ.

Does this calculator include Class 2 National Insurance?

No. Class 2 National Insurance no longer applies to most self-employed people with profits above the Small Profits Threshold. If your profit is below the threshold, you can choose to pay Class 2 voluntarily to protect your State Pension record — speak to us if you’re unsure whether this applies to you.

What if I’m both employed and self-employed?

Enter your self-employment profit in the first field and your PAYE salary or other taxable income in the second. The calculator uses your combined income to work out the correct Income Tax band, but calculates Class 4 NI on your self-employment profit only, since your employer already deducts Class 1 NI from your salary separately.

When is my Self Assessment tax bill due?

For the 2025/26 tax year, online returns and payment are due by 31 January 2027. Many self-employed people also need to make a “payment on account” toward the following year’s tax bill, due the same date and again by 31 July.

What are Payments on Account and will they apply to me?

If your tax bill is over £1,000 and less than 80% of your income is taxed at source, HMRC requires two advance payments toward next year’s bill — one on 31 January alongside your current bill, and one on 31 July — each roughly half your previous year’s liability.

How do the different Income Tax bands work?

Income up to your Personal Allowance (£12,570) is tax-free. The next portion up to £50,270 total income is taxed at 20% (basic rate), the portion between £50,270 and £125,140 at 40% (higher rate), and anything above £125,140 at 45% (additional rate). Your Personal Allowance also tapers away entirely once total income exceeds £125,140.

Does this calculator work for limited company directors?

No, this calculator is built for sole traders and self-employed profit taxed via Self Assessment. Limited company directors are taxed differently, through a combination of salary and dividends — see our limited company director tax guide for that calculation instead.

Get Help With Your Self Assessment

If your tax situation is more complex than a straightforward profit estimate, or you’d rather have an accountant handle your return, get in touch for a fixed-fee quote. See our pricing guide for typical costs.

Sources

• GOV.UK — Income Tax rates and Personal Allowances
• GOV.UK — National Insurance rates and categories (Class 4)
• GOV.UK — Understand your Self Assessment tax bill (Payments on Account)
• GOV.UK — Self Assessment tax returns (deadlines)

Tax rates and thresholds shown are for the 2025/26 and 2026/27 tax years and are checked against GOV.UK guidance. Rates are subject to change — always confirm current figures on GOV.UK or with your accountant before relying on this estimate.

About the Author

Written and reviewed by Shamayun Chowdhury
Senior Accountant at Major Accountancy and Lecturer in Accounting at Nottingham Trent University. CIMA qualified, with 15+ years of UK practice experience supporting sole traders, contractors, and small companies with Self Assessment and tax planning.

Last reviewed: August 2026