A tax refund is often treated as a windfall. It is the return of money that was taken during the year in excess of what was owed, and its size is set almost entirely by withholding accuracy.
Withholding is an estimate made in advance
Employers remit tax from each paycheck based on information the employee provides about filing status, dependants and other income.
That information is used to estimate the annual liability and spread it across the pay periods, so the system is forecasting a figure that is not yet known.
The final liability is computed on the return. Whether the year ends in a refund or a balance due depends on how far the estimate drifted from reality.
The withholding form drives the calculation
The form filed with an employer translates household circumstances into a per-paycheck withholding amount, accounting for expected credits and other income.
Most people complete it once when they are hired and never revisit it, even though the assumptions behind it change with almost any life event.
Marriage, a second job, a working spouse, a new dependant or a large bonus all move the correct figure, and none of them updates the form automatically.
Multiple income sources are the common cause of error
Each employer withholds as though its wages were the household's only income, applying the lower brackets to its own payments.
With two jobs or two earners, the combined income sits in a higher bracket than either employer assumed, and too little is withheld overall.
The opposite happens with irregular bonus payments, which are frequently withheld at a flat supplemental rate that can exceed the household's actual marginal rate.
A large refund is an interest-free loan to the government
Over-withholding means the household lived on less during the year and recovered the difference months later without compensation.
The same money held in a deposit account through the year would have earned something, and would have been available in an emergency.
Against that, some people prefer over-withholding precisely because it is a forced saving they cannot access, which is a behavioural argument rather than a financial one.
Adjustment is available at any point in the year
A revised withholding form can be filed with an employer whenever circumstances change, and the effect begins with the next payroll run.
Because the change applies only to remaining pay periods, an adjustment made late in the year has to be larger to correct the same annual gap.
Rules, forms and thresholds are set by tax authorities and change over time, so anyone with a complex situation should check current guidance or take professional advice.