Short-term gains, long-term gains, or none: how your tax situation changes the effective rate

TL;DR: The effective cost of a box spread loan depends heavily on what capital gains the borrower has available to offset. Short-term capital gains are the most valuable to offset (taxed at ordinary income rates), so a box spread capital loss against STCG saves the most per dollar. Long-term capital gains are taxed at lower rates, so the saving is smaller. If there are no capital gains to absorb — just a $3,000/year ordinary income offset — the benefit nearly disappears. Matching the loan to a year when the borrower has significant taxable gains multiplies the effective benefit.

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The 2-minute version

A box spread generates Section 1256 capital losses — 60% long-term, 40% short-term. Those losses are most valuable when they offset gains taxed at the highest rates.

Think of capital losses as currency. Their face value is the dollar amount of the loss. Their purchasing power is the tax rate of whatever gains they offset. A $10,000 capital loss that offsets a $10,000 short-term capital gain (taxed at 37%) saves $3,700. The same $10,000 loss that offsets a $10,000 long-term capital gain (taxed at 20%) saves only $2,000. The same loss with no gains to offset saves $370 per year ($3,000 × federal marginal rate of ~12%, if ordinary income is low enough).

The box spread’s 60/40 split adds a structural benefit: 60% of the loss is automatically long-term. Even if the borrower has only short-term capital gains available, 60% of their Section 1256 losses match against long-term gains (which may be harder to find). This forces some portfolio planning to use the losses most effectively.

The bottom line is practical: the strategy works best for investors who have meaningful taxable capital gains each year. It works well but slightly less so for investors with only long-term gains. It provides marginal benefit for investors with no significant capital gains.

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The nerdy version

Four scenarios with worked examples

The following analysis uses a $500,000 box spread loan at a 4% nominal annual rate. Annual implied interest (simplified, pre-tax): $20,000. Section 1256 split: $12,000 long-term (60%) / $8,000 short-term (40%).

Rates used:

• Federal STCG rate: 37% (top bracket)
• Federal LTCG rate: 20%
• NIIT: 3.8% (on net investment income over threshold)
• California state rate: 13.3% (treats LTCG as ordinary income)
• Combined LTCG + CA + NIIT: 20% + 3.8% + 13.3% = 37.1%
• Combined STCG + CA: 37% + 13.3% = 50.3%

For non-California taxpayers, replace 13.3% with your state rate. Texas/Florida residents subtract that component.

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Scenario 1: Borrower has significant short-term capital gains (STCG)

Emma is a hedge fund employee with $500,000 in W-2 income and another $200,000 in short-term capital gains from her personal trading portfolio. She’s in the 37% federal bracket, with STCG taxed at 37%.

Annual Section 1256 losses: $20,000 ($12,000 LTCG + $8,000 STCG portion)

Loss component	Amount	Tax rate (CA resident)	Tax saved
Long-term (60%)	$12,000	37.1% (LTCG + NIIT + CA)	$4,452
Short-term (40%)	$8,000	50.3% (STCG + CA)	$4,024
Total tax saved			$8,476

Effective annual interest cost: $20,000 - $8,476 = $11,524 Effective annual rate on $500,000 principal: 2.30%

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Scenario 2: Borrower has significant long-term capital gains (LTCG) only

Tony is a long-term buy-and-hold investor. Each year he sells some appreciated index fund positions to rebalance, generating $150,000 in long-term capital gains. No STCG. Top LTCG federal rate: 20%. California: 13.3%.

Annual Section 1256 losses: $20,000 ($12,000 LTCG + $8,000 STCG portion)

Loss component	Amount	Offsets	Tax rate	Tax saved
Long-term (60%)	$12,000	LTCG	37.1%	$4,452
Short-term (40%)	$8,000	LTCG (no STCG available, carries forward or offsets LTCG at lower rate)	37.1%	$2,968
Total tax saved				$7,420

Note: Short-term capital losses can offset long-term capital gains if no short-term gains are available, at the LTCG rate.

Effective annual interest cost: $20,000 - $7,420 = $12,580 Effective annual rate: 2.52%

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Scenario 3: Borrower has both STCG and LTCG

Maya sold concentrated employer stock (one-year holding, generating STCG) and also rebalances long-held index funds (LTCG). She has $100,000 in STCG and $100,000 in LTCG available each year.

Here the 60/40 split works slightly against efficiency: 60% of losses are long-term (lower value), while more of her gains are short-term (higher value). The 40% short-term losses offset 40% of her STCG at the full 50.3% combined rate.

Loss component	Offsets	Tax rate	Tax saved
$12,000 LTCG losses	LTCG gains	37.1%	$4,452
$8,000 STCG losses	STCG gains	50.3%	$4,024
Total			$8,476

Effective annual rate: 2.30% (same as Scenario 1 when full mix is available)

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Scenario 4: No capital gains (no offsetting gains available)

Sarah is early in her career, has a small portfolio, and doesn’t have significant taxable capital gains. Her Section 1256 losses will first offset any capital gains (zero), then offset up to $3,000 of ordinary income per year, with the remainder carrying forward.

