Your Self Assessment bill can feel like a mystery when it lands. You file a return, HMRC spits out a number, and you're left wondering where it came from.
It isn't magic. HMRC follows a set order: add up your income, take off allowances and expenses, apply the tax rates, then knock off anything you've already paid. Once you can see that order, the bill stops being a surprise.
This guide walks through exactly how the calculation works for the 2025/26 tax year, what affects the final figure, and when you have to pay. It's written for sole traders, landlords and company directors who file their own return or want to sense-check what their accountant produces.
<h2 id="what-is-a-self-assessment-tax-bill">What exactly is a Self Assessment tax bill?</h2>
Your Self Assessment tax bill is how HMRC collects Income Tax and, where it applies, National Insurance on income that hasn't already been taxed at source.
Think of it as your annual reconciliation. Some of your income (a salary, for example) has already had tax taken off through PAYE. Other income (self-employment profits, rent, dividends, savings interest) usually hasn't. Self Assessment pulls the whole picture together so HMRC can check you've paid the right amount overall.
That's why you report income HMRC already knows about, like employment earnings, as well as the untaxed bits. Without the full picture, the rates can't be applied correctly.
<h2 id="who-needs-to-file">Who needs to file a Self Assessment tax return?</h2>
Not everyone does. You'll usually need to file for a tax year if any of these applied:
- You were self-employed as a sole trader and earned more than £1,000 (the trading allowance) before expenses.
- You were a partner in a business partnership.
- You had untaxed income, for example from renting out property, tips, commission, savings, investments or dividends.
- You had to pay the High Income Child Benefit Charge.
- You wanted to claim certain reliefs, or you had Capital Gains Tax to pay.
If you're not sure, gov.uk has a free checker tool, and it's the safest place to confirm your own position. You can also talk to a Zmartly accountant if you'd rather have someone review your circumstances.
Some people file voluntarily even when they don't strictly have to, often to reclaim tax they've overpaid.
<h2 id="how-hmrc-calculates">How does HMRC calculate your Self Assessment tax bill?</h2>
The calculation has two possible parts: Income Tax, and (for the self-employed) National Insurance. HMRC works through them in a set order.
Step by step
- Add up your taxable income. Employment income, self-employment profits, rental profits, dividends, savings interest and other taxable receipts.
- Take off allowable expenses. If you're self-employed, that's genuine business costs. Landlords deduct allowable property costs, and finance costs such as mortgage interest are handled as a 20% basic-rate tax reducer rather than a straight deduction.
- Apply your Personal Allowance. For 2025/26 the Personal Allowance is £12,570, so the first £12,570 of income is usually tax-free.
- Apply capital allowances if relevant. These let you write off the cost of business assets like equipment, computers or machinery.
- Apply the tax rates. HMRC charges the relevant rate on each slice of your remaining income.
- Deduct tax already paid. PAYE, tax taken off at source, and any payments on account come off last.
The number you're left with is your balancing payment, or your refund if you've overpaid.
<h2 id="income-tax-bands">What income tax bands apply for 2025/26?</h2>
The rate you pay depends on which band each slice of income falls into. The bands below apply to England, Wales and Northern Ireland 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% |
Your income is taxed slice by slice. You don't pay 40% on everything once you cross £50,270; you only pay 40% on the part above it.
What about the £100,000 trap?
Once your adjusted net income passes £100,000, your Personal Allowance is reduced by £1 for every £2 over that line, disappearing entirely at £125,140. That tapering creates an effective marginal rate of around 60% on income between £100,000 and £125,140, which is worth planning around if you're close to it.
A note for Scottish taxpayers
Scotland sets its own Income Tax rates and bands, which differ from the rest of the UK. The bands above don't apply if you're a Scottish taxpayer, so check the Scottish rates for your year. National Insurance and the rates of tax on dividends and savings are the same across the UK.
<h2 id="national-insurance">Do you pay National Insurance through Self Assessment?</h2>
If you've been self-employed during the year, National Insurance is usually the second part of your bill. It counts towards your State Pension and certain benefits.
Class 4 National Insurance
Class 4 is charged on your self-employment profits. For 2025/26 you pay:
- 6% on profits between £12,570 (the Lower Profits Limit) and £50,270 (the Upper Profits Limit).
- 2% on profits above £50,270.
HMRC calculates this automatically as part of your return and adds it to your Income Tax.
Class 2 National Insurance
The rules changed from 6 April 2024. Class 2 is no longer payable as a flat weekly charge if your profits are above the Small Profits Threshold (£6,845 for 2025/26). Your NI record is treated as maintained, so your State Pension entitlement is protected without paying it.
If your profits are below the Small Profits Threshold, you can still pay Class 2 voluntarily, at £3.50 a week for 2025/26, to keep that year counting towards your State Pension. For many people on low profits, that's a cheap way to protect a qualifying year.
If you're also employed and paying Class 1 through PAYE, those contributions are taken into account, so you won't be charged twice on the same earnings.
<h2 id="what-reduces-your-bill">What reduces your final tax bill?</h2>
HMRC doesn't just hand you the gross figure. Several things come off before you get to what you actually owe.
Tax already paid at source
Tax deducted from your salary through PAYE comes off your bill. So does tax taken from certain other income before it reaches you. This is the most common reason a final balancing payment is smaller than the headline tax figure.
