This treasury bills vs I bonds calculator compares the after-tax interest from rolled T-bills, an I bond and a savings account over the period you choose. Enter your own rates and tax brackets, and the table updates at once. Nothing you type is stored or sent anywhere.
How this treasury bills vs I bonds calculator works
Treasury bills. The calculator assumes you buy a 26-week bill, hold it to maturity and roll the proceeds into a new one at the same yield, so interest compounds twice a year. Interest is taxed at your federal rate and is exempt from state and local income tax.
I bond. The composite rate is the fixed rate plus an inflation component. The calculator uses the formula fixed + 2 x semiannual inflation + fixed x semiannual inflation, with the semiannual inflation rate set to half your annual assumption. Interest compounds every six months. An I bond cannot be redeemed in the first 12 months, and cashing in before five years costs the last three months of interest, which the calculator includes. Federal tax is shown at redemption and the interest is exempt from state and local tax.
Savings account. Interest compounds monthly at the APY you enter and is taxed at federal plus state rates.
Example: $10,000 over 12 and 60 months
These hypothetical inputs are a 4.0% bill yield, a 0.90% I bond fixed rate, 3.0% annual inflation, a 4.0% savings APY, a 24% federal rate and a 5% state rate. The 0.90% matches the fixed rate for I bonds issued from May to October 2026; new rates are announced around November 1, so check TreasuryDirect before you buy.
| After-tax interest on $10,000 | 12 months | 60 months |
|---|---|---|
| Treasury bills, rolled | $307 | $1,664 |
| I bond | $225 | $1,625 |
| Savings account | $289 | $1,569 |
At 12 months the I bond trails because of the three-month interest penalty. At 60 months the penalty is gone, and the three options end up close together. A different inflation figure, a different fixed rate or a lower state tax rate would change the order, which is the reason to run your own numbers.
What the calculator does not know
It holds rates constant. Real bill yields change at every auction, and the I bond inflation component resets every six months. It does not include brokerage fees or fund expenses, and it assumes you stay under the I bond limit of $10,000 per person per calendar year. Treat the results as an illustration, and use the treasury bills vs I bonds calculator again whenever rates change.
Learn more before you buy
Our full guide to treasury bills vs I bonds covers how each works, the purchase limits, the tax rules and when to use which. For background, read what a bond is and how bonds work, and see how tax brackets work to pick the right federal rate. If you want a bank option for comparison, see the high-yield savings account guide, or the CD ladder strategy for money you can lock up.
You can buy both at TreasuryDirect, where current I bond rates and bill auction results are published.
Frequently asked questions
Which is better, treasury bills or I bonds?
It depends on how long you can leave the money alone, the rates at the time and your tax bracket. The calculator compares them with your own numbers.
Why does the I bond show as locked?
I bonds cannot be redeemed during the first 12 months, so periods shorter than that are not a fair comparison.
Is the inflation rate I enter an official figure?
No. It is your assumption. The real I bond inflation component is set twice a year from CPI-U.
Does the calculator include state tax?
Yes for the savings account. Treasury bill and I bond interest is exempt from state and local income tax, so only federal tax applies.
Calculators are for education only. They give estimates based on the numbers you enter, not personalized financial, tax or legal advice.