US Tax Preparation for American Expats: Form 1116 Deductions

Form 1116 does not credit UK tax against US tax pound for dollar. It first reduces foreign source income by a share of your US deductions, then rescales dividends and gains taxed at preferential rates. Accurate US tax preparation for american expats depends on those lines, and Jungle Tax finds them wrong on most self-prepared returns.

This guide deals with one part of the form only: lines 2 to 7 of Part I, and the adjustments at lines 16 and 18 of Part III. It is written for Americans resident in the UK with substantial earnings, investment income, a mortgaged home and, often, a lingering US state filing. Line references are to the 2025 Form 1116 filed during 2026. It assumes you already know that foreign income is split into separate categories and that unused credits carry over; neither topic is repeated here.

Why does UK tax at 45% still leave US tax to pay?

The foreign tax credit is capped. For each category of income, the credit cannot exceed total US tax multiplied by a fraction: foreign source taxable income in that category over worldwide taxable income. UK tax actually paid is irrelevant to the size of that cap. A UK additional-rate taxpayer can pay HMRC far more than the cap and still owe the IRS, because the excess UK tax is simply not creditable in that year.

Everything turns on the numerator. The IRS does not let you put gross UK salary or gross UK dividends into it. You must first subtract:

• deductions that definitely relate to that foreign income;
• a rateable share of deductions that relate to no particular income, including the standard deduction;
• a share of home mortgage interest and of other interest expense;
• foreign source losses, and in some years losses from other categories or from US sources.

Each dollar moved onto those lines lowers foreign source taxable income, lowers the fraction, and lowers the credit. Preparers who leave lines 2 to 5 blank report a credit that is too large. That is the most common Form 1116 error we see on returns prepared without specialist help, and it is an understatement of US tax, not a harmless presentation point.

What goes on lines 2 to 7 of Form 1116?

The IRS instructions for Form 1116 set out a fixed order. The table summarises each line and the method the IRS requires.

Line	What it captures	Method
2	Expenses definitely related to the foreign income on line 1a, including state and local income tax imposed on that income	Direct allocation, with a statement attached
3a	Standard deduction, or certain itemised deductions (medical expenses, general sales taxes, real property taxes on the home, personal property taxes)	Apportioned by gross income (line 3d over line 3e)
3b	Other deductions not related to any specific income, such as adjustments to income on Schedule 1	Apportioned by gross income
4a	Deductible home mortgage interest, including points	Gross income method, on a separate worksheet
4b	Investment, business and passive activity interest, student loan interest	Asset method, each type separately
5	Losses from foreign sources	Direct, after capital loss adjustments
6 and 7	Total deductions, and net foreign source taxable income	Line 1a less line 6

Line 2: which expenses are definitely related to foreign income?

A deduction is definitely related to a class of income when it is incurred to produce that income. For a UK-resident American the usual entries are expenses of a UK sole-trader business, the deductible half of self-employment tax where it applies, and expenses of UK rental property other than interest. Interest never goes on line 2. The instructions require an attached statement itemising what is included, and self-prepared returns almost never have one.

State income tax belongs here too, and it is frequently overlooked. If you still file a resident return in a state that taxes worldwide income, the part of that state tax imposed on your UK income is definitely related to foreign source income. It reduces the numerator even though the same UK income has already borne UK tax. The amount is restricted to what was actually deductible on Schedule A after the federal cap on state and local taxes, which was raised to $40,000 for 2025 and phases down for higher earners.

Lines 3a to 3g: how is the standard deduction apportioned?

The standard deduction is not treated as a US-only item. It is entered on line 3a and multiplied by the ratio of gross foreign source income in the category (line 3d) to gross income from all sources (line 3e). For 2025 the standard deduction is $15,750 for a single filer and $31,500 for a joint return. Someone whose income is 95% UK-source therefore loses roughly 95% of it from foreign source income, spread across the categories.

Three technical points decide whether the ratio is right:

• Gross means gross. Lines 3d and 3e use gross receipts, gains before losses and all other income before deductions. Using net rental profit or net gains distorts the ratio.
• Excluded earnings stay in. Foreign earned income excluded on Form 2555 is included in both lines 3d and 3e, although deductions connected with excluded income are kept off lines 2 to 5.
• Dividends and gains are not rescaled here. Where line 1a has been reduced for the rate adjustment described below, lines 3d and 3e still use the unadjusted figures.

Line 3e is a constant. The same worldwide gross income figure appears on every Form 1116 in the return, and a mismatch between the general and passive forms is an immediate sign that the schedules were prepared independently.

If you itemise, only certain Schedule A items go on line 3a: medical expenses, general sales taxes, real property taxes on the home and personal property taxes. The instructions also exclude the enhanced deduction for seniors from line 3b; it is added back at line 18 instead.

Line 4a: when must home mortgage interest be apportioned?

