Self-Assessment Tax Calculator UK 2025/26

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.