A contractor whose net income jumps from CAD $85,000 to CAD $140,000 partway through the year often keeps paying instalments sized to the CRA reminder that arrived in February, calculated from the prior year’s smaller return. By the time the increase shows up in bookkeeping or a mid-year review, one or two instalment due dates have already passed at the old, lower amount, and interest has started accruing on the gap.
This is a different problem from the mechanics covered in the quarterly tax instalments guide, which explains the three calculation methods and when instalments are required in the first place. This guide is for contractors who are already in the instalment system and need to catch up mid-year after income rises faster than expected.
Why the Gap Appears
CRA’s no-calculation reminders are built from the two most recently assessed returns. A contractor whose income was CAD $85,000 last year gets reminders sized to that figure, regardless of what the current year actually looks like. Following the reminder amount protects against instalment interest under the no-calculation safe harbour, but only because CRA absorbs the mismatch between the reminder and the higher actual liability, not because the amount was ever meant to track the current year.
The gap becomes a problem specifically for contractors who stop using the no-calculation method partway through the year, whether deliberately or by accident, for example by paying a rounded amount that does not match either the reminder or a proper current-year estimate. Once a payment departs from the no-calculation figure, the safe harbour it provides no longer applies to that instalment, and CRA compares the payment against the actual amount that should have been paid.
Switching to the Current-Year Method Mid-Year
The three calculation methods are not chosen once for the whole year. A contractor can pay the March instalment using the no-calculation reminder, then switch to a current-year estimate for June, September, and December once the income increase is clear.
The switch only protects the instalments paid after it happens. If the increase became apparent after the June due date, the March and June instalments are assessed against whatever method actually applied to them at the time, and any shortfall at those dates accrues interest from that date forward, independent of what happens at September and December.
Worked example. A contractor’s prior-year net tax owing was CAD $18,000, generating no-calculation reminders of CAD $4,500 per quarter. A single large project in Q2 raises expected full-year net tax owing to CAD $34,000. The March instalment (CAD $4,500, paid before the income jump was apparent) stands as filed. From June forward, the contractor recalculates: CAD $34,000 minus the CAD $4,500 already paid, divided across the three remaining due dates, works out to roughly CAD $9,833 per quarter for June, September, and December. This does not eliminate interest on any shortfall relative to what a full-year current-year estimate would have required at each date, but it stops the gap from widening further.
Sizing the Catch-Up Payment
A common mistake is treating the September or December instalment as the place to make up the entire year’s shortfall in one payment. Interest at each due date is calculated on the amount that was actually required and unpaid at that date. A large payment in September does not reduce interest that already accrued on the March and June shortfalls; it only affects what continues to accrue on the September balance going forward.
The more useful calculation is a revised full-year estimate, run as soon as the income increase is reasonably certain, that resets what each remaining instalment should be. This keeps future due dates aligned with the actual liability instead of leaving them sized to a February reminder that is now several months out of date.
Where the income increase happened early in the year, for example a signing bonus or a large Q1 contract that resolved before March, running the current-year estimate before the March due date, rather than catching up after, avoids the shortfall altogether.
When the Increase Is Temporary
Not every income jump is a permanent step up. A one-time project or an unusually large Q1 that will not repeat next year is still counted at full value for the current year’s instalment calculation, but it is worth flagging as a one-time item when setting next year’s instalment expectations, since the no-calculation reminders for next year will otherwise be built from this year’s inflated figure.
A contractor who does not expect to repeat this year’s income level has a case for using the current-year method again next year, sized to the lower expected liability, rather than defaulting to reminders based on an unusually high year. That approach carries the same underpayment interest risk as any current-year estimate if the projection turns out too low, so it works best when the reason for the drop (a contract ending, a planned reduction in hours) is clear rather than speculative.
Related Articles
- Quarterly Tax Instalments for Canadian IT Contractors covers the underlying instalment threshold, due dates, and the three calculation methods this guide assumes as background.
- Quebec Quarterly Tax Instalments for IT Contractors covers the parallel Revenu Québec instalment track, which follows the same catch-up logic on its own due dates.
- CRA Penalties and Interest covers how instalment interest fits alongside other CRA interest and penalty categories.