Form 1116: Why Isn't My Foreign Tax Credit Dollar for Dollar?

Because the credit is capped, not at the Canadian tax you paid, but at the US tax attributable to your foreign-source income. IRC 904(a) sets that ceiling. When Canadian rates are higher than the effective US rate on the same income (and for an employed Canadian in the US, they often are), the excess Canadian tax generates unused credits rather than a dollar-for-dollar offset. Those unused credits carry forward, and a separate problem starts when the Canadian tax you claimed changes after you filed: IRC 905(c) forces a redetermination, and IRC 6689 penalizes a late notification of it.

How does the foreign tax credit limitation work?

The formula is in IRC 904(a). Your foreign tax credit for any category of income cannot exceed your US tax liability multiplied by the ratio of your foreign-source taxable income to your worldwide taxable income.

In a formula:

Credit limit = US tax × (foreign-source taxable income / worldwide taxable income)

If your Canadian employment income is your only foreign-source income, and the Canadian tax on it exceeds that limit, the excess does not reduce your US tax further. It becomes an unused credit.

Why the limit binds for Canadians: combined Canadian federal-provincial tax rates on employment income run from roughly 20% to over 50% depending on the province and bracket. The effective US federal rate on the same income, after the standard deduction and graduated brackets, is often lower. The gap between what you paid Canada and what the US would have charged on the same slice is the excess credit.

Piece	What it is
US tax liability	Your total federal income tax before credits (the number the limit runs against)
Foreign-source taxable income	Income sourced to Canada (or another country) under the US source rules, including treaty overrides
Worldwide taxable income	All your taxable income, US and foreign combined
Credit allowed	The lesser of the Canadian tax paid and the limit from the formula
Excess credit	Canadian tax paid minus the credit allowed, when the Canadian tax is higher

The limit runs separately for each category of income that Form 1116 tracks. General category (most employment and business income) and passive category (investment income) are the two that cross-border filers see most often.

What happens to excess credits?

They carry forward. IRC 904(c) allows unused foreign tax credits to be carried to the succeeding 10 taxable years. The one-year carryback was repealed for tax years beginning after December 31, 2017, so the carry runs forward only.

Excess credits from a high-tax year (say, a year where you worked in Ontario and paid combined federal-provincial tax at 46%) can offset US tax in a later year where the limit has room, which happens when your foreign-source income ratio rises or your US tax goes up. For a cross-border filer who moves to the US and keeps earning both US-source and Canadian-source income, the ratio shifts over time, and excess credits from the move year can absorb tax in later years when the Canadian income shrinks.

Schedule B of Form 1116 reconciles the carryover from year to year. If you have carried credits into a year and also have a current-year credit, the current year is used first, which preserves the older credits for future use within the 10-year window.

What happens when my Canadian tax changes after I filed?

IRC 905(c) requires a redetermination. If the amount of foreign tax you claimed as a credit on Form 1116 changes for any reason, whether by CRA reassessment, an amended Canadian return, a provincial refund, or a tax treaty adjustment, your US foreign tax credit must be recalculated, and you must notify the IRS.

The statute is broad about what triggers it: any change in the amount of foreign tax paid, accrued, or deemed paid. A $500 CRA reassessment that reduces your Canadian tax by $500 is a redetermination event. So is a refund from an amended return, a carryback of losses that reduces provincial tax, or a reduction triggered by a Canadian objection or appeal.

The practical issue for cross-border filers is timing. You file your US return in April (or October with an extension), claiming a credit for the Canadian tax you expect to owe or have paid. The CRA processes your Canadian return on its own timeline, and a notice of reassessment may change the numbers months or years later. A redetermination event can happen long after the US return is filed.

The IRS introduced Schedule C of Form 1116 for reporting redeterminations, replacing the older requirement to file an amended return in all cases. Under the regulations finalized in 2022 (T.D. 9959, applicable to tax years beginning after December 28, 2021), the reporting method depends on the size and nature of the change. For many adjustments, the redetermination is reported on Schedule C attached to the next return you file, rather than by amending the original year. For larger or more complex changes, the instructions to Schedule C and the underlying regulations govern the specific procedure.

Is there a penalty for not reporting the change?

Yes. IRC 6689 imposes a penalty for failure to timely notify the Secretary of a foreign tax redetermination. The penalty is generally a portion of the additional US tax due from the redetermination, based on how late the notification is, and it can run from 5% to 25% of the deficiency. Reasonable cause can excuse it, but the excuse is not automatic.

The penalty is not well known, and that is part of the problem. A CRA reassessment that drops Canadian tax by a few thousand dollars may seem like good news, but if the US side is not adjusted, the taxpayer is carrying a foreign tax credit that overstates the tax actually paid, and the notification clock is running.

Why does the Canadian tax change so often?

Because the Canadian and US filing systems do not run in parallel. A few common patterns:

The sequence gap. A Canadian who moved to the US files the US return first (due April 15) and the Canadian return second (due April 30 for the departure year, or June 15 if a business was carried on). The US return claims a credit for Canadian tax that may be estimated, because the Canadian return has not been assessed yet. When the CRA assessment comes back different, a redetermination is triggered.

CRA reassessment. The CRA routinely reassesses returns, especially on cross-border files with foreign income, treaty claims, or departures. A reassessment that changes tax payable triggers 905(c).

Amended Canadian returns. If you amend your Canadian return (T1-ADJ or a new T1) and the tax changes, the credit you claimed on the US side no longer matches.

Provincial adjustments. Provincial tax is a separate computation on the Canadian return, and changes to it flow through to the total Canadian tax that feeds the US credit.

That mirror effect is the thing that surprises people. A Canadian refund after claiming a foreign tax credit does not always leave you better off, because the credit adjustment can claw back most or all of the benefit on the US side.

What should I do next?

Keep your CRA notices of assessment and reassessment for every year you claimed a foreign tax credit. When a CRA reassessment changes your Canadian tax, compare the new number against what your Form 1116 claimed. If the numbers differ, the change needs to be reported on Schedule C of Form 1116 with your next US return, and if the difference is large or the return is old, read the Schedule C instructions for the specific procedure that applies. If you have years of unchecked mismatches, sort them before the IRS finds them, because the penalty under IRC 6689 runs on the notification delay.

• What does the streamlined procedure actually cost for a Canadian?, because every year in a catch-up package includes Form 1116
• Form 8938 vs FBAR: do I file both?, the other annual filings that travel with Form 1116
• Getting the 15% treaty rate on IRA and 401(k) income in Canada, where the credit runs in the other direction

This guide is general information, not tax advice for your specific situation. Which points apply, and how, depends on your facts.