Tax relief claims
Some claims reduce your taxable income or give you back tax. Personal pension contributions and Gift Aid donations both extend your basic-rate band, which can pull income out of the higher-rate band. Pension contributions in particular are one of the most effective levers for higher earners.
Payments on account
If you made payments on account in advance (more on these below), they're credited against the year's bill.
The practical lesson we see most often: people forget to claim reliefs they're entitled to, and overpay. A quick review before you file usually pays for itself. Our self-assessment service is built around exactly that check.
<h2 id="payments-on-account">What are payments on account, and do you need to make them?</h2>
Payments on account catch people out more than anything else in Self Assessment. They're advance payments towards next year's bill, made in two instalments.
You'll have to make them if both of these apply:
- Your Self Assessment bill (Income Tax plus Class 4 NI) for the year is more than £1,000, and
- Less than 80% of your tax was collected at source (for example through PAYE).
How the timing works
Instead of paying everything in one go, the calendar looks like this:
- 31 January: your balancing payment for the year just gone, plus your first payment on account for the current year.
- 31 July: your second payment on account for the current year.
Each payment on account is half of your previous year's tax bill.
Illustrative example. Suppose Daniel, a self-employed designer, has a 2024/25 Self Assessment bill of £4,000 and almost none of it was collected at source. By 31 January 2026 he pays the £4,000 balancing payment plus a first payment on account of £2,000 (half of £4,000), so £6,000 in total. He then pays a second £2,000 on account by 31 July 2026. Those two £2,000 payments are credited against his eventual 2025/26 bill.
That first January can sting, because you're effectively paying one and a half years at once. After that, it smooths out.
If you expect to earn less this year, you can apply to reduce your payments on account. Be careful, though: cut them too far and HMRC charges interest on the shortfall once the real figure is known.
<h2 id="when-and-where">When and where can you view your tax bill?</h2>
Filing online
File through HMRC's online system and your calculation appears straight away once you've entered everything. Look for the "View your calculation" section to see the full breakdown. That instant figure is one of the best reasons to file online.
Filing on paper
Paper returns are processed by hand, so your calculation arrives by post afterwards. Note the earlier deadline too: paper returns are due by midnight on 31 October following the tax year, against 31 January for online.
The payment deadline
Whichever way you file, the payment deadline is the same: midnight on 31 January following the end of the tax year. Tax for the 2024/25 tax year was due by 31 January 2026, and your 2025/26 bill will be due by 31 January 2027. Miss it and HMRC charges interest, with penalties on top if it drags on.
If paying on time is going to be a struggle, contact HMRC as early as you can. A Time to Pay arrangement can spread the cost over several months, and HMRC is usually more approachable than people expect.
<h2 id="how-zmartly-helps">How can Zmartly help with your Self Assessment?</h2>
Understanding the calculation is one thing. Staying on top of it all year, and being sure you've claimed everything, is another.
That's where we come in. At Zmartly, our qualified accountants:
- Keep your records straight from day one, so your return is built on clean data rather than a January scramble.
- Estimate your liability through the year, so you can set money aside instead of being ambushed in January.
- Review your return before it's filed, spotting missed deductions and reliefs and checking the figures stack up.
- Handle the whole submission if you'd rather not touch it, whether you're a sole trader, a company director or a landlord.
Want to stop guessing what you'll owe? Book a free consultation with a Zmartly accountant and we'll make your next Self Assessment something you're actually ready for. If you just want a quick estimate first, try our self-employed tax calculator.
<h2 id="faqs">Frequently asked questions</h2>
How do I calculate my Self Assessment tax bill?
Add up all your taxable income, take off allowable expenses and your Personal Allowance, then apply the relevant Income Tax rates and Class 4 National Insurance. Finally, deduct tax you've already paid through PAYE or at source. HMRC does this automatically when you file online and shows it under "View your calculation".
When do I need to pay my Self Assessment tax?
By midnight on 31 January following the end of the tax year. The 2024/25 bill was due by 31 January 2026. If you make payments on account, you'll also have an instalment due on 31 July.
What is a payment on account?
It's an advance payment towards next year's bill. You make them if your bill is over £1,000 and less than 80% of your tax was paid at source. Each one is half of your previous year's bill, paid on 31 January and 31 July.
Do I still pay Class 2 National Insurance?
From 6 April 2024, Class 2 is no longer payable as a flat weekly charge if your profits are above the Small Profits Threshold (£6,845 for 2025/26); your NI record is still protected. If your profits are below that, you can pay Class 2 voluntarily at £3.50 a week for 2025/26 to keep the year qualifying for your State Pension.
How can I reduce my Self Assessment tax bill legally?
Claim every allowable business expense, make pension contributions, use allowances you're entitled to, and claim relevant reliefs such as Gift Aid. A quick professional review before you file is the simplest way to make sure nothing legitimate is missed.
What happens if I can't pay my Self Assessment tax bill?
Contact HMRC straight away. They may agree a Time to Pay arrangement so you can spread the cost over several months. Don't ignore it, because interest and penalties only make the total bigger.
Where can I view my Self Assessment tax bill online?
After you file online, it appears under "View your calculation". You can also see it any time in your HMRC personal tax account by logging in to your online services.
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