What is a synthetic loan, and why would I want one?

TL;DR: A synthetic loan lets someone borrow money against their stock portfolio at rates closer to a mortgage than a credit card, without selling the stocks and triggering a capital gains tax bill. A traditional margin loan does something similar, but a box-spread approach uses a corner of the options market to get a better rate. The effective cost, after tax, can land somewhere around 3-4% when a standard margin loan costs 8-13%.

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The 2-minute version

Imagine Maya. She’s been receiving stock compensation at her tech job for years and never sold, or maybe she’s been steadily buying index funds since her twenties and hasn’t touched them. Either way, she now has $2 million in stock sitting in her brokerage account with a very low cost basis. She wants to buy a house.

The obvious move is to sell the stock and use the proceeds for a down payment. That would hand the IRS somewhere north of $400,000 in capital gains tax. She’d be writing a check to the government before she’d written a check to a real estate agent.

The less obvious move: borrow against the stock instead. The stock stays invested. The portfolio keeps growing. The loan sits against it as collateral, like a mortgage sits against a house.

Banks offer a version of this, called a securities-backed line of credit. The problem is the rate. Most of the big custodians charge 8 to 13 percent on these loans, closer to a personal loan than to the 6-7% mortgage rate she’d get on a comparable property.

A synthetic loan works differently. Instead of walking into a bank, think of it this way: you’re standing in front of the entire U.S. options market and announcing, “I need to borrow money, I have $2 million in stock as collateral, who wants to lend?” Every institutional player in that market can compete for your business simultaneously. They undercut each other. The best rate wins. That’s peak price competition, the kind of rate shopping most borrowers never get access to. The implied interest rate that emerges from that process runs far closer to Treasury yields than to consumer lending rates. As of early 2026, that rate was around 3.70-3.95%, depending on the term.

The word “synthetic” just means the loan is constructed from financial instruments rather than written at a bank desk. It creates the same economic outcome: cash arrives, interest accrues, principal is due later. Everything else about the experience looks ordinary.

The house example above is intentionally large to make the math vivid, but nobody tells you what to do with the proceeds. People use these loans for things far more modest: a bridge loan between properties, a small renovation, part of a down payment, a temporary cash crunch, a car purchase or down payment, a tax bill. If you need liquidity for six months and don’t want to sell, this is a tool. Nobody asks what you’re spending the money on. That’s one of the genuinely useful parts of the structure.

The twist that makes the math even more interesting is a tax provision called Section 1256. The IRS classifies these options positions as capital assets with a specific treatment, which means the interest cost shows up as an annual capital loss deduction. Depending on the borrower’s tax situation, that deduction cuts the effective interest rate by a third to a half. More on that in the tax series.

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The nerdy version

The mechanism is a structure called a “box spread.” A box spread is a four-leg options position on the S&P 500 options market that is mathematically equivalent to a zero-coupon bond.

Here’s the intuition without the options theory. When the position is opened, a net cash credit arrives in the account. That’s the loan proceeds. When the position closes at expiration, a fixed amount leaves the account. That’s the repayment. The difference is implied interest, and that interest rate is set by the market, not by a bank.

Because multiple institutional players can enter and exit these positions freely, the implied rate tracks the prevailing risk-free rate closely. Right now that means something close to short-term Treasury yields. You can track live box spread rates at BoxTrades. That rate is structurally lower than what any bank will offer a retail borrower on a margin loan, for the same reason that T-bill rates are lower than personal loan rates: counterparty risk is near zero (the positions clear through the Options Clearing Corporation, a federally regulated entity that has never failed to settle in its history, though that history, however long, is not a guarantee about the future), and there’s no bank overhead, no credit underwriting, and no spread added for profit.

The rate comparison as of April 2026:

Borrowing method	Approximate rate
Box spread (fixed, 1-5 year)	~3.70%
Box spread (floating, SOFR + 0.20%)	~3.95%
30-year jumbo mortgage	~6.47%
Schwab Pledged Asset Line	6-8%
Fidelity margin loan	7.50-10.58%
Schwab margin loan	8-13%

Rates change constantly. Verify before acting.

