Small Business Tax Calculator UK: How to Work Out What You Owe

Tax Queen

Tax Expert @ Pie

11 min read

Let's Break This Down Together

Sorting small business taxes can feel like a puzzle. There are different rules, deadlines and terms, and the answer depends heavily on how your business is structured.

In this guide, we'll walk through the main taxes you might face, how to use a small business tax calculator effectively, which expenses can affect your taxable profit, and how savings interest can fit into your wider tax position.

We'll also look at tax deadlines, Making Tax Digital and why checking your figures throughout the year can be more useful than waiting until the Self Assessment deadline.

By the end, you'll have a clearer idea of what a calculator can tell you, what it cannot, and how Pie Tax can help you stay organised.

Your Structure Decides Everything Else

Before you open any calculator, be clear which of these describes you, because the taxes, rates and deadlines follow from your structure.

Sole trader or partnership. You generally pay Income Tax and, where applicable, Class 4 National Insurance on your share of taxable profits through Self Assessment. Profit, rather than turnover, is generally what matters for Income Tax and National Insurance, so allowable business expenses can reduce the amount on which you are taxed. Class 2 has not been a compulsory weekly charge since April 2024, although voluntary contributions remain available in certain circumstances to protect your National Insurance record. See Class 2 and Class 4 National Insurance.

Limited company. There are two separate tax layers to consider. The company pays Corporation Tax on its taxable profits, while you may have personal tax to pay on salary, dividends or other income you receive from the company. A calculator that only looks at one layer may not show your full personal and business tax position.

Both, or moving between them. This is where tax calculations can become more complicated. If you are considering incorporation, our guide to sole trader or limited company works through the main differences.

VAT is separate again. VAT registration is based on taxable turnover rather than profit or business structure. For the current rules, compulsory registration generally applies when taxable turnover goes over £90,000 in the previous 12 months or you expect it to exceed £90,000 in the next 30 days. See how to become VAT registered.

What Taxes Do Small Businesses Need to Calculate?

Most small businesses can face several different taxes rather than one single business tax, and they do not all use the same rates, thresholds or deadlines.

For sole traders, the main calculation is usually Income Tax on taxable profits, together with any Class 4 National Insurance due.

For limited companies, Corporation Tax is calculated on the company's taxable profits. For 2026, the small profits rate is 19% for companies with profits of £50,000 or less, while the main rate is 25% for companies with profits above £250,000. Companies with profits between £50,000 and £250,000 may qualify for Marginal Relief. The thresholds can be reduced where a company has associated companies or a short accounting period.

VAT is based on taxable turnover rather than profit. The current VAT registration threshold is £90,000, so a business can potentially need to register even if its profit margin is relatively small.

There may also be employer National Insurance if you employ staff, and other taxes can apply depending on what your business does and the income or assets involved.

Accountant and Business owner talking while holding a phone and using a laptop

Limited Company Taxation

For limited companies, understanding the difference between company tax and personal tax is essential.

A company generally pays Corporation Tax on its taxable profits. For 2026, companies with profits of £50,000 or less generally use the 19% small profits rate, while companies with profits above £250,000 generally use the 25% main rate. Marginal Relief can apply between those thresholds.

The company may also have employer National Insurance obligations where it pays salaries to employees or directors.

If the company disposes of an asset and makes a chargeable gain, the gain is generally dealt with within the company's Corporation Tax calculation rather than being subject to personal Capital Gains Tax.

For the owner, personal tax is a separate calculation. Salary is generally taxed as employment income, while dividends are taxed under the dividend tax rules once any available Dividend Allowance has been taken into account.

That is why a small business tax calculator needs to distinguish between company profits and the money you personally receive.

What Is a Small Business Tax Calculator?

A small business tax calculator is a digital tool that estimates your potential tax liability using information such as income, business expenses, tax allowances and your business structure.

For a sole trader, the calculation will generally start with business income and allowable expenses to establish taxable profit. The calculator can then estimate Income Tax and National Insurance based on the relevant tax year.

For a limited company, the calculation is different because Corporation Tax is charged on the company's taxable profits, while your personal tax position depends on what you receive from the company and your other income.

A calculator is useful for planning, but it is only as accurate as the information you enter. It should therefore be treated as an estimate rather than a substitute for your actual tax return or HMRC's calculation.

How to Use a Tax Calculator Effectively

Start with accurate figures from your records, because everything downstream depends on them.

For a sole trader, work out your total business income and deduct allowable business expenses before estimating Income Tax and National Insurance.

For a company director, separate the company's taxable profits from your own personal income. Salary, dividends, savings interest and other income can all affect your personal tax position in different ways.

It is also important to select the correct tax year. A calculator using 2025/26 figures can produce a different result from one using 2026/27 figures.

The most useful approach is to run different scenarios rather than relying on one final number. You can see what happens if your income changes, expenses increase, or you take money from your company in a different way.

Self-employed man calculating with calculator and notebook

Four Things That Make a Calculator's Answer Wrong

It is set to the wrong tax year. Tax rates and allowances can change. A calculator using an earlier tax year can quietly apply outdated figures, so check the tax year before relying on the result.

You have entered turnover instead of profit. For a sole trader, Income Tax and Class 4 National Insurance are generally based on taxable profits rather than simply the amount of money coming into the business. Entering gross income without considering allowable expenses can significantly overstate the estimate.

It only models one layer. Limited company owners may need to consider Corporation Tax at company level as well as personal tax on salary, dividends and other income.

Your expenses are incomplete. A calculator cannot identify expenses you have not recorded. Keeping accurate records throughout the year gives you a much better starting point for estimating what you owe.