Annual Section 1256 losses: $20,000 Annual ordinary income offset: $3,000 (statutory limit) Remaining carried forward: $17,000

Tax saved in Year 1: $3,000 × 37% (federal) × (applicable rate) = approximately $1,110 (federal only; assume no state LTCG benefit)

Effective annual interest cost: $20,000 - $1,110 = $18,890 Effective annual rate: 3.78%

This is still better than Schwab’s Pledged Asset Line (6-8%) but the advantage over a bank margin loan depends on the margin loan’s rate. For a borrower in Scenario 4, the strategy is primarily a rate arbitrage, not a tax efficiency play.

The $17,000 carryforward isn’t lost — it will offset future capital gains. But if those future gains are years away, the time-value of the deferred benefit reduces its present value.

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The SALT zone overlay

For borrowers in the SALT phase-out zone ($500K-$600K MAGI), an additional layer of benefit applies on top of the scenario analysis above. Each dollar of capital loss that reduces MAGI from within the phase-out band also restores SALT deductions worth up to 30 cents per dollar × 37% = 11.1 cents in additional federal savings.

Scenario 1 (STCG) + SALT zone:

Component	Amount	Tax saved
LTCG loss benefit (60%)	$12,000	$4,452
STCG loss benefit (40%)	$8,000	$4,024
SALT recapture ($20K × 30% × 37%)	$20,000	$2,220
Total		$10,696

Effective annual rate: $20,000 - $10,696 = $9,304; rate = 1.86%

This is the theoretical minimum — a top-bracket California taxpayer with significant STCG in the SALT phase-out zone, borrowing at 4% nominal. It’s an unusual set of circumstances but a real one.

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Practical planning: timing the deductions

Because Section 1256 positions are marked to market annually, the annual loss recognition is fixed — it happens every December 31 regardless of the borrower’s preference. What the borrower can control is the composition of capital gains in that year.

A borrower who knows they will have a significant STCG event in a given year (stock options exercising, a business sale, a property sale) can use a box spread opened before or during that year to generate offsetting capital losses. The losses flow through at year-end and offset the high-rate gains.

A borrower with no anticipated capital gains in the near term is better served by a floating-rate loan rather than a fixed multi-year loan — they can close the position when a taxable event occurs and capture the deduction in the same year.

Applying the 60/40 split strategically

The 60/40 split cannot be changed — it’s statutory. But the borrower can plan their portfolio to maximize absorption of both the long-term and short-term components:

• LTCG realization (harvesting long-term gains from rebalancing) absorbs the 60% LTCG loss component efficiently
• Any STCG events (options, trading, <1 year positions) absorb the 40% STCG component at the highest possible rate

A portfolio with a mix of both gain types each year uses the Section 1256 losses at maximum efficiency. A portfolio generating only LTCG each year leaves the 40% short-term component partially underutilized (it will offset LTCG at LTCG rates rather than STCG at STCG rates).

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What this isn’t

This analysis doesn’t include state-specific rate differences. A Texas resident with no state income tax sees smaller total benefits than a California resident. A New York City resident may see larger combined state + city rates. Run the math with your specific state rates.

Carryforward value is real but time-discounted. In Scenario 4, the $17,000 annual carryforward isn’t lost — it offsets future gains. But a deduction in five years is worth less than a deduction today. The present value of the carryforward depends on when the borrower expects to have absorbing gains.

These calculations change every year. Tax rates, SALT phase-out parameters, state rates, and individual income composition all change. Run a fresh analysis — or have a CPA run one — before each tax year.

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If you want to actually do this

This article completes the tax series. For the mechanics of how capital losses flow through the return (Form 6781, Schedule D), see “How the IRS treats box spreads: Section 1256 and the 60/40 rule”.

For the SALT phase-out specifics, see “The SALT torpedo”.

For the full loan mechanics, start with “What is a synthetic loan”.

When ready to model this with a professional — and talk to both a CPA and an adviser who executes the strategy — request a referral. See full disclosure.

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Sources

1. IRC §1256 — 60/40 split; mark-to-market; Section 1256 contract definition
2. IRS Publication 550, Chapter 4 — Net capital loss; $3,000/year ordinary income offset; carryforward rules
3. IRS Schedule D Instructions — Capital loss carryforward rules
4. IRS Form 6781 Instructions — Annual mark-to-market reporting
5. California Franchise Tax Board: Capital Gains — CA taxes LTCG as ordinary income at 13.3%
6. BOX_SPREAD_STRATEGY_KNOWLEDGE_BASE.md — Four-scenario analysis, effective rate formulas, SALT zone calculations
7. SpreadWise, “The Tax Benefits of SpreadWise Securities-Backed Loans” — SBLOC vs. box spread; effective rates by scenario
8. SpreadWise, “4 Ways Fortress Financial Helps Business Owners Get Faster, Cheaper Capital” — After-tax effective rate ~2.95%; rate comparison table