The shape of the problem
An employee never thinks about this. Tax comes out before the money arrives, every month, automatically. The salary that lands in the account is theirs, and the arithmetic has already happened somewhere upstream.
Self-employment removes that machinery entirely. The full invoice arrives in your account. All of it looks like yours. The tax on it is due at some point between four and eighteen months later, depending on where you are and when in the year you earned it, and in the meantime it sits in the same account as your rent money, indistinguishable from it.
Add irregular income and the problem sharpens. A good March followed by a thin April and a dead May is a completely normal freelance quarter. The natural response — spend more in March because the money is there, spend less in May because it is not — is exactly wrong, because March's income is what generates the bill, and by May the reserve has already been eaten covering the quiet month.
The failure is never a tax-rate problem. It is a timing problem, and it is fixed by separating the money on the day it arrives rather than on the day it is due.
The two mistakes behind almost every tax shock
Mistake one: reserving on revenue instead of profit. Someone hears "put aside 30%" and applies it to everything that lands. That over-reserves if expenses are significant, feels punitive, and gets abandoned within a few months precisely because it feels wrong. Tax is charged on profit — revenue minus allowable expenses — so the reserve should be a percentage of the profit on each invoice, not of the invoice.
Mistake two: forgetting that tax is not the only thing due. This is the one that actually causes the disasters, because it is invisible until the bill lands. Most jurisdictions charge self-employed people two things on the same profit: income tax and a social contribution — self-employment tax, National Insurance, CPP, whatever it is called locally. Reserve for income tax alone and you can be 30–50% short on the total without having made an arithmetic error anywhere.
If you have set a reserve percentage, confirm it covers both the income tax and the social contribution in your jurisdiction. This single omission is behind a large share of self-employed tax shortfalls, and it does not surface until the assessment arrives.
Working out what to hold back
The reserve percentage is your effective rate on profit — total tax plus contributions, divided by total profit. Not your marginal band, which is the rate on your next pound and will over-reserve substantially, and not the basic rate, which ignores everything above it.
The rough procedure, which you should have an accountant sanity-check once:
- Estimate the year's profit. Revenue minus expenses. Use last year if you have one; be conservative if you do not.
- Apply your jurisdiction's bands to that figure — income tax after any tax-free allowance, plus the social contribution, which often has its own separate thresholds and its own ceiling.
- Divide the total by the profit. That percentage is your reserve rate.
- Apply it to the profit on every payment, on the day it arrives.
Two adjustments worth making. If profit is growing, reserve at next year's expected rate rather than last year's actual — progressive bands mean a good year is taxed harder than the year before it, and the reserve set on last year's rate will be short. And if your jurisdiction requires payments on account or quarterly estimates, the first year of those requires roughly one and a half years of tax inside twelve months. That is the single largest cash-flow event in most self-employed careers and it is entirely predictable.
Worked example: a lumpy year
A consultant with four big months and eight thin ones
Revenue £82,000, expenses £11,000, profit £71,000. Suppose the combined income tax and social contribution on that profit works out to £20,590 — an effective rate of 29%.
Here is the year as it was actually lived, with a 29% reserve taken on the profit share of each month's receipts:
| Month | Received | To reserve (29% of profit) | Spendable |
|---|---|---|---|
| March | £24,000 | £6,025 | £17,975 |
| April | £2,000 | £502 | £1,498 |
| May | £0 | £0 | £0 |
| June | £18,000 | £4,519 | £13,481 |
| Rest of year | £38,000 | £9,544 | £28,456 |
| Total | £82,000 | £20,590 | £61,410 |
The reserve lands almost exactly on the bill, because it was taken proportionally as the money arrived rather than estimated at the end.
Now the version without a reserve. March feels like a £24,000 month, so the year is planned around a £82,000 income. Come January, £20,590 is due and the account holds whatever survived twelve months of spending against a number that was never really yours. The money was not wasted — it was spent normally, against an income figure that was overstated by 29% every single month.
Note what the reserve actually does to March: it turns a £24,000 month into an £18,000 month, on the day the money lands. That is the entire mechanism. Everything else is bookkeeping.
Where the money should sit
The reserve needs to be somewhere you will not spend it by accident, and the bar for "accident" is lower than people think.
- A separate account, at minimum. Same bank is fine. The important property is that the balance in your working account is a number you can spend without doing arithmetic first.
- Ideally a different bank. Not visible in the app you open every day. Transferring out takes an extra step, and the extra step is the point.
- Interest-bearing if possible. The reserve sits for months. There is no reason for it to sit at zero, though liquidity matters more than yield — it must be available on the due date without notice or penalty.
- Never in anything that can fall. The reserve is not an investment. Its job is to be exactly the right size on a known date, and an instrument that can be down 20% on that date has failed at the only thing it was for.
- Move it on the day the payment lands, not weekly, not monthly. Any delay is a window in which the money looks spendable, and a window is all it takes.
And when the bill is paid, do not treat a surplus as a windfall. If the reserve over-collected, that is next year's cushion — especially if income is growing, because next year's effective rate will be higher.
The first year is the dangerous one
Two things converge in year one and they converge badly.
First, income tends to be lowest while every business cost is being incurred for the first time. Reserving feels least affordable exactly when the habit needs to form.
Second, in jurisdictions with payments on account or quarterly estimates, the first bill is often larger than a full year's tax — it settles the year just finished and makes an advance payment against the year in progress. Someone expecting to owe £14,000 receives a demand for £21,000, on a date they had not planned for, with a reserve built for the smaller number.
The defence is dull and effective: find out, in your first month of trading, exactly what is due and when, for your jurisdiction and structure. One conversation with an accountant, before there is anything complicated to discuss, is the cheapest it will ever be — and the answer is a date and a number you can plan around rather than a surprise you absorb.
What this cannot tell you
Everything above is a method, not a set of figures. The figures depend on things only your own situation determines.
- Rates and thresholds are jurisdiction-specific and change annually. Any percentage in this article is illustrative. Yours comes from current rules applied to your profit.
- Business structure changes everything. Sole trader, partnership and incorporated company are taxed on different bases, at different times, with different contribution rules. Advice for one can be actively wrong for another.
- Sales tax is a separate reserve. VAT, GST and their equivalents are money you collected on someone else's behalf. They were never income, they are not part of this calculation, and they need their own reserve.
- Reliefs and allowances can move the effective rate a long way. Pension contributions, capital allowances, trading allowances and loss relief all change the answer, sometimes substantially, and all of them are worth an accountant's time.
The half of this problem that comes earlier — making sure the invoice was large enough to carry the reserve in the first place — is covered in working backwards from take-home to an hourly rate.