The most common sole trader cashflow problem is a January Self Assessment bill that arrives larger than expected. The solution is not to deal with it in January — it is to reserve the correct monthly amount throughout the year, starting in April. This guide explains how to calculate the right figure and why the correct percentage varies significantly with profit level.
Updated 2026/27 · SoleTraderTaxCalculator.co.uk · Editorial standards · Methodology
The oft-cited advice to set aside 25–30% of profit for tax comes from a reasonable average across common profit levels. However, the actual effective rate — combined income tax and Class 4 NI as a percentage of gross profit — varies from under 10% at very low profits to over 40% at higher profit levels. A flat 25% figure is not wrong for profits around £40,000–£50,000, but it overstates the requirement below £25,000 and understates it above £70,000.
The reason for the variation is progressive taxation. The Personal Allowance means the first £12,570 of profit is tax-free, so a sole trader at £20,000 profit pays tax only on £7,430. At £70,000 profit, a large portion sits in the higher rate band (40% income tax plus 2% Class 4 NI). Using a flat percentage without considering which band your profit sits in will either leave you short or leave you over-reserving for no reason.
At £25,000 profit (England), the combined income tax and Class 4 NI bill is approximately £3,209. As a percentage of £25,000 profit, the effective rate is approximately 12.8%. Setting aside 25% would mean £6,250 in the reserve — nearly double what is needed. The correct monthly set-aside at £25,000 is approximately £267.
At £50,000 profit, the combined bill is approximately £12,498 — income tax of approximately £7,540 plus Class 4 NI of approximately £2,262 plus additional higher-rate tax on the £50,000 level. As a percentage of £50,000 profit, the effective rate is approximately 25%. A 25% set-aside is broadly right here. The correct monthly set-aside at £50,000 is approximately £1,041.
At £70,000 profit, the higher rate kicks in significantly above £50,270. The combined bill rises to approximately £22,580, representing approximately 32% of gross profit. A 25% set-aside would create a shortfall of approximately £4,500. The correct monthly set-aside at £70,000 is approximately £1,882. This is why the calculator's precise output matters more than a rough percentage estimate at higher profit levels.
Enter your expected annual taxable profit into this calculator to get the precise monthly set-aside figure. This figure is calculated as (income tax + Class 4 NI) divided by 12, based on the 2026/27 rates. If your profit is relatively predictable — you have an annual contract, a stable client roster, or reasonably consistent monthly income — this figure is reliable and should be transferred to your tax reserve account each month.
If your income is variable, recalculate each quarter using your updated year-to-date profit and a projection of the remaining months. A January-to-March surge that pushes annual profit above your initial estimate will change the set-aside figure meaningfully. Catching this at the April or July review point gives you several months to top up the reserve rather than scrambling in December.
The monthly set-aside covers income tax and Class 4 NI. If you also owe student loan repayments, the calculator includes these when a plan is selected — and these should also form part of the monthly transfer. Pension contributions are handled separately as a direct cash outflow from your income, not from the tax reserve.
The single most effective step is opening a dedicated savings account for the tax reserve and treating it as untouchable outside of HMRC payments. Many banks offer instant-access savings accounts with no penalty for withdrawal — the goal is not to lock the money away, but to separate it from spending.
Transfer the set-aside on the day income arrives, or on a fixed monthly date, whichever suits your income pattern. If income is irregular, transfer a proportion of each payment received rather than waiting for a monthly trigger. The act of moving the money immediately makes it psychologically less available for spending and ensures you cannot accidentally use it.
Review the reserve balance at the end of each quarter. Compare the actual balance against your cumulative monthly target. A gap at the quarterly review indicates either that set-asides have been missed, that income has grown more than projected, or that expenses have been lower than expected (increasing taxable profit). Any of these is an opportunity to correct the trajectory before January.
Neither figure is universally correct. The right percentage depends on your actual profit level and tax position. Use this calculator to get the precise figure. At £25,000 profit the effective rate is around 13%; at £50,000 it is around 25%; at £70,000 it is around 32%. A flat percentage will either over- or under-reserve depending on your profit.
Set aside a percentage of each payment received rather than a fixed monthly amount. A conservative estimate of 20–25% applied immediately to each income receipt provides a reasonable buffer that can be corrected at quarterly reviews as the year's profit picture becomes clearer.
The set-aside covers your estimated annual tax bill. If on-account payments apply, the January demand includes both the prior year's balance and the first on-account payment. Building slightly more than the direct annual estimate — or simply accumulating twelve months before touching the reserve — provides the buffer needed.
An instant-access savings account separate from your main business or personal account. Some banks offer accounts with named pots or earmarked savings features that make this separation visually clear. The key requirement is that it is not your spending account — the money should feel notionally spent.
The sole trader tax calculator turns this guidance into a concrete monthly take-home and tax reserve estimate, based on 2026/27 HMRC rates. Enter taxable profit — not turnover.