Methodology
How We Calculate Your Rate
Every number on this calculator comes from public data sources and standard tax law. Here's exactly how we do the math.
Treasury Yield Data
We fetch daily yields from the St. Louis Federal Reserve (FRED) for four maturities:
| FRED Series | Maturity | Used For |
|---|---|---|
DGS3MO | 3-Month T-Bill | 30 / 60 / 90-day box spreads |
DGS6MO | 6-Month T-Bill | 180-day box spreads |
DGS1 | 1-Year T-Note | 365-day box spreads (default) |
DGS2 | 2-Year T-Note | 730-day LEAPS box spreads |
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.
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:
Example: 1-Year Box Spread, $100,000 Face Value
The spread between strikes is $100 × 100 multiplier = $10,000 max value per contract. Sell 10 contracts → $100,000 box. The market pays you approximately $95,250 today (at 4.77% gross rate), and you owe exactly $100,000 at expiration. The $4,750 difference is your implied interest cost.
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.
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):
| Filing Status | MAGI Threshold |
|---|---|
| Single / Head of Household | $200,000 |
| Married Filing Jointly | $250,000 |
| Married Filing Separately | $125,000 |
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
Effective Rate = Gross Rate × (1 − Total Shield)
Total Shield = Federal Shield + State Rate + Local Rate
Federal Shield = (0.60 × LTCG Rate) + (0.40 × Ordinary Rate) + NIIT (if applicable)Example — NYC resident, 37% federal bracket, NIIT applicable:
| Gross box spread rate | 4.77% | |
| Federal shield | 30.60% | (0.60 × 20%) + (0.40 × 37%) + 3.8% |
| New York State | 9.65% | Ordinary income treatment at state level |
| NYC local | 3.88% | NYC resident local income tax |
| Total shield | 44.13% | |
| Effective rate | 4.77% × (1 − 0.4413) = 2.66% |
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.
Data Sources & Update Frequency
| Data | Source | Frequency |
|---|---|---|
| Treasury yields | FRED API (St. Louis Federal Reserve) | Daily (cached 24h) |
| State income tax rates | State tax authority publications | Updated as laws change |
| NYC local tax rate | NYC Dept. of Finance (3.876%) | Updated as laws change |
| NIIT threshold | IRS Publication 550 | Annual |
| Section 1256 rules | 26 U.S.C. § 1256 | As amended |
This calculator is for educational purposes only and does not constitute financial, tax, or legal advice. Consult a qualified CPA or financial advisor before implementing any borrowing strategy. Results may differ from actual execution due to bid-ask spreads, broker commissions, and your individual tax situation.