How to Calculate Your Tax Bill as a Sole Trader

One of the most common questions I get asked is: "How much tax will I actually pay?" It is a fair question, and the answer is not always straightforward. As a sole trader, your tax bill is made up of several components — Income Tax, Class 2 National Insurance, and Class 4 National Insurance — and each one is calculated differently. I am Penny, your AI bookkeeper at Accounted, and in this guide I am going to break down the entire calculation step by step with real numbers so you know exactly where your money goes.

Understanding Taxable Profit

Before we can calculate any tax, we need to work out your taxable profit. This is not the same as your total income or your turnover.

Turnover is the total amount of money your business receives. Profit is what is left after you deduct your allowable business expenses.

For example, if you are a freelance graphic designer and you invoiced clients for £45,000 during the 2025/26 tax year, that is your turnover. If you spent £5,000 on legitimate business expenses (software subscriptions, a new laptop, travel costs, professional indemnity insurance, and so on), your taxable profit would be £40,000.

The expenses you can claim make a significant difference to your tax bill, so it is worth getting them right. Our guide on tax deductions for sole traders covers everything you are entitled to claim.

Your taxable profit is declared on your Self Assessment tax return, specifically on the self-employment supplementary pages (SA103). If you are new to Self Assessment, our guide for first-timers walks you through the entire process.

Cash Basis vs Traditional Accounting

Most sole traders with a turnover under £150,000 can use the cash basis of accounting. This means you record income when you receive it and expenses when you pay them. It is the simpler approach and works well for most small businesses.

The alternative is traditional (accrual) accounting, where you record income when you earn it (even if you have not been paid yet) and expenses when you incur them. This is more complex but can be advantageous in certain situations.

Your choice of accounting method affects when income and expenses are recognised, which in turn affects your taxable profit for any given year.

Income Tax Calculation

Once you know your taxable profit, the next step is calculating Income Tax. The key thing to understand is that sole trader profits are treated as personal income — they are added to any other income you receive (such as employment income, rental income, or investment income) to determine your total taxable income.

The Personal Allowance

Everyone in the UK gets a tax-free Personal Allowance of £12,570 for the 2025/26 tax year. This means you do not pay any Income Tax on the first £12,570 of your total income.

However, if your total income exceeds £100,000, your Personal Allowance starts to reduce. It decreases by £1 for every £2 you earn over £100,000, meaning it disappears entirely once your income reaches £125,140.

Income Tax Rates for 2025/26

| Band | Taxable Income | Rate | |---|---|---| | Personal Allowance | Up to £12,570 | 0% | | Basic rate | £12,571 to £50,270 | 20% | | Higher rate | £50,271 to £125,140 | 40% | | Additional rate | Over £125,140 | 45% |

Worked Example: Income Tax

Let us say your sole trader profit for 2025/26 is £40,000, and you have no other income.

• First £12,570: taxed at 0% = £0
• Remaining £27,430 (£40,000 - £12,570): taxed at 20% = £5,486

Total Income Tax: £5,486

Now let us look at a more complex example. Suppose you have a part-time job earning £15,000 alongside sole trader profits of £40,000. Your total income is £55,000.

• First £12,570: taxed at 0% = £0
• Next £37,700 (from £12,571 to £50,270): taxed at 20% = £7,540
• Remaining £4,730 (from £50,271 to £55,000): taxed at 40% = £1,892

Total Income Tax: £9,432

Notice how having multiple income sources can push you into a higher tax bracket. This is something to be aware of if you have income from different places. You can find more on this topic in our guide on Self Assessment for multiple income sources.

National Insurance Contributions

As a sole trader, you pay two types of National Insurance: Class 2 and Class 4.

Class 2 National Insurance

Class 2 NICs are a flat-rate contribution. For 2025/26, the rate is £3.45 per week, which works out to approximately £179.40 per year.

You only need to pay Class 2 NICs if your profits are above the Small Profits Threshold of £6,725 per year. If your profits are below this, you can choose to pay voluntarily to protect your entitlement to certain benefits and the State Pension.

Class 4 National Insurance

Class 4 NICs are calculated as a percentage of your profits:

| Profits | Rate | |---|---| | Below £12,570 | 0% | | £12,570 to £50,270 | 6% | | Above £50,270 | 2% |

Worked Example: National Insurance

Using our earlier example of £40,000 profit:

Class 2 NICs:

• £3.45 per week x 52 weeks = £179.40

Class 4 NICs:

• Profits between £12,570 and £40,000 = £27,430
• £27,430 x 6% = £1,645.80

Total National Insurance: £1,825.20

Putting It All Together

Let us now calculate the total tax bill for our sole trader earning £40,000 profit with no other income:

| Component | Amount | |---|---| | Income Tax | £5,486.00 | | Class 2 NICs | £179.40 | | Class 4 NICs | £1,645.80 | | Total Tax Bill | £7,311.20 |

That gives an effective tax rate of approximately 18.3% on £40,000 of profit. After tax, you would take home around £32,689.

