Looking for an inflation bond calculator

[early]

Has anybody ever found a calculator that will analyze the after-tax effective yields on TIPs and I-bonds? I was trying to decide whether to buy 5-year TIPs at the upcoming auction (likely to sell at a discount to par with a 0.875% coupon and expected yield of 1.15%) to the alternative of buying TIPs in the secondary market, where bonds with about the same maturity date are selling with a much higher coupon (4.25%) (so I'd pay a premium). The secondary market bonds, of course, will have a higher initial inflation-adjusted principal than the new auction bonds. So comparing the two is tricky because the coupon rate is applied to the inflation-adjusted principal, not to the par amount, which means you can't just run a standard bond calculator like the SIA's. Then, of course, to complicate matters further, there's the alternative of buying I-bonds. A fair comparison would seem to have to take into account your tax rate and whether you're buying them in an IRA or not, your assumed reinvestment rate, an assumed inflation rate, as well as the usual price, coupon and maturity factors. I keep thinking that somebody out there must have developed a calculator or spreadsheet that would handle this. Anybody found one?

 

[Mr._johngalt]

Yep! I use an old envelope (the side without
the coffee stains). Works excellent

JG