How we calculate your rate

Treasury Yield Data

We fetch daily yields from the St. Louis Federal Reserve (FRED) for five maturities:

We add a +0.25% convenience spread to each yield to approximate the typical bid-ask friction observed in real SPX box spread markets. Actual execution costs vary by broker, strike selection, and market conditions.

We also fetch SOFR, CPIAUCSL, CPILFESL, PCEPI, and PCEPILFE from FRED. The CPI/PCE series use FRED's units=pc1 setting to return year-over-year percentage changes directly.

Cboe Box Rate Benchmark

The calculator links to Cboe's official SPX Box Rate Index dashboards as an exchange-published benchmark. Example: BOX18M26, the Cboe Box Rate Index for the June 18, 2026 SPX option series.

We do not scrape Cboe data. Until there is a stable licensed/API feed, the calculator uses FRED treasury yields plus an explicit friction estimate for executable-rate modeling, while Cboe is used as a methodology and benchmark reference.

How a Box Spread Generates Cash

A box spread is a four-leg options position on the SPX index that creates a synthetic fixed-income instrument. Here's a concrete example:

Because SPX options are European-style (no early exercise) and cash-settled (no delivery of shares), there is no early assignment risk. The payoff is mathematically locked regardless of where the S&P 500 moves.

How Box Spread Loans Work

An SPX box spread combines four options legs: buy a call and sell a put at one strike, then sell a call and buy a put at another strike. The difference between the strikes creates a fixed cash-settled payoff at expiration.

Selling the box generates cash today. The interest cost is embedded in the discount between the cash received upfront and the fixed amount repaid at expiration. That is why the calculator treats the result like a synthetic fixed-term loan, while still warning that account collateral and broker margin rules remain critical.

Section 1256 Tax Treatment

SPX index options are Section 1256 contracts under the Internal Revenue Code (IRS Publication 550, 26 U.S.C. § 1256). This has two important consequences:

• 60/40 split: All gains and losses — regardless of how long you hold the position — are treated as 60% long-term capital gain/loss and 40% short-term. This is more favorable than ordinary income treatment for most high earners.
• Mark-to-market: Open Section 1256 positions are marked to market at year-end, meaning unrealized gains/losses are recognized annually. For a box spread held to expiration within one calendar year, this is irrelevant.

The implied interest you pay on a box spread is realized as a capital loss, not as investment interest expense. This is a critical distinction: investment interest expense deductions are limited to net investment income, while capital losses can offset capital gains dollar-for-dollar.

The Net Investment Income Tax (NIIT)

The NIIT is a 3.8% federal surtax on net investment income (NII) for taxpayers above certain income thresholds (IRS NIIT FAQ):

Box spread losses are deductible against NII, so high earners subject to NIIT receive an additional 3.8% federal shield on top of their regular capital gains rates. We apply NIIT in our federal shield calculation when the checkbox is checked.

The After-Tax Effective Rate Formula

Example — NYC resident, 37% federal bracket, NIIT applicable:

State rates are sourced from official state tax authority publications for tax year 2026. Local rates are based on known city income taxes (NYC: 3.876%). We do not attempt to model the full SALT interaction at high income levels — a disclaimer is shown in those cases.

Inflation and Real Borrowing Cost

The real-rate callout compares your after-tax effective box spread rate against Core PCE inflation (PCEPILFE), the inflation measure most closely associated with the Federal Reserve's policy target.

If your after-tax rate is below Core PCE, the loan has a negative real cost in purchasing-power terms. This does not make the strategy risk-free; it means inflation is eroding the real value of the fixed obligation faster than the after-tax interest cost accrues.

Data Sources & Update Frequency

Institutional Sources

• CME Group — Index Options Box Spreads as Financing Tool
• OCC / Options Education — Box Spreads: Synthetic Lending and Borrowing
• Cboe — SPX Box Spreads: What Every Advisor Should Know
• Cboe — Long-Dated Box Spreads: A Better Way to Buy a Home
• Alpha Architect BOXX ETF