Employees have tax removed from every paycheck. Self-employed workers receive gross payments and are expected to remit tax themselves during the year, in instalments, rather than in one settlement afterwards.
The system requires payment as income is earned
Tax is due as income arises, not when the return is filed. Withholding satisfies this automatically for employees and there is no equivalent for freelance income.
Someone earning without withholding therefore has to make the payments directly, on a schedule set by the tax authority.
Failing to do so can trigger an underpayment charge even where the full liability is paid on time at filing, because the timing itself was wrong.
Quarterly instalments follow an uneven calendar
The instalment deadlines are spaced through the year, and the periods they cover are not all three months long despite being described as quarterly.
Each payment is meant to cover tax on income earned in the corresponding period, which matters for anyone whose income is seasonal.
Paying the same amount four times is simpler but can produce an underpayment for an early period if income was concentrated there.
Self-employment tax is the part that surprises people
Employees split payroll contributions with an employer. A self-employed worker is treated as both parties and owes the full amount on net earnings.
That obligation sits on top of income tax and applies from a low earnings threshold, so it is often the larger of the two for modest freelance income.
A portion of it is deductible in computing income tax, which softens the effect without removing it.
Safe harbour rules cap the exposure
Tax authorities generally provide a threshold: pay at least a set proportion of the current year's liability, or a set proportion of the prior year's, and no underpayment charge applies.
The prior-year measure is the more practical one for variable income, since the figure is already known and does not require forecasting.
The specific proportions differ depending on income level and change from time to time, so the current published rules are what matter.
Deductions reduce the base the payments are calculated on
Estimated payments should be based on net income after allowable business expenses, not on gross invoiced amounts.
Overpaying because expenses were ignored ties up cash for months, while ignoring them at filing time overstates the liability outright.
Keeping a running record through the year is what makes an accurate estimate possible, and a professional is worth consulting where the position is complicated.