Why You Still Owe Tax When Tax Instalments and Payroll Deductions Do Not Match

Tax taken at source can miss side income, investments, or multiple jobs. Tax instalments cover the gap, so payroll deductions alone may not clear your balance.

By Promissory.ca Editorial Team Reviewed against official Canadian sources Last updated: 2026-09-16

Tax instalments are prepayments you make directly to the Canada Revenue Agency when payroll deductions are not expected to cover your final tax bill. They exist because tax deducted at source is a withholding mechanism, not a final assessment. Your employer withholds based on payroll forms and withholding tables. Your actual tax bill is calculated later on your tax return, using all income, deductions, and credits. If those figures do not match, you can owe more even though every paycheque showed tax taken off.

Why payroll deductions are only an estimate

Source deductions are designed to collect tax throughout the year, but they rely on assumptions. An employer knows the pay it processes, the federal and provincial claims you report on a payroll form, and the withholding tables that apply. It does not know your full financial picture. That is why a tax deduction at source can feel accurate on every paycheque yet still leave a balance when you file.

Multiple income sources

If you work more than one job, each employer may withhold tax as though it is your only income. The combined income can push part of your earnings into a higher marginal tax bracket, but neither payroll system may withhold enough to reflect that combined total. The same issue can arise with pensions, employment insurance benefits, Canada Pension Plan or Quebec Pension Plan payments, and other regular taxable amounts.

Income without payroll withholding

Self-employment income, gig work, contract work, rental income, taxable investment income, and some trust or estate income generally do not have tax deducted at source. Even when withholding applies, such as on certain registered plan withdrawals, the withholding rate may be lower than your final marginal tax rate. That gap becomes a balance owing.

Credits and deductions change

Payroll withholding may not keep up with changes in your life. A new eligible dependant, a disability tax credit, a tuition transfer, a large charitable donation, or a change in child care expenses can affect your final tax. Conversely, losing a credit or adding income can increase tax owing. Your payroll form can be updated, but it is not a substitute for a year-end calculation.

Withholding is not a settlement

The amount shown in the income tax deducted box on a T4 slip is a credit against your tax liability. It is not a final determination. When you file, the CRA calculates total tax, subtracts non-refundable credits and refundable credits, then subtracts tax already paid through payroll and instalments. Only then is the final balance known.

When the CRA expects tax instalments

The CRA generally expects individual tax instalments when your net tax owing is more than $3,000 for the tax year and either of the two prior tax years. In Quebec, the threshold is $1,800. The instalment due dates are 15 March, 15 June, 15 September, and 15 December. Farmers and fishers have a single due date of 31 December. These are administrative rules, not a penalty by themselves, but missing the dates can lead to instalment interest.

Instalments are not a separate tax. They are payments on account of the tax you expect to owe. When you file your return, the CRA credits both payroll deductions and instalment payments against your total tax. If you paid too much, you may receive a refund. If you paid too little, you may owe a balance. The instalment system simply asks you to prepay some of that expected balance during the year.

How tax instalments interact with payroll deductions

Tax instalments and payroll deductions are not alternatives. They work together as two credits against the same tax bill. Payroll deductions happen automatically each pay period. Tax instalments are separate payments you send to the CRA, usually quarterly unless you are a farmer or fisher. If payroll deductions already cover most of your expected tax, your instalment requirement may be smaller. If payroll deductions fall, your instalment requirement may rise.

Payroll deductions can reduce the instalment amount

When you estimate net tax owing, you subtract the source deductions you expect for the year. If your pay increases, your employer may withhold more tax, which can reduce the gap. If your pay decreases, source deductions may fall, leaving a larger gap. A bonus, retroactive pay, or a second job can also change the calculation. The instalment amount is not fixed forever; it should be reviewed when income or payroll changes.

Payroll changes can create an instalment gap

Suppose you change jobs and have a period without pay, then receive a signing bonus or severance. Payroll withholding on the bonus may use a method that does not reflect your annual income. You may still owe tax at filing, and instalments may be requested. Similarly, if you stop having tax withheld from a pension or start receiving self-employment income, the payroll system no longer covers the full bill. The instalment system is designed to fill that gap.

Common situations that create a balance despite source deductions

• You hold more than one job, and each payroll department withholds as if it is your only source of income.
• You have self-employment, gig, or contract income with little or no withholding.
• You receive rental, investment, or trust income that is not fully taxed at source.
• You withdraw from a registered plan and the withholding tax is lower than your final marginal rate.
• You have a large one-time payment, such as a bonus, severance, or retroactive pay.
• Your eligible credits or deductions are lower than the amounts used for payroll withholding.
• You sold an asset and realised a taxable capital gain that was not subject to payroll withholding.

In each case, the common thread is that the payroll system sees only part of your tax picture. The final return sees the whole picture. That is why owing tax after source deductions is normal for many people, not a sign that payroll made a mistake.

Estimating your instalment requirement

The CRA may send instalment reminders or show an amount in your online account. You can also estimate your own position. The goal is to compare expected total tax with expected credits and payments already made through payroll. A tax professional can help if your income is complex, but the general steps are straightforward.

1. Estimate your total taxable income from all sources for the year.
2. Estimate total federal and provincial or territorial tax after non-refundable credits.
3. Subtract expected payroll deductions and any other tax already paid or credited.
4. Compare the remaining net tax owing with the instalment threshold and due dates.
5. Review the estimate after a pay change, job change, large purchase, or investment sale.

If the result suggests you will owe more than the threshold, instalments may be required. If payroll deductions increase later, you may be able to reduce later instalments. If payroll deductions decrease, you may need to increase them. The key is to avoid treating the first estimate as permanent.

Consequences of paying too little or too late

If you are required to pay tax instalments and you pay less than required, the CRA may charge instalment interest. The amount depends on how much was due, when it was paid, and the prescribed rate for the period. Interest can also compound, which makes a small shortfall more expensive over time. In some cases, a penalty may apply if the interest is high enough, but the exact outcome depends on your situation.

Paying too much is not usually a problem in the same way. The excess becomes a credit on your return and may be refunded. The practical risk is cash flow: money sent to the CRA is not available for other uses until the return is assessed. That is why instalment planning is as much about budgeting as it is about tax calculation.

This is general information only, not tax advice.