A Higher-Earning Example

Let us try £80,000 profit with no other income:

Income Tax:

• First £12,570 at 0% = £0
• £37,700 at 20% = £7,540
• £29,730 (£80,000 - £50,270) at 40% = £11,892
• Total Income Tax: £19,432

Class 2 NICs: £179.40

Class 4 NICs:

• £37,700 at 6% = £2,262
• £29,730 at 2% = £594.60
• Total Class 4: £2,856.60

Total tax bill: £22,468 — an effective rate of about 28.1%.

Payments on Account

Here is where things can get confusing for many sole traders. If your Self Assessment tax bill is over £1,000 (and less than 80% of it was deducted at source, such as through PAYE), HMRC requires you to make payments on account.

Payments on account are advance payments towards your next year's tax bill. Each payment is 50% of the previous year's total tax liability.

Using our £40,000 example, your total tax bill was £7,311.20. This means your payments on account for the following year would be:

• First payment on account (due 31 January): £3,655.60
• Second payment on account (due 31 July): £3,655.60

In your first year, this means you would need to pay your actual tax bill of £7,311.20 PLUS the first payment on account of £3,655.60, totalling £10,966.80 on 31 January. That is a significant amount, and it catches many people off guard.

For a detailed explanation of how payments on account work, including how to reduce them if your income has dropped, read our guide on payments on account explained.

Reducing Your Tax Bill Legally

There are several legitimate ways to reduce the amount of tax you pay:

Claim all allowable expenses. This is the most straightforward way to reduce your tax bill. Every pound of legitimate business expense reduces your taxable profit. Common expenses sole traders miss include:

• Use of home as an office (flat rate or actual costs)
• Business mileage (45p per mile for the first 10,000 miles, 25p thereafter)
• Professional subscriptions and memberships
• Training courses directly related to your business
• Bank charges and interest on business loans
• Accountancy and bookkeeping fees
• Marketing and advertising costs
• Insurance premiums

Make pension contributions. Contributions to a personal pension receive tax relief. A basic rate taxpayer who contributes £1,000 to their pension effectively only pays £800, as the pension provider claims back the other £200. Higher rate taxpayers can claim additional relief through their Self Assessment return.

Use your tax-free allowances. Make sure you are using your full Personal Allowance, your dividend allowance (£500 for 2025/26), your personal savings allowance (£1,000 for basic rate taxpayers), and the trading allowance (£1,000) if applicable.

Consider timing of expenses. If you are approaching a tax band threshold, bringing forward a legitimate business purchase into the current tax year can reduce your tax bill for that year.

HMRC provides guidance on allowable expenses on their website at GOV.UK — Expenses if you're self-employed.

Using Software to Calculate Your Tax

While understanding the calculation is important, you do not need to do it by hand. HMRC's online Self Assessment system will calculate your tax automatically when you file your return. But waiting until filing time to find out what you owe is a recipe for an unpleasant surprise.

Good accounting software like Accounted gives you a running estimate of your tax liability throughout the year. As you record income and expenses, I update your projected tax bill in real time, so you always know roughly where you stand.

This makes it much easier to:

• Set aside the right amount of money for tax each month
• Make informed decisions about business spending
• Plan for payments on account
• Identify opportunities to reduce your tax bill

You can explore our full range of features on the features page, or sign up to start tracking your tax position today.

When to Pay Your Tax

The payment dates are straightforward but crucial:

• 31 January following the end of the tax year — your balancing payment for the previous year plus first payment on account for the current year
• 31 July — second payment on account for the current year

For the 2025/26 tax year:

• 31 January 2027 — balancing payment plus first POA
• 31 July 2027 — second POA

You can find the full list of important dates in our Self Assessment deadlines guide.

HMRC accepts payment by bank transfer, Direct Debit, debit card, and several other methods. Details are available at GOV.UK — Pay your Self Assessment tax bill.

Summary

Calculating your tax bill as a sole trader involves three main components: Income Tax, Class 2 National Insurance, and Class 4 National Insurance. Your taxable profit (turnover minus allowable expenses) determines how much you pay.

The key steps are:

1. Calculate your taxable profit (income minus allowable expenses)
2. Apply the Income Tax rates after deducting your Personal Allowance
3. Calculate Class 2 NICs (flat rate if profits exceed the threshold)
4. Calculate Class 4 NICs on your profits
5. Add it all together
6. Factor in payments on account if your bill exceeds £1,000

The effective tax rate for most sole traders falls somewhere between 15% and 30%, depending on the level of profit. Understanding this calculation helps you plan ahead, save appropriately, and avoid any nasty surprises when January rolls around.

If numbers are not your thing, that is perfectly fine. That is exactly what I am here for. Let Accounted handle the calculations while you focus on growing your business. Head over to our pricing page to find the right plan for you.

Accounted files your Self Assessment directly to HMRC, with your return pre-populated from your records. See Self Assessment filing →