I whipped out my ancient how-to-make-a-website book, and made a simple but handy JavaScript calculator for calculating the equivalent bank CD rate for a given T-Bill or Savings Bond rate, as the interest from them are exempt from local and state taxes. This uses the rate conversion formula previously given. Remember, marginal means the tax rate at which your last earned dollar is taxed. Please try it out and let me know if something’s broken:
Calculator:
For example, at my 25% Fed and 9% State tax rates, the current 4.14% rate for a 4-week T-Bill is the equivalent of a 1-month bank CD earning 4.70% annualized.
Note: The above calculator does not assume that you will itemize deductions and deduct your state taxes from your federal taxes. Even if you do itemize, I would note that everyone gets the standard deduction, so it’s not necessarily fully deductible.
Useful Resources:
Recent T-Bill auction results
2006 Federal Tax Rates
State Income Tax Rates