Methodology

Data sources

• T-bills: US Treasury Daily Treasury Bill Rates (coupon-equivalent yield).
• T-bill ETFs (SGOV, BIL): 30-day SEC yield from the issuer's page.
• Money market funds: 7-day yield from the fund company (Vanguard uses its official price endpoint).
• Savings accounts and CDs: APY read automatically from each bank's public rates page. CDs are 1-year or 12-month terms.

Every rate shows its source link and as-of date. If a rate can't be read from the official page, we mark it missing. We never guess or carry an old number forward. Bank and CD values outside a plausible range (2.5–5.5%) are rejected.

After-tax formulas

Bank interest (savings, CDs, most money market funds): after-tax = yield × (1 − federal rate) × (1 − state rate)

US Treasuries (T-bills, T-bill ETFs, Treasury money market funds such as VUSXX): after-tax = yield × (1 − federal rate). Treasury interest is exempt from state and local income tax.

Default federal bracket: 22% (adjustable on the home page). Simplifications: we use the state's top marginal rate, and we ignore local income tax, the state deduction of federal tax, and partial state exemptions for government money funds.

State tax rates

Top marginal state income tax rates come from the Tax Foundation, State Individual Income Tax Rates and Brackets, 2026 (rates as of Jan 1, 2026). South Carolina uses 6.2% after its scheduled July 1, 2026 reversion. See each state page for its rate.

Update cadence

Rates refresh weekly. Each run saves a dated snapshot, which powers the "Δ 1 wk" column, and regenerates every state and comparison page. The newsletter goes out every Monday.

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Every Monday: who raised, who cut, and the best after-tax spot for your cash.

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