Deductible home mortgage interest is apportioned by a gross income method on the Worksheet for Home Mortgage Interest. It makes no difference where the property is. Interest on a London home, and interest on a retained US home, are pooled and split between foreign and US income by the same fraction.

The threshold is low. If gross foreign source income, including income excluded on Form 2555, does not exceed $5,000, all interest expense may be allocated to US source income. Above $5,000, apportionment is mandatory. No UK-resident high earner is under it.

One trap: the mortgage worksheet uses a different ratio from line 3f. It expressly leaves income excluded on Form 2555 out of both the numerator and the denominator, whereas lines 3d and 3e include it. Copying the line 3f percentage onto line 4a is wrong for anyone who also claims the exclusion.

Line 4b: how does the asset method work for other interest?

Investment interest, business interest and passive activity interest are each apportioned separately by reference to assets, not income. The IRS example is straightforward: $2,000 of investment interest, with assets of $40,000 producing US source income and $60,000 producing foreign source income by adjusted basis, puts $1,200 on line 4b.

Two UK situations matter in practice. A margin or Lombard facility secured on a mixed portfolio must be traced to the assets held, so a portfolio weighted to UK and European holdings pushes most of the interest against foreign passive income. And interest on borrowing to acquire UK rental property is passive activity interest, apportioned by passive activity assets, which are usually wholly foreign.

Which deductions are not charged against foreign income at all?

Charitable contributions are the main one. They are allocated to US source income under the regulations and do not appear in the line 3a list. Expenses definitely related to US source income, such as costs of a US rental property, also stay off the form. Getting this wrong in the other direction, by apportioning everything on Schedule A, understates the credit and overpays US tax.

How do the capital gain and qualified dividend rate adjustments work?

Dividends from UK companies are generally qualified dividends, taxed in the US at 15% or 20% instead of ordinary rates. Because that income bears less US tax, the IRS requires it to be scaled down before it enters the limitation fraction. The IRS foreign tax credit compliance tips identify this as a recurring error.

• Foreign source qualified dividends and capital gain distributions taxed at 15% are multiplied by 0.4054 before entry on line 1a.
• Those taxed at 20% are multiplied by 0.5405.
• Those taxed at 0% are left off line 1a altogether.
• The denominator is scaled as well. On the Worksheet for Line 18, worldwide 20% income is reduced by 0.4595 of its amount and worldwide 15% income by 0.5946.

There is an adjustment exception, but it is narrow. For 2025 it applies only if line 5 of the Qualified Dividends and Capital Gain Tax Worksheet does not exceed $197,300 ($394,600 on a joint return) and foreign source qualified dividends plus net capital gain total less than $20,000. Readers of this guide will rarely meet both tests. The election is also all-or-nothing: you cannot adjust line 1a and leave line 18 unadjusted.

Filers with a Schedule D face further steps: a US capital loss adjustment where US losses have absorbed foreign gains, and a rate differential adjustment for each rate group. A foreign source capital loss on line 5 is adjusted in the same way. These are set out in IRS Publication 514, and they are the point at which most consumer-grade preparation gives up.

What happens when another category or US income shows a loss?

Line 15 carries down the net figure from Part I. Line 16 then adjusts it. A loss in one category, for example a UK rental loss in the passive category, must be allocated against income in the other foreign categories before it touches US income. A net US source loss, which is common where a US rental property or a US business runs at a deficit, is allocated against foreign source income in every category and reduces each limitation. Both create loss accounts that are recaptured in later profitable years, so an error persists beyond the year it is made.

Worked example: a London executive with dividends and a mortgage

Take a single US citizen resident in London filing a 2025 return, figures in dollars:

• UK salary, general category: $600,000
• UK qualified dividends, passive category: $100,000
• US source bank interest: $50,000
• Itemised deductions: $40,000 deductible mortgage interest on the London home and $10,000 to US charities

Taxable income is $700,000 and US tax before credits is about $199,547, of which $20,000 is tax on the dividends at 20%.

Self-prepared version. Lines 2 to 5 are blank and nothing is rescaled. Foreign taxable income is shown as $700,000, the fraction is 100%, and the credit eliminates the entire US liability. The return shows nil tax due.

Correct version.

• Mortgage interest is split by gross income of $750,000: $32,000 to the general category (600/750), $5,333 to passive (100/750) and $6,667 to US income. Charitable contributions stay with US income.
• General category line 7: $600,000 less $32,000 = $568,000.
• Passive category line 1a: $100,000 x 0.5405 = $54,050. Line 7: $54,050 less $5,333 = $48,717.
• Line 18: $700,000 less ($100,000 x 0.4595) = $654,050.
• General limitation: $568,000 / $654,050 x $199,547 = $173,294.
• Passive limitation: $48,717 / $654,050 x $199,547 = $14,863.

