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 creates an interesting interaction with the borrower’s gain composition. If the borrower has only long-term capital gains, the 60% long-term component offsets at LTCG rates and the 40% short-term component flows against whatever gains remain — both components end up saving at the LTCG rate, which is lower. But if the borrower has only short-term capital gains and no long-term gains, something counterintuitive happens: the 60% long-term component has no LTCG to absorb it, so under the capital loss netting rules it flows against short-term gains instead. Both components end up saving at the STCG rate — the highest rate. This makes the STCG-only scenario the most efficient of all.

The bottom line is practical, and slightly counterintuitive: the strategy is most tax-efficient for investors with only short-term capital gains (both components save at the top rate), less so for investors with a mix or only long-term gains, and provides marginal benefit for investors with no realized capital gains at all.

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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 + NIIT + CA: 37% + 3.8% + 13.3% = 54.1%

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) only — no LTCG

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 has no long-term capital gains — all her realized gains are short-term. She’s in the 37% federal bracket.

Annual Section 1256 losses: $20,000 ($12,000 long-term component + $8,000 short-term component)

The key mechanic here: Emma has STCG but no LTCG. Under capital loss netting rules (IRC §1222), long-term losses net against long-term gains first. Because Emma has no LTCG, the $12,000 long-term capital loss has no long-term gains to absorb it — so the net long-term loss flows against her net short-term gain. Both components end up offsetting short-term gains at the STCG rate.

Loss component	Amount	Offsets	Tax rate (CA resident)	Tax saved
Long-term (60%)	$12,000	STCG (no LTCG available; LTCL flows to net STCG under §1222)	54.1% (STCG + NIIT + CA)	$6,492
Short-term (40%)	$8,000	STCG	54.1% (STCG + NIIT + CA)	$4,328
Total tax saved				$10,820

Effective annual interest cost: $20,000 - $10,820 = $9,180 Effective annual rate on $500,000 principal: 1.84%

This is the most tax-efficient scenario. When the borrower has only short-term gains, the entire Section 1256 loss — both the 60% long-term component and the 40% short-term component — ends up offsetting gains taxed at the highest combined rate (54.1% for California residents). The 60/40 statutory split becomes irrelevant when no long-term gains exist to absorb the long-term component separately.

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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: When no short-term capital gains are available, short-term capital losses offset long-term capital gains — saving tax at the LTCG rate. The character of the loss does not change; its tax value depends on what it offsets.

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 against full efficiency: 60% of losses are long-term and must offset long-term gains first (at the lower 37.1% combined rate), while only 40% of losses hit STCG at the full 54.1% combined rate. Unlike Scenario 1 where all losses cascade onto STCG, Maya’s LTCG absorbs the long-term losses at a lower rate.

Loss component	Offsets	Tax rate	Tax saved
$12,000 LTCG losses	LTCG gains	37.1%	$4,452
$8,000 STCG losses	STCG gains	54.1%	$4,328
Total			$8,780

Effective annual rate: 2.24%

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

Sarah has substantial W-2 income but has not sold any positions — her portfolio gains are entirely unrealized. She has no realized capital gains this year. Her Section 1256 losses will first offset any capital gains (zero this year), 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 (California resident, ordinary income offset): $3,000 × 50.3% (37% federal + 13.3% CA) = $1,509

Effective annual interest cost: $20,000 - $1,509 = $18,491 Effective annual rate: 3.70%

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 only) + SALT zone:

As in the base scenario, Emma has no LTCG — both loss components offset STCG at 54.1%.

Component	Amount	Tax saved
Long-term loss (60%), offsets STCG	$12,000	$6,492
Short-term loss (40%), offsets STCG	$8,000	$4,328
SALT recapture ($20K × 30% × 37%)	$20,000	$2,220
Total		$13,040

Effective annual rate: $20,000 - $13,040 = $6,960; rate = 1.39%

This is the theoretical minimum — a top-bracket California taxpayer with only short-term capital gains in the SALT phase-out zone, borrowing at 4% nominal. Both the STCG-only benefit and the SALT torpedo stack on each other. It’s an unusual convergence of circumstances but a real one for active traders or employees with large RSU/option vesting events in a high-income year.

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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%