Treaty-tier withholding and foreign tax credit modeling

25% on a Canadian dividend at statutory rate, 15% under the US-Canada treaty for a qualified holder, 0% net US tax with a correctly filed Form 1116 — and 25% double tax if the Form 1116 is wrong or absent. The same pair of numbers applies in slightly different combinations to every cross-border equity dividend a US holder receives, and the bridge between the source-country withholding and the US filing is the foreign tax credit, separated by passive-versus-general basket, sourced country-by-country, with a ten-year carryforward.

This is also where domestic-only tax software has its largest correctness gap on cross-border holdings. The rest of the article walks through the withholding side (what each country takes, by treaty pair), the FTC side (basket separation, sourcing, carryforwards), and the synthetic-data shapes a platform needs to exercise both layers without shipping the 25-vs-0% bug.

The withholding side: what each country takes

Default statutory withholding rates by country are typically high (25-30% on dividends, 15-30% on interest). Tax treaties between the US and the source country reduce these for qualified holders to negotiated rates — typically 15% on dividends, 0-10% on interest, with reduced rates for substantial-ownership cases.

The "treaty rate" column is what should be withheld for a US holder who has properly documented their treaty status (typically by filing a W-8BEN with the foreign withholding agent). The "statutory rate" is what gets withheld if documentation is missing or invalid. The difference is real money — and it's a real production bug for platforms whose customers haven't filed their W-8BENs correctly. The platform either has to flag the documentation gap proactively or surface the over-withholding so the customer can recover it via reclaim (a separate, often-painful process administered by the source country).

// Foreign dividend payment with treaty handling
{
  "ca_id": "CA-2025-RY-DIV-Q3",
  "symbol": "RY.TO",
  "issuer_country": "CAN",
  "ca_type": "qualified_dividend",
  "ex_date": "2025-09-25",
  "pay_date": "2025-11-12",
  "gross_amount_local": 1.34,                    // CAD per share
  "gross_amount_base": 0.991,                    // USD per share at FX
  "withholding_rate_applied": 0.15,              // treaty rate; would be 0.25 if W-8BEN missing
  "withholding_amount_local": 0.201,             // CAD per share withheld
  "withholding_amount_base": 0.149,              // USD per share withheld
  "net_amount_base": 0.842,                      // USD net to holder
  "treaty_invoked": "US-Canada",
  "ftc_eligible_amount_base": 0.149              // foreign tax for FTC purposes
}

The FTC machinery: Form 1116 baskets and limits

Foreign tax credits are claimed on Form 1116. The form is structured by "basket" — a categorization of foreign-source income into types that are subject to separate FTC computation. The two baskets that matter for most retail and institutional wealth-tech use cases:

• Passive category income. Dividends, interest, capital gains, royalties, and other passive-investment income.
• General category income. Active business income, wages, and certain other categories.

A holder with both passive and general foreign-source income files two separate Form 1116s — one per basket. FTCs computed in one basket cannot offset US tax on income in the other basket; the limitation calculation is per-basket.

The FTC limit per basket is:

Excess foreign tax paid (beyond the basket's FTC limit) carries forward 10 years and back 1 year, separately by basket. The carryforward tracking is a non-trivial data structure: per-basket, per-year, per-source-country rolling balance.

Country-by-country sourcing — the part that gets missed

The FTC computation requires knowing the source country of every dollar of foreign-source income. This sounds straightforward — a Canadian dividend is Canadian-source — and is genuinely complicated for some categories:

• Foreign mutual fund distributions. A foreign fund holding equities from many countries distributes income that came from many sources. Some funds report by country (allowing pass-through sourcing); many don't. The default treatment for unsourced fund distributions can be unfavorable.
• ADR (American Depositary Receipt) dividends. The dividend is paid via the US depositary bank but the underlying issuer is foreign. Sourcing follows the underlying issuer's country.
• Forced-paid taxes. Some countries (notably Spain, Italy, and certain emerging markets) have minimum withholding rates that exceed treaty rates, with reclaim procedures that are difficult to use. The "actual foreign tax paid" for FTC purposes is the amount withheld, but the over-withholding is technically not FTC-eligible to the extent it exceeds the treaty rate.
• Resourced-by-treaty income. Certain US-source income is "resourced" as foreign-source under specific treaty provisions; this affects the FTC limit calculation.

Synthetic data has to model country-of-source per income item, not just at the portfolio level. A test corpus that classifies all foreign-fund distributions as a single "international" source can't exercise the per-country sourcing logic that actual Form 1116 preparation requires.

The high-tax kick-out and the substantial-presence rule

Two specific FTC rules that synthetic test data has to model because they're the source of common bugs:

The high-tax kick-out (§904(d)(2)(B)): Foreign income that bears a high effective rate of foreign tax can be moved from the passive basket to the general basket, where it's typically easier to use the FTC against. The election is made annually; it can prevent the passive-basket FTC limitation from being binding.

The substantial-presence rule (§7701(b)): Affects how a non-US-citizen is taxed in the US. A non-citizen present in the US 183+ days in the current year (or by the weighted-day formula across 3 years) is a resident alien for tax purposes and subject to worldwide-income taxation, including the FTC for foreign-source income. A non-citizen present fewer days is a non-resident alien and taxed only on US-source income.

Test data for cross-border platforms has to include some households where the substantial-presence determination matters — international-assignment employees, dual-residents, multi-year transitionary periods.

What synthetic data has to model

A realistic FTC-aware test corpus needs:

How this shows up in our catalog

The cross-border bundles in the WealthSynth catalog include households with realistic foreign-dividend income across the major treaty partners (UK, Canada, Germany, France, Japan, Switzerland, Australia, Netherlands). Withholding rates are calibrated to the treaty rates for properly-documented holders, with a small subset (~5%) of households modeled as having documentation gaps that produce statutory-rate withholding. FTC carryforwards are carried in the longitudinal data so the multi-year carryforward consumption logic is exercised.

For the broader cross-border context, see Cross-Border & Multi-Currency Wealth. For the foreign-fund-specific tax interaction, see PFIC tracking and excess-distribution modeling. For the multi-currency value translation that pairs with FTC computation, see Multi-currency portfolio modeling in synthetic households.