int.drew.app

Convert any interest rate into its true tax-equivalent — so every account, loan, and bond speaks the same language.

Your rate

Rate

Quoted as

Account / instrument type Compounding (for APR ↔ APY)

Your federal marginal tax bracket Your state marginal tax bracket (CA)

Add 3.8% Net Investment Income Tax (NIIT)

After-tax yield

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Taxable-equivalent rate

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Type		Federal	State	After-tax	Equivalent plain rate

Equivalent plain rate is the single comparable number: for earnings it's the taxable-equivalent yield (what a fully-taxable account would have to pay to match after tax); for borrowing it's the after-tax cost.
Tax math runs on the effective annual rate (APY); an APR input is converted using the compounding period above.
Investment interest deduction (portfolio loan, invested) is limited to your net investment income; unused amounts carry forward. It also assumes state conformity with the federal rule.
Combined marginal rates are summed simply (federal + state); SALT-deduction and state-credit interactions are not modeled. Not tax advice — brackets and rules change.