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 back down to $10,000 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 1.80% after tax (California resident, short-term capital gains, 20% LTCG bracket). 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 — but not to zero. The phase-out rate is $0.30 per dollar of income over $500,000. By the time income reaches $600,000, the $40,000 SALT deduction has been phased back to $10,000 — the floor set by the prior TCJA cap. Above $600,000 it stays at $10,000.

This creates a hidden problem. At $499,999 income, a taxpayer gets the full $40,000 SALT deduction. At $600,001, they get $10,000. 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 California taxpayers in this band reaches approximately 61.4% (48.1% federal + 13.3% CA).

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
• Floor: $10,000 (the prior TCJA cap — SALT does not go below $10,000)
• Phase-out complete at: $600,000 MAGI ($40,000 − $10,000 = $30,000 of SALT at risk ÷ $0.30 per dollar = $100,000 phase-out range above $500,000)
• Above $600,000: SALT deduction remains at $10,000

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. Using the statutory 60/40 split and California rates, with both LTCG and STCG available to offset:

• Long-term component (60% = $30,000) × 37.1% (20% federal LTCG + 3.8% NIIT + 13.3% CA) = $11,130
• Short-term component (40% = $20,000) × 54.1% (37% federal + 3.8% NIIT + 13.3% CA) = $10,820
• SALT recapture: $50,000 × 30% = $15,000 of SALT deduction restored × 37% federal = $5,550

Total savings: $11,130 + $10,820 + $5,550 = $27,500

On a $50,000 capital loss deduction, the total tax saving is roughly $27,500 — an effective benefit rate of 55.0%.

Against a 4% nominal borrowing rate: the after-tax effective rate is approximately 4% × (1 − 0.550) = 1.80%.

This example uses the 20% federal LTCG rate, which applies to single filers above approximately $533,400 (2025) and to married filing jointly above approximately $600,000 (2025). Single filers in the lower half of the SALT zone (or most MFJ filers in the zone) face 15% federal LTCG instead of 20%, reducing the LTCG component to 15% + 3.8% NIIT + 13.3% CA = 32.1% and the effective rate to approximately 1.92%. The SALT recapture and STCG components are unchanged. This example also assumes the taxpayer has sufficient capital gains — both LTCG and STCG — to absorb the full Section 1256 loss in the current year. See “Short-term gains, long-term gains, or none” for what happens when capital gains are limited.

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.

California resident, 4% nominal rate, $20,000 annual interest:

Situation	Federal LTCG rate	Combined subsidy rate	Effective rate at 4% nominal
SALT zone ($500K–$600K) + STCG available	20% (single above ~$533K)	55.0%	1.80%
SALT zone + STCG available	15% (most MFJ; single below ~$533K)	52.0%	1.92%
SALT zone + LTCG only (no STCG)	20%	48.2%	2.07%
SALT zone + LTCG only (no STCG)	15%	43.2%	2.27%
Outside SALT zone + STCG available	20%	43.9%	2.24%
Outside SALT zone + STCG available	15%	40.9%	2.36%
Outside SALT zone + LTCG only	20%	37.1%	2.52%
Outside SALT zone + LTCG only	15%	32.1%	2.72%
No capital gains (ordinary income offset, $3,000/year cap)	—	7.5%	3.70%

The 20% federal LTCG rate applies to taxable income above approximately $533,400 (single filer, 2025) or $600,000 (married filing jointly, 2025). Most MFJ filers and single filers in the lower half of the SALT zone are at the 15% rate. Non-California residents: subtract 13.3% from each row’s components (e.g., outside SALT zone + LTCG for a Texas or Florida resident at 20% LTCG = 23.8% subsidy, ~3.05% effective rate). The SALT recapture component ($2,220 on $20,000 of losses in the zone) is purely federal and unchanged regardless of state.

A worked example: Tony in the SALT zone

Tony is a California single filer earning $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 also has capital gains from rebalancing his investment portfolio each year.

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 (his current SALT deduction is $25,000).

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 ($11,100 long-term at 60%; $7,400 short-term at 40%).

Those $18,500 in losses, applied to Tony’s California tax situation (single filer above the ~$533K 20% LTCG threshold):

Component	Amount	Rate	Tax saved
Long-term loss (60%)	$11,100	37.1% (20% + 3.8% NIIT + 13.3% CA)	$4,118
Short-term loss (40%)	$7,400	54.1% (37% federal + 3.8% NIIT + 13.3% CA)	$4,003
SALT recapture: $18,500 × 30% = $5,550 restored × 37% federal	—	—	$2,054
Total tax saved			$10,175

Net tax saving: $10,175 — an effective subsidy rate of 55.0%.

Effective interest cost in Year 1: $18,500 − $10,175 = $8,325. Against $500,000 borrowed, that’s a 1.67% effective rate.

(If Tony were at the 15% federal LTCG rate — e.g., if married, the 15% bracket extends to ~$600K for MFJ — the LTCG component drops to 32.1%, reducing total savings to approximately $9,620 and the effective rate becomes approximately 1.78%.)

The same loan to a California resident outside the SALT zone with LTCG exposure would produce a Year 1 effective cost of $18,500 × (1 − 0.371) = $11,641, a 2.33% effective annual rate — still compelling, but the SALT torpedo adds roughly 0.6 percentage points of additional benefit for Tony.

Verification caveat

The SALT torpedo provisions are from the One Big Beautiful Budget Act (OBBBA) of 2025. The parameters described above — $40,000 cap, $10,000 floor, $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 $40,000 SALT deduction is already in place and there is nothing to recapture. Above $600,000, the deduction has reverted to $10,000 (the TCJA floor) — significantly less than the $40,000 cap, but not zero — and there is no further torpedo recapture available. The window is $500,000 to $600,000 MAGI.

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