The maximum credit is $188,157, leaving $11,390 of US tax payable. UK tax on those dividends at the 39.35% additional rate is $39,350, yet only $14,863 is usable against a US charge of $20,000 on the same income. UK income tax on the salary, in the region of $250,000 at recent exchange rates, is similarly capped at $173,294. The surplus is not lost, but it cannot be used this year, and it cannot cross from one category to the other.

US and UK treatment compared

Item	US return (IRS)	UK return (HMRC)
Interest on a main home mortgage	Deductible if itemising, within the acquisition debt limit; apportioned to foreign income on line 4a	No relief
Interest on residential rental borrowing	Deductible against rental income; apportioned by assets on line 4b	No deduction; a basic rate tax reduction only
Basic allowance	Standard deduction, apportioned to foreign income on line 3a	Personal allowance of £12,570, withdrawn above £100,000 and nil from £125,140
Dividends	15% or 20% if qualified; rescaled in the limitation	Ordinary rates up to 39.35%, with the lower dividend rates rising from April 2026
Top rate on earnings	37%	45%
Relief for the other country's tax	Credit limited by category on Form 1116	Foreign tax credit relief, limited to UK tax on the same income and to treaty rates

The mismatch is structural. HMRC taxes a wider base with almost no personal deductions, so the UK liability is high. The IRS then measures your foreign income on a narrower, US-computed base. The higher the UK tax and the larger your US deductions, the wider the gap between tax paid and credit allowed. HMRC's own guidance on income taxed twice works the same way in reverse for US source income: relief is capped at the UK tax on that income.

What are the errors most often found on self-prepared returns?

• Lines 2 to 5 left blank, so gross foreign income is treated as foreign taxable income.
• The standard deduction omitted from line 3a on the assumption that it is a domestic allowance.
• Mortgage interest on a UK home ignored, or the line 3f ratio reused on line 4a.
• Qualified dividends entered at full value on line 1a, with line 18 taken straight from Form 1040.
• Line 1a adjusted but line 18 not, or the reverse.
• Different line 3e figures on the general and passive forms.
• State income tax on UK earnings not placed on line 2.
• Charitable contributions wrongly apportioned to foreign income, understating the credit.
• No separate computation for alternative minimum tax, which requires its own Form 1116.
• No attached statements for lines 2, 3a and 3b.

How is this put right on amended or late returns?

The correction is a recomputation, not a disclosure. Each affected year needs Part I rebuilt from gross income, the interest worksheets completed, line 18 recalculated and the credit restated. Because the limitation for one year determines the excess carried to the next, the years must be redone in sequence, earliest first.

Returns already filed. An amended return on Form 1040-X restates the credit and pays the additional tax with interest. An amended return filed before the IRS makes contact about the issue can generally be treated as a qualified amended return, so that the accuracy-related penalty is measured against the corrected figure. Where the correction runs in your favour, for instance because deductions were over-allocated to foreign income, claims attributable to foreign tax credits benefit from an extended ten-year refund period in place of the usual three.

Returns never filed. Under the IRS streamlined filing procedures, the three most recent years of returns are submitted together. Those returns must be right on first submission, and a credit computed without apportionment undermines the certification that the tax shown is correct. We prepare the three years as one linked computation so that each year's limitation and excess reconcile to the next.

UK side. A corrected US liability can alter the relief claimed on US source income in a UK self assessment return, which is why our US-UK tax accountants and UK tax services team reconcile both returns together. Clients with larger portfolios and several properties are handled through our high net worth preparation service, and further technical notes are in our guides library.

What records does a correct Form 1116 require?

• Gross income by source and category, before any expenses or losses, in dollars.
• Schedule A as filed, with mortgage interest statements for every property and confirmation that each loan is secured on a qualifying home.
• Loan statements for investment and property borrowing, with a schedule of the assets each facility financed and their adjusted basis.
• State returns for any year a state taxed UK income.
• Dividend vouchers and broker reports identifying qualified dividends and the holding periods behind them.
• The Qualified Dividends and Capital Gain Tax Worksheet or Schedule D Tax Worksheet for each year.
• Prior-year Forms 1116 and loss account balances.

Key points

• The credit is capped by net foreign source taxable income computed under US rules, not by UK tax paid.
• The standard deduction, home mortgage interest and other interest all reduce foreign income by formula.
• The $5,000 exception for interest is irrelevant to almost every UK-resident filer.
• Qualified dividends and long-term gains must be rescaled in both the numerator and the denominator.
• Errors compound through carryovers and loss accounts, so corrections are made year by year in order.

If your returns were prepared without lines 2 to 5 completed, or you are bringing several years up to date and want the foreign tax credit computed properly the first time, please contact our cross-border team for a confidential consultation. We will review the filed returns, quantify any exposure or refund, and prepare the corrected US and UK returns together.