Now layer on the tax treatment. Under IRC §1256, S&P 500 index options positions are “Section 1256 contracts.” They are marked to market at year-end, and any resulting gain or loss is split 60% long-term / 40% short-term. For a borrower, the box spread position produces losses (the “interest cost”) that flow through as annual capital loss deductions on Form 6781, regardless of what the loan proceeds are used for.

That last part is worth underlining. A standard margin loan’s interest is only tax-deductible if the borrowed money is used to buy investments. Use it for a house, a car, a tax bill: no deduction. A box spread’s capital loss deduction has no such restriction. The capital loss flows through regardless.

At a 40% combined federal + state marginal rate, a 4% nominal borrowing rate becomes roughly 2.4% after the capital loss benefit. At the top federal bracket with California, it can get lower still.

What about professional management?

Most borrowers don’t execute this themselves. The strategy requires margin approval, some fluency with options order entry, and ongoing position management as expirations approach. Firms like SpreadWise, SpreadWise, and others manage the positions on the client’s behalf, charging around 0.5% annually on the loan balance. That fee is not deductible, so it adds back to the effective rate. But even at 4% + 0.5% = 4.5% gross, the after-tax effective rate is still comfortably below a conventional margin loan.

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What this isn’t

This is not a free lunch. The borrowing cost is real. The positions are collateralized. If the portfolio falls enough, a margin call forces liquidation. At a 50% loan-to-portfolio ratio, a decline of 28.57% would trigger a call. That math matters more when the pledged portfolio is concentrated in a single stock.

Portfolio size shapes what you can do, not whether you can do it at all. The strategy can be used responsibly at many portfolio sizes. A $50,000 portfolio can support a modest borrow, a $500,000 portfolio gives you more room, and a $3 million portfolio opens up much larger and more flexible positions. The question isn’t whether you’re “big enough” to participate; it’s whether the size and use case make the mechanics worth the overhead. Portfolio composition matters just as much as size: a concentrated position in a single volatile stock carries more margin call risk than a diversified index portfolio of the same value. Both factors, how much you have and how diversified it is, determine what you can safely do.

This is not a substitute for a financial plan. Borrowing against a portfolio to fund consumption, perpetually and without a repayment plan, is debt accumulation, not wealth building. The strategy makes most sense when there is a clear use for the cash and a credible plan to repay.

This does not eliminate taxes. It defers them. The capital gains embedded in the portfolio are still there. At some point, when the portfolio is sold, those gains will be realized. The bet is that the gains compound faster than the loan accrues, and that the eventual tax bill (at long-term capital gains rates, or at stepped-up basis at death) is less than what selling today would cost.

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If you want to actually do this

One more thing worth knowing: this is not a novel strategy. Institutions have been using box spreads to borrow cheaply for decades. What this site explains is how people with regular brokerage portfolios can use the same mechanism, one that has historically been reserved for professional desks.

The firms that execute this are SEC-registered investment advisers who manage the options positions on behalf of clients. If reading this made the concept click and the next step is talking to someone who actually does it, request a referral. I’ll send a referral code and an intro.

Talk to your CPA first. The tax benefit depends on your specific situation: your marginal rate, the composition of your gains, your state. See full disclosure.

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Sources

1. IRC §1256. Statutory basis for Section 1256 contract treatment (IRS/Cornell LII)
2. IRS Form 6781 Instructions. How Section 1256 gains and losses are reported
3. IRS Publication 550. Investment Income and Expenses; covers Section 1256 in Chapter 4
4. Cboe SPX Options Specifications. Primary source for SPX contract specs (European-style, cash-settled)
5. OCC About. OCC’s role as central counterparty and SIFMU status
6. Joseph Wang, “Box Spreads: A Better Alternative to Mortgages,” Cboe (April 22, 2025). Rate and use-case analysis from SpreadWise’s co-founder
7. SpreadWise, “Tax Deductibility of Box Spreads.” Annual deduction mechanics, Section 1256 treatment
8. Bogleheads: Let’s Talk SPX Box Spreads. Community thread, 800+ posts; rate and broker discussion