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.
Written and reviewed by James Whitfield · Updated August 2026 · Checked against 2026/27 HMRC rates · Editorial standards · Methodology
There is no single right percentage to set aside — your effective tax rate rises with profit, from about 13% at £25,000 to roughly 26% at £70,000.3 The reliable method is to take your estimated annual Income Tax and Class 4 NI, divide by 12, and move that amount into a separate account every month.1 The popular '25–30%' rule of thumb only really fits profits around £40,000–£55,000; below that it over-reserves, above it it leaves you short. Recalculate each quarter if your income is variable.
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 profit — varies from under 10% at very low profits to over 30% at high profit levels.3 A flat 25% figure is not wrong for profits around £45,000–£55,000, but it overstates the requirement below £40,000 and understates it well 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 meaningful slice sits in the higher rate band (40% income tax, with Class 4 NI dropping to 2% above £50,270).3 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.
Here is what the combined Income Tax and Class 4 NI bill actually works out to at common profit levels for 2026/27 (England/Wales/NI), and the monthly amount to reserve. Every figure comes straight from the calculator on this site.
| £25,000 profit — 12.9% effective | £269 / month |
| £30,000 profit — 15.1% effective | £378 / month |
| £40,000 profit — 17.8% effective | £594 / month |
| £50,000 profit — 19.5% effective | £811 / month |
| £70,000 profit — 25.8% effective | £1,507 / month |
Notice the gap the rule of thumb creates. At £25,000, setting aside 25% would bank £6,250 against a real bill of £3,232 — nearly double what is needed, tying up cash you could use. At £70,000, 25% banks £17,500 against a £18,089 bill — a small shortfall, but a shortfall. The higher your profit, the more the flat percentage lets you down, and the more it pays to use the exact figure.
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 19%; at £70,000 it is around 26%. 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.
Yes, if you are on a repayment plan. Student loan repayments are collected through Self Assessment alongside your tax, so they form part of the January bill. Plan 2, for example, takes 9% of profit above £29,385 for 2026/27. The calculator adds this in when you select your plan, and the resulting monthly set-aside already includes it — so reserve the full figure, not just the tax portion.
The rates, thresholds and rules in this guide are drawn from the official HMRC and GOV.UK sources below, using the confirmed 2026/27 figures. Each link opens the relevant official page in a new tab.
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.
Self Assessment checklist, expense tracker and payments on account calendar — all in one practical PDF. Updated for 2026/27.
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