What it means
Employers withhold income tax from wages in instalments through the year, based on estimates made from forms the employee fills in. If those estimates are too low, the amount withheld falls short of the final tax liability.
The employee finds out only when the year-end calculation is done. Common causes include having two jobs where each employer withholds as if it were the only one, claiming more allowances than you are entitled to, and receiving extra income that has no tax deducted, such as freelance fees or investment gains.
A bonus or a pay rise part-way through the year can also push total tax above what the earlier withholding assumed. The consequence is a balance due.
Many tax systems also charge interest or a penalty if the shortfall is large or the person did not make adequate payments during the year, though the exact thresholds and safe harbours are set by the tax authority and change over time. Fixing it is straightforward when caught early.
The employee updates the withholding form so that more is deducted from each remaining pay period, or makes a separate estimated tax payment. Spreading the shortfall over the remaining months is much easier on cash flow than facing one lump sum.
Employers have a compliance interest as well. If a payer withholds less than the law requires, the authority can hold the payer responsible for the missing amount, so payroll teams check their tables and processes carefully.
Employees who expect a large change in income can avoid the problem by running a quick estimate before the year begins. Most tax authorities provide calculators for this, and payroll or finance teams can often help.
A five-minute check early in the year costs far less than a surprise bill at the end.
In practice
Real-world examples.
Example
A software engineer takes a second job at a consulting firm. Both employers withhold tax as though their salary were the only income, so by April the engineer owes several thousand dollars more than was deducted. A short conversation with payroll, asking each employer to withhold at a higher rate, would have prevented the surprise.
Example
A restaurant manager receives a $15,000 year-end bonus that is paid with very little tax taken off. The manager sets aside part of the bonus and increases deductions in the following year to avoid a repeat shortfall.
Example
A retired teacher draws a pension and sells shares at a gain, but neither source has tax withheld at the right level. She makes a quarterly estimated payment to close the gap instead of waiting for the annual return. She also adds a reminder to her calendar each July to compare her year-to-date tax with her expected liability.
Formula
Calculation
Underwithholding = Total tax liability - Total tax withheld
Monthly catch-up = Underwithholding / Remaining pay periods
An employee earns $120,000 and ends up owing $24,000 of income tax for the year. Her employer withheld $1,700 a month, so total withholding is $1,700 x 12 = $20,400.
Underwithholding = $24,000 - $20,400 = $3,600
If she spots the problem with 12 months to go, she can ask for an extra $3,600 / 12 = $300 a month to be withheld, which brings the total to $2,000 a month and exactly covers her liability of $2,000 x 12 = $24,000.
The arithmetic checks: $300 x 12 = $3,600, and $1,700 + $300 = $2,000 a month. If she waited until only three months remained, the extra deduction would be $3,600 / 3 = $1,200 a month, which shows why acting early matters.Case study
Seen in the real world.
Tallgrass Logistics is an illustrative, fictional haulage company that runs payroll through a small in-house team. When tax rates changed mid-year, the team forgot to update the withholding table for 40 employees on higher pay bands.
Each affected employee had about $60 a month too little withheld for six months, a total of $360 per person. At year end the shortfall appeared as unexpected tax bills, and several staff complained that they had planned their budgets around expected refunds.
The company paid for a payroll review, corrected the table and offered affected staff the option to spread the extra deduction over the following months. The illustrative lesson is that a small monthly error becomes a visible problem once it is added up across a full year. Staff said the clear explanation mattered more to them than the correction itself, and the payroll team now sends a short reminder each January about checking withholding after any change in circumstances.
Watch out
Common mistakes.
- Assuming that tax withheld by an employer always equals the tax you actually owe.
- Ignoring other income, such as side work or investment gains, when setting withholding on your main job.
- Waiting until the tax return is due to discover a shortfall, instead of checking mid-year when it can be corrected cheaply.
Questions
People also ask.
What causes underwithholding most often?
Multiple jobs, too many allowances claimed, untaxed side income and large bonuses are the usual causes.
Can underwithholding lead to a penalty?
In many tax systems it can if the shortfall is large or payments during the year were inadequate, but the thresholds are set by the tax authority.
How do I fix underwithholding?
Update your withholding form to increase deductions, or make estimated payments directly to the tax authority.
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