Self-Assessment Tax Calculator UK 2025/26
Estimate your self-assessment tax bill across all income sources. Covers employment, self-employment, rental, dividends, capital gains, pensions, and student loans.
Self-assessment is how you tell HMRC about income that is not taxed at source — things like freelance earnings, rental income, dividends, or capital gains. If you owe tax that was not collected through PAYE, this is the return you need to file by 31 January each year.
The thing that catches people out is payments on account. HMRC does not just ask for last year's tax — they also want half of next year's estimated bill upfront. So if your first self-assessment comes to £6,000, you will actually need to pay £9,000 that January (the full £6,000 plus a £3,000 payment on account). It is a nasty surprise if you are not expecting it.
HMRC reported that 12.1 million people were expected to file a self-assessment return for the 2023/24 tax year (HMRC, January 2025). Our calculator covers every major income source so you can estimate your bill before the deadline hits.
How to estimate your self-assessment tax bill
- Enter all your income sources — employment, self-employment, rental, dividends, and interest
- Add any tax reliefs such as pension contributions or Gift Aid donations
- Select your student loan plan if applicable
- See your total tax bill, payment on account amounts, and key filing deadlines
Written by the CalcStack team · Last updated March 2026
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Marriage allowance transfer
Frequently asked questions
Who needs to file a self-assessment tax return?▾
You need to file if you're self-employed and earned over £1,000, if you're a company director, if you earned over £100,000 from any source, if you received rental income, or if HMRC has specifically asked you to. If your only income is a normal PAYE salary from one employer, you almost certainly don't need to bother.
When is the self-assessment deadline?▾
For 2024/25: paper returns by 31 October 2025, online returns by 31 January 2026. The tax payment is due on 31 January too. Miss it and you get an automatic £100 fine even if you owe nothing. Don't leave it until January — the system always crashes in the last week.
What are payments on account?▾
This catches a lot of people out in their first year. HMRC assumes you'll earn roughly the same next year, so they make you pay in advance — two instalments, each 50% of last year's bill. The first lands on 31 January (same day as your final payment for the previous year, so it's a double hit), and the second on 31 July. If you know your income is dropping, you can apply to reduce them through your online tax account.
How is self-employment tax calculated?▾
Take your self-employment income, subtract allowable expenses, and you've got your profit. That profit gets taxed at your marginal income tax rate. On top of that, you'll pay Class 2 NI (£3.45/week if profit exceeds £12,570) and Class 4 NI (6% on profits between £12,570 and £50,270, 2% above that). So if you made £40,000 profit, you'd owe income tax plus roughly £1,646 in Class 4 NI and £179 in Class 2.
How does the dividend allowance work?▾
Your first £500 of dividends each year is tax-free. After that, it's 8.75% if you're a basic-rate taxpayer, 33.75% at higher rate, or 39.35% at additional rate. One thing to be aware of: the allowance used to be £2,000 and was slashed to £500 from 2024/25. It doesn't reduce your overall income for band calculations either — it just means you don't pay tax on that first slice.
Can I deduct pension contributions from my tax bill?▾
Yes — and it's one of the best ways to reduce your self-assessment bill. Your pension provider automatically claims 20% tax relief from HMRC. If you're a higher or additional rate taxpayer, you claim the extra through your tax return. So a £10,000 pension contribution only costs a 40% taxpayer £6,000 after all the relief is applied.
How does Gift Aid affect my tax return?▾
The charity claims an extra 25% on top of your donation from HMRC. If you're a higher-rate taxpayer, you can also claim back the difference between your rate and the basic rate on your return. So a £100 Gift Aid donation to charity actually costs a 40% taxpayer just £75. Gift Aid donations also extend your basic rate band, which can pull you below a threshold if you're on the edge.
What is the marriage allowance?▾
If one of you earns under £12,570 and the other is a basic-rate taxpayer, you can transfer £1,260 of personal allowance across. That saves the higher earner up to £252 a year. You can backdate it 4 years too, so a new claim could be worth over £1,000 in one go. Apply on the gov.uk website — it takes 5 minutes.
How are student loan repayments calculated on self-assessment?▾
It's 9% of your total income above the plan threshold: £22,015 for Plan 1, £27,295 for Plan 2, £27,660 for Plan 4, £25,000 for Plan 5. Postgraduate loans are 6% above £21,000. Anything you already paid through PAYE during the year gets deducted from the total. If HMRC overcharges you, you'll get a refund.
What happens if I miss the self-assessment deadline?▾
It gets expensive fast. Day one: £100 automatic fine. After 3 months: £10 per day up to £900. After 6 months: 5% of the tax you owe or £300, whichever is higher. After 12 months: another 5% or £300. And interest starts building from the original due date on any unpaid tax. Even if you can't pay the bill, always file on time — the filing penalty and the payment penalty are separate.