Going self-employed is brilliant — until January arrives and HMRC sends you a tax bill you weren't quite prepared for. It happens to a staggering number of people every year. The good news? It's entirely avoidable. If you know how self-employed tax works and set aside the right amount from the start, that January bill is nothing more than a direct debit you already planned for.
How Self-Employment Tax Works in the UK
As a self-employed person in the UK, you pay tax through the Self Assessment system. This means you file a tax return each year (deadline: 31 January for online submissions), declaring your income and expenses. HMRC then calculates what you owe. Use our self-employment tax calculator to get an estimate before the bill lands.
You pay two types of tax on your self-employed income:
- Income Tax: the same rates as employed people — 20% basic rate, 40% higher rate
- National Insurance Contributions (NICs): specifically, Class 4 NICs on profits above the threshold (currently 9% on profits between £12,570 and £50,270, then 2% above that)
Your Personal Allowance: The Free Bit
The first £12,570 of your income (as of 2025/26) is tax-free — this is your Personal Allowance. So if you earn £20,000 profit in a year, you only pay tax on £7,430. This is a key number to remember when estimating your bill.
Our estimated tax calculator factors in the Personal Allowance automatically so you get an accurate picture of your actual liability.
A Real Example
Say you earn £35,000 in profit from your self-employed work in the 2025/26 tax year.
- Taxable income: £35,000 − £12,570 = £22,430
- Income Tax at 20%: £22,430 × 0.20 = £4,486
- Class 4 NICs: (£35,000 − £12,570) × 9% = £2,018.70
- Total tax bill: approximately £6,505
That's 18.6% of your gross income going to HMRC. Not catastrophic, but you need to have had that money sitting waiting for the bill.
The Payments on Account Trap
Here's the bit that catches most newly self-employed people off guard: HMRC operates a system called payments on account. If your tax bill exceeds £1,000, HMRC requires you to pay in advance towards next year's bill as well. On top of your current year's bill, you'll pay 50% of it as a deposit for the following year. Then another 50% in July.
In practice, this means your first big tax bill can be 1.5 times what you expected. If your bill is £6,500, you'll pay £9,750 in January (£6,500 + £3,250 on account). Plan for this from day one.
What to Set Aside: The Rule of Thumb
A widely-used guideline is to set aside 25-30% of every payment you receive. This covers Income Tax and NICs for most people earning in the basic rate band. If you earn more and approach the higher rate (above £50,270), set aside 40-45%.
Create a separate savings account labelled "Tax" and transfer the relevant percentage every time a payment lands. Treat it as money that was never yours. That discipline makes self-employment tax entirely painless.
Allowable Expenses: Your Best Friend
You only pay tax on your profit, not your gross income. Profit = income minus allowable business expenses. These can include:
- Equipment and tools used for work
- Home office costs (heating, electricity — a proportion)
- Travel and mileage for work purposes
- Professional subscriptions and training
- Accountancy fees (yes, those are deductible)
Good record-keeping throughout the year — not a frantic scramble in January — is what lets you claim everything you're entitled to. Use accounting software or even just a well-maintained spreadsheet.
When to Register
You must register with HMRC for Self Assessment by 5 October following the tax year in which you started earning self-employed income. So if you started in April 2025, register by October 2025. Missing this deadline can result in a fine.
Self-employment tax is genuinely manageable — it just requires planning. Set aside the right percentage, keep your expenses records clean, and use a calculator to estimate your bill well before January. Future you will be grateful.
Further reading: GOV.UK's self-employed section covers registration, deadlines, and allowable expenses in detail. Read the official HMRC guidance on self-employed NICs.