The SALT torpedo: why $500K–$600K earners get a bonus tax break from box spreads

TL;DR: Under the One Big Beautiful Budget Act (2025), the SALT deduction cap is $40,000 but phases out at 30 cents per dollar of income between $500,000 and $600,000. This creates an invisible surcharge — the effective marginal tax rate in that income band is roughly 8-9% higher than the nominal rate, due to lost deductions. A box spread capital loss can reduce income into or through this band, reversing the phase-out and cutting the effective borrowing rate to as low as 2.14% after tax. This is the most powerful tax-efficiency argument for the strategy, and it only applies to a specific income band.

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The 2-minute version

In 2025, Congress reset the SALT deduction cap to $40,000 — a significant increase from the prior $10,000 cap. For most taxpayers, this was straightforwardly good.

But the legislation included a phase-out. Taxpayers earning between $500,000 and $600,000 in modified adjusted gross income (MAGI) lose the $40,000 SALT deduction gradually. The phase-out rate is $0.30 per dollar of income over $500,000. By the time income reaches $600,000, the full $40,000 SALT deduction has been phased out to zero.

This creates a hidden problem. At $499,999 income, a taxpayer gets the full $40,000 SALT deduction. At $600,001, they get nothing. Each dollar earned between $500,000 and $600,000 costs 30 cents of SALT deduction. Each lost cent of SALT deduction — if the taxpayer is itemizing — increases taxable income by 30 cents and tax by 30 cents × marginal rate.

At a 37% federal marginal rate, each dollar of income in this band costs an additional 0.30 × 0.37 = 11.1 cents in federal tax on top of the regular marginal rate of 37 cents. The blended federal effective marginal rate in this band is 48.1%.

Add California’s 13.3% on top and the combined effective marginal rate for some taxpayers in this band approaches 55%.

Now reverse it. A capital loss from a box spread reduces MAGI. If the capital loss pulls income down from $580,000 to $480,000, it reverses the phase-out on $80,000 of the SALT cap. The deduction is recaptured. The effective cost of the capital loss (what it “saves” per dollar) is substantially higher than just the face value of the marginal rate.

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The nerdy version

The mechanics of the SALT phase-out

The One Big Beautiful Budget Act (OBBBA) sets the SALT deduction as follows:

• Maximum: $40,000
• Phase-out begins at: $500,000 MAGI
• Phase-out rate: $0.30 per dollar of MAGI over $500,000
• Phase-out complete at: $633,333 MAGI ($40,000 / $0.30 = $133,333 phase-out range above $500,000)

Note: Some analyses cite $600,000 as the complete phase-out; the exact figure depends on implementation. Verify current-year parameters with a CPA.

Each dollar of income in the phase-out range has a hidden multiplier. To quantify it:

Per dollar of income in the $500K-$600K band:

• Regular federal marginal rate: 37%
• SALT deduction lost: $0.30
• Value of lost SALT deduction (at 37% rate): $0.30 × 37% = $0.111
• Net effective federal marginal rate: 37% + 11.1% = 48.1%

Add California state income tax (13.3%, which also has no SALT offset because CA doesn’t conform to the federal SALT cap):

• Combined effective marginal rate in the phase-out zone: 48.1% + 13.3% = ~61.4%

This is not theoretical. A California taxpayer at $550,000 income is paying roughly 61 cents of combined tax on each additional dollar, or on each dollar they might have reduced income by had they taken steps to do so.

How the box spread capital loss interacts

A box spread generates Section 1256 capital losses. Capital losses reduce net capital gain, which reduces adjusted gross income and ultimately MAGI.

If a California taxpayer has $550,000 in income (after all other deductions), a $50,000 Section 1256 capital loss pulls MAGI to $500,000 — exactly at the start of the phase-out band. The $50,000 of income reduction was worth:

• Regular capital gains savings: $50,000 × 23.8% (20% LTCG + 3.8% NIIT) federal
• SALT recapture: $50,000 × 30% = $15,000 of SALT deduction restored × 37% = $5,550 additional federal savings
• California savings: $50,000 × 13.3% = $6,650
• Total savings: $11,900 + $5,550 + $6,650 = ~$24,100

On a $50,000 capital loss deduction, the total tax saving is roughly $24,100 — an effective benefit rate of 48.2%.

Against a 4% nominal borrowing rate: the after-tax effective rate is approximately 4% × (1 - 0.482) = 2.07%.

The four-situation matrix

The tax benefit varies significantly depending on the taxpayer’s situation. Not everyone is in the SALT torpedo zone, and not all capital gains are equivalent.

