Box Spread Taxes — The 60/40 Deduction, Step by Step

What this page does

This page is the calculator’s show-your-work. It uses the exact example the homepage calculator opens with: $1,000,000 borrowed for one year at today’s market rate of 4.55%, $20,000 of short-term capital gains taxed at 35%, and $18,000 of long-term capital gains taxed at 15%.

One idea makes the whole thing work: for tax purposes, the “interest” on a box spread is not interest at all. It is a capital loss. And capital losses can cancel out capital gains — which means they can cancel out the tax those gains would have owed.

The interest for the year

A box spread loan has no stated interest rate and no monthly bill. You receive cash today and owe a fixed, larger amount on a fixed date. The gap between the two is your cost — economically, interest.

The options market quotes that cost as a rate compounded daily on a 360-day calendar (the money-market convention). Turning it back into plain dollars just inverts the quotation:

$1,000,000 × ((1 + 4.55%/360)³⁶⁰ − 1) = $46,548

That $46,548 is what you economically pay for the year. Now comes the tax twist.

The 60/40 split

A box spread is four options, and options on broad indexes are Section 1256 contracts. Section 1256 has a special rule: these contracts are treated as sold on the last day of the year, and every dollar of gain or loss is split 60% long-term, 40% short-term — no matter how long you actually held them. (The IRS Publication 550 chapter on Section 1256 contracts is the government source.)

So the $46,548 of financing cost becomes a capital loss in two buckets:

$46,548 × 60% = $27,929 long-term loss
$46,548 × 40% = $18,619 short-term loss

Then the loss goes looking for gains, in a fixed order:

1. Same side first. The $18,619 short-term loss wipes out your $20,000 of short-term gains. The $27,929 long-term loss wipes out your $18,000 of long-term gains.

2. Then across. Whatever loss is left on one side can eat gains on the other side. In our example both gain piles are already empty, so this step finds nothing — but it is the reason the system is forgiving:

As long as you have capital gains somewhere — long or short — the loss will find them. The 40% short-term slice is never stranded just because your gains happen to be long-term, and the 60% long-term slice is never stranded just because your gains happen to be short-term. The character of your gains doesn’t decide whether the write-off happens. It only decides the blended rate: the short-term slice of the loss saves tax at your short-term rate, the long-term slice at your long-term rate.

3. The extra. After every gain is gone, $8,548 of loss is still unused. Up to $3,000 of it can offset ordinary income this year — salary, business income — taken from the short-term bucket first. The remaining $5,548 isn’t lost: it carries forward to future tax years, keeping its long/short character, waiting for future gains.

One more thing worth knowing: the deduction doesn’t wait for the loan to end. On a multi-year box spread with no cash payments until maturity, the open position is still marked to market every December 31 — so each year’s accrued interest becomes that year’s capital loss.

The tax saved

Every dollar of loss that lands on a dollar of gain erases the tax that gain would have owed. Three buckets:

$20,000 of short-term gains × 35% = $7,000
$18,000 of long-term gains × 15% = $2,700
$3,000 of ordinary income × 35% = $1,050

Total tax saved = $10,750

That’s the “estimated tax saved” at the top of the calculator. (The $5,548 carryforward will cut a future tax bill too — the calculator doesn’t count it, because it can’t know when.)

The true borrowing rate

$46,548 of gross cost minus $10,750 of tax savings = $35,798 net. But dollars can’t be compared to the Loan section’s 4.55%, which is quoted as a 360-day daily-compounding nominal rate. So we convert the net cost back into that same quotation — the nominal rate r that turns $1,000,000 into $1,035,798 when compounded daily for 360 days:

r = 360 × ((1 + $35,798 / $1,000,000)^1/360 − 1) = 3.52%

That is the true borrowing rate: 3.52% against the market’s 4.55% — the value of the deduction, expressed in the same units as the rate you started with.

Flowchart

The whole calculation, at a glance.

Every number below comes straight from the calculator’s inputs — change them up top and this chart follows.

Inputs — straight from the calculator

$1,000,000 borrowed · 4.55% rate · $10,000 ST gains @ 35% · $20,000 LT gains @ 15%

Step 1 — Interest · daily compounding, 360-day

$1,000,000 × ((1 + 4.55%/360)³⁶⁰ − 1) = $46,548 gross cost

Step 2 — Section 1256 · 60/40 split

60% → long-term loss $27,929
40% → short-term loss $18,619

Step 3 — Netting · same character first, then cross

$20,000 of ST gains sheltered @ 35% · $18,000 of LT gains sheltered @ 15%

Loss left over after netting?

Fully absorbed
by gains

Yes

Unused loss

$3,000 offsets ordinary income
$5,548 carried forward

Step 4 — Tax saved

$10,750
sheltered gains × their rates, plus the ordinary offset @ 35%

Step 5 — True borrowing rate

3.52%
360-day daily-compounding nominal — vs 4.55% market

23.1% of the interest comes back as tax savings.

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FAQ

Is box spread interest tax deductible?+

Not as interest — for tax purposes it isn't interest at all. The financing cost of a box spread is a Section 1256 capital loss (60% long-term, 40% short-term) that can offset your capital gains, up to $3,000 of ordinary income per year, with the rest carried forward. The calculator's “tax saved” is the tax those offset gains would otherwise have owed.

What is Section 1256 60/40 treatment?+

Under Section 1256 of the tax code, certain contracts — including the broad-index options in a box spread — are marked to market at year-end, and every dollar of gain or loss is treated as 60% long-term and 40% short-term, no matter how long you actually held them.

Do I need capital gains for the deduction to work?+

Mostly, yes. The loss first offsets your capital gains — same character first, then across. Without any gains, up to $3,000 per year can offset ordinary income and the remainder carries forward to future years, so the benefit waits rather than disappears.

What happens to losses I can't use this year?+

They carry forward indefinitely, keeping their long-term/short-term character, until there are gains — or future $3,000 ordinary-income offsets — to absorb them.

Does it matter whether my gains are short-term or long-term?+

Yes — it sets your blended savings rate, and the ranking is counterintuitive. The greatest savings come from having only short-term gains: the 60% long-term slice has no long-term gains to absorb it, so it cascades onto your short-term gains and the entire loss saves at your highest rate. A mix of both lands in the middle — each slice saves at its own rate. Long-term gains only is the least efficient, since everything saves at the lower long-term rate. And with no gains at all, only the $3,000 annual ordinary-income offset applies, so most of the benefit waits in a carryforward.

What is the “true borrowing rate”?+

Your net cost after the tax savings, converted back into the market's quotation — a 360-day daily-compounding nominal rate — so it compares apples-to-apples with the Loan section's market rate.

Sources

IRS Publication 550, Chapter 4 — Section 1256 contracts; the 60/40 rule
IRS Form 6781 and instructions — reporting Section 1256 gains and losses

The other half

New to box spreads? Start with the loan itself.

How four ordinary options become one fixed repayment — and why the rate comes from the market, not a bank. Read the explainer →

Box spread taxes, step by step.