Try Our Free Tax Calculators

If you'd rather skip some of the manual maths, Pie Tax's calculators can help you estimate different parts of your tax position.

Use the income tax calculator to estimate Income Tax on your income, or try the self-employed calculator for a calculation based on self-employed income and expenses.

If you receive dividends, the dividend tax calculator can help you understand the potential tax on dividend income, while the salary after tax calculator can help estimate your take-home pay.

Pie Tax also gives you a way to keep your tax information organised rather than treating your calculator result as a one-off number.

Interest Tax Calculator: How Much Tax Do You Pay on Savings Interest?

An interest tax calculator can be useful if your business or personal finances include savings interest.

Savings interest is not automatically tax-free. Your tax position depends on your total income and the allowances available to you.

For 2026/27, the Personal Savings Allowance is generally £1,000 for basic rate taxpayers and £500 for higher rate taxpayers. Additional rate taxpayers do not receive a Personal Savings Allowance.

There is also a starting rate for savings of 0% on up to £5,000 of savings interest in certain circumstances. The amount available depends on your other taxable income.

An interest tax calculator can therefore help you estimate how much of your savings interest may be taxable after taking your allowances into account. To work out the correct position, you need to consider your total income rather than looking at interest in isolation.

If you run a limited company, interest received by the company is a company income item and can form part of its taxable profits for Corporation Tax purposes. Your company's interest should therefore not simply be put through a personal interest tax calculation.

Common Small Business Tax Deductions to Include

Don't forget to factor in eligible business expenses when using your small business tax calculator.

Office costs can include relevant business costs for premises and, where the rules allow, a proportion of certain household costs if you work from home.

Business travel can also be deductible where the relevant conditions are met. Mileage claims may be available using HMRC's approved mileage allowance payments where the rules apply.

Equipment and other assets may qualify for capital allowances rather than being treated as an ordinary revenue expense. The treatment depends on the asset and the circumstances.

Professional fees for services such as accounting and legal work can be allowable where they relate to the business. Marketing, insurance and qualifying staff costs may also be deductible.

Not every cost is automatically allowable, however. Mixed personal and business costs may need to be apportioned, and some types of expenditure have specific tax rules.

Our guides to the home office allowance and business expenses explain the rules in more detail.

Timing Beats Cleverness

The most useful thing a calculator can do is show you where you stand while you still have time to act.

A figure checked during the year can help you budget for your eventual tax bill and identify areas where your records need attention. Waiting until a payment deadline leaves much less time to deal with missing expenses, unexpected income or cash-flow problems.

That is why checking your figures regularly can be more useful than running one calculation immediately before a deadline.

For sole traders, remember that Payments on Account may also increase the amount you need to pay at a Self Assessment deadline. Payments on account are generally made in 2 instalments, due by 31 January and 31 July, unless an exception applies. Each instalment is normally half of the previous year's tax bill.

Legitimate tax planning means claiming reliefs and deductions you are entitled to and choosing between options allowed by the rules. Keep thorough records and make sure your calculations reflect your actual circumstances.

When to Seek Professional Tax Advice

While tax calculators are useful for day-to-day planning, some situations call for additional advice.

Major business changes such as incorporation can have tax consequences that a basic calculator may not capture. If you're approaching the VAT registration threshold, it is important to understand the registration rules and timing.

International business activities can create additional tax considerations, while property investments, inheritance matters and retirement planning can involve rules beyond a standard small business tax calculation.

A calculator is most useful when you understand what it is calculating. Where your circumstances are more complicated, professional advice can help you interpret the figures.

Important Tax Deadlines for Small Businesses

Sole traders. The main Self Assessment deadline for paying tax owed for the previous tax year is 31 January, with the second payment on account normally due on 31 July. Payments on account can apply depending on your circumstances. See payments on account for more detail.

Making Tax Digital for Income Tax is also being introduced in stages. From 6 April 2026, it applies to sole traders and landlords with qualifying income over £50,000 based on the relevant earlier tax year. The threshold falls to over £30,000 from 6 April 2027, and over £20,000 from 6 April 2028. Those within the rules need compatible software to keep digital records and send quarterly updates to HMRC. See Making Tax Digital for sole traders and landlords.

Limited companies. Corporation Tax is generally payable 9 months and 1 day after the end of the accounting period, while the Company Tax Return is generally due within 12 months. See Corporation Tax late payment penalties.

VAT returns are commonly submitted quarterly, although the exact deadlines depend on your VAT accounting period and circumstances.

Final Thoughts

Small business tax calculations don't need to be overwhelming when you have accurate records and the right tools.

A small business tax calculator can help you estimate your Income Tax, National Insurance or Corporation Tax position, while an interest tax calculator can help you understand how savings interest may affect your wider personal tax position.

The important thing is to use the correct tax year, enter accurate figures and understand what the calculator is actually measuring.

Pie Tax helps you keep your income, expenses and tax information organised throughout the year, so you can spend less time trying to piece your figures together.

To learn more about Pie Tax, see why Pie Tax or find out about us. Get started with Pie Tax for free or see how it works for yourself.

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Frequently Asked Questions

Am I taxed on turnover or profit?

Profit. Turnover matters for VAT registration, but Income Tax, National Insurance and Corporation Tax all apply after allowable expenses.

Why does my calculator give a different figure to my accountant?

Usually because it has fewer expenses in it, or it is set to a different tax year, or it is modelling only one layer of a company's tax.

How often should I check?

Quarterly. Annually tells you what happened. Quarterly lets you do something about it.

Do I need an accountant as well?

For routine years, a calculator and good records are usually enough. For incorporation, property, international work or selling the business, the advice tends to cost less than the mistakes.