How Much to Set Aside for Tax on £30k Profit
The exact percentage to set aside for tax on £30,000 of self-employed profit, plus where to keep the reserve and how payments on account change it.
Direct answer: On £30,000 of self-employed profit, set aside about 15.1% of every payment — roughly £378 a month — to cover the £4,532 Self Assessment bill.
Assumptions: England, Wales or Northern Ireland; 2026/27 rates; profit after allowable expenses; standard Personal Allowance; no pension or student loan unless stated.
The calculated reserve
| Item | Amount |
|---|---|
| Annual profit | £30,000 |
| Income Tax | £3,486 |
| Class 4 NI | £1,046 |
| Total Self Assessment bill | £4,532 |
| Monthly set-aside | £378 (15.1% of income) |
Why a fixed percentage beats a fixed amount
Sole-trader income is lumpy, so the reserve should scale with it. Skimming the same percentage of every payment — into a separate account on the day money arrives — funds the bill in proportion through quiet and busy months alike. The calculated percentage above covers Income Tax and Class 4 National Insurance at this profit level; a fixed monthly amount only works if income never moves.
Where to keep the tax reserve
Any easy-access savings account works; the important property is separation from spending money, because a balance sitting in the current account gets absorbed. Many business accounts offer pots that automate the transfer every time a payment lands. The reserve is your money until January, so let it earn interest somewhere sensible rather than under the mattress of your current account.
- Transfer the percentage on the day each invoice is paid, not monthly.
- Round the percentage up slightly to pre-fund payments on account.
- Keep the reserve in instant access — the bill date is fixed.
- Review the percentage after any lasting change in profit.
- Never plug a quiet month from the tax pot; adjust drawings instead.
Why your result may differ
The balancing payment is due by 31 January, and once the bill exceeds the payments-on-account threshold an advance towards the following year is due the same day. A reserve funded at the right percentage absorbs both without borrowing. If you also have PAYE income, a student loan or pension contributions, run the self-employed calculator for a personal figure — Scottish taxpayers should use the Scottish setting, because the Income Tax line changes.
Run the numbers for your own situation
Official references: Figures use the 2026/27 Income Tax rates, employee National Insurance rates and self-employed National Insurance rates. Figures are estimates; payslip timing and individual circumstances can change the result.