Situation	Subsidy rate	Effective rate at 4% nominal
SALT zone ($500K-$600K MAGI) + short-term capital gains to offset	~46-50%	~2.14%
SALT zone + long-term capital gains to offset	~32-38%	~2.50-2.72%
Outside SALT zone + short-term capital gains	~37-40%	~2.40-2.52%
Outside SALT zone + long-term capital gains	~23-28%	~2.88-3.07%
No capital gains, standard deduction	~10-15%	~3.40-3.60%

These ranges are approximate. Actual rates depend on state, filing status, deductions, and specific income composition.

A worked example: Tony in the SALT zone

Tony earns $300,000 in W-2 salary. He exercises $250,000 in stock options, creating $250,000 in ordinary income (a real-world income event for tech employees). Total MAGI: $550,000.

He’s squarely in the SALT phase-out zone. His $550,000 puts him $50,000 into the phase-out band; he has lost $15,000 of his $40,000 SALT deduction.

Tony opens a $500,000 box spread loan to fund a home renovation instead of selling appreciated portfolio holdings. The 3-year fixed rate is 3.70%. Year 1 mark-to-market generates approximately $18,500 in Section 1256 capital losses (first year of a 3-year loan with annual recognition of implied interest).

Those $18,500 in losses:

1. Offset $18,500 of Tony’s investment income — at a combined rate including SALT recapture of approximately 48%
2. Pull $18,500 of income out of the phase-out zone, restoring $5,550 of his SALT deduction
3. Net tax saving: $18,500 × 48% = $8,880

Effective interest cost in Year 1: $18,500 nominal interest - $8,880 in tax savings = $9,620. Against $500,000 borrowed, that’s a 1.92% effective rate.

The same loan to a taxpayer outside the SALT zone with LTCG exposure would produce a Year 1 effective cost of roughly $18,500 × (1 - 0.238) = $14,100. Still good, but meaningfully higher.

Verification caveat

The SALT torpedo provisions are from the One Big Beautiful Budget Act (OBBBA) of 2025. The parameters described above — $40,000 cap, $500,000-$600,000 phase-out range, 30% phase-out rate — reflect the OBBBA as enacted. Congress can and does change these parameters.

Before planning any transaction around the SALT torpedo, verify the current-year phase-out parameters with a CPA. The strategy works as described here only if the phase-out provisions remain in effect.

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What this isn’t

This bonus is narrow. It only applies to taxpayers in the specific SALT phase-out income band. Below $500,000, the full SALT deduction is already captured and there’s nothing to recapture. Above the phase-out range, the deduction is already fully lost and the torpedo has fully fired. The window is $500,000 to $600,000 MAGI (approximately).

This does not help non-itemizers. The SALT deduction only exists for taxpayers who itemize. Standard deduction filers get no SALT benefit and therefore get no SALT torpedo recapture. The strategy still provides a capital loss benefit, but the torpedo component is zero.

This is not a planning justification by itself. The SALT torpedo recapture is one input in the effective rate calculation, not a reason to borrow. If the underlying purpose (home purchase, bridge, tax bill) doesn’t make sense at the base rate of 4%, a marginal tax benefit doesn’t change the fundamentals.

State matters enormously. California’s 13.3% rate at the top bracket makes the combined effective marginal rate calculation substantially more favorable for CA residents in the SALT zone. A Texas or Florida resident (no state income tax) gets a smaller benefit from the same box spread capital loss. The numbers in this article are California-specific unless stated otherwise.

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If you want to actually do this

The next article shows the full scenario analysis across all four major taxpayer situations — STCG, LTCG, both, and neither: “Short-term gains, long-term gains, or none: how your tax situation changes the effective rate”.

For the mechanics of how Section 1256 capital losses flow through the tax return, start with “How the IRS treats box spreads: Section 1256 and the 60/40 rule”.

When ready to talk to a firm that executes this and a CPA who models the exact savings for your situation, request a referral. See full disclosure.

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Sources

1. One Big Beautiful Budget Act (OBBBA), 2025 — SALT cap ($40,000) and phase-out provisions (verify current-year parameters)
2. IRC §164(b)(6) — SALT deduction limitation statutory basis
3. IRS Publication 17 — Itemized deductions; SALT deduction calculation
4. California Franchise Tax Board — CA top rate 13.3%; CA non-conformity to federal SALT cap
5. BOX_SPREAD_STRATEGY_KNOWLEDGE_BASE.md — SALT torpedo formula, effective rate calculation, four-scenario tables; formula: Effective_rate ≈ nominal_rate × (1 - combined_marginal_rate_including_SALT_recapture)
6. SpreadWise, “The Tax Benefits of SpreadWise Securities-Backed Loans” — After-tax effective rate ~2.88-3.24% before SALT consideration
7. SpreadWise, “4 Ways Fortress Financial Helps Business Owners Get Faster, Cheaper Capital” — After-tax effective rate ~2.95%; Section 1256 60/40 calculation