What limits does this respect?

$10,000 electronic I bonds per Social Security number per calendar year, plus $5,000 paper via a federal tax refund. T-bills have no $10k cap (TreasuryDirect noncompetitive is much larger).

Are both state-tax free?

Usually yes — they are Treasury obligations. Federal tax still applies unless an I bond qualifies for the education exclusion.

Why are rates inputs?

I bond composites reset every 1 May and 1 November. T-bill auction yields move constantly. A hardcoded rate would rot.

I bonds lock for 12 months and lose the last 3 months of interest if cashed before 5 years. T-bills mature in weeks to a year and can be sold on the secondary market.

Runs in your browser · Last updated 7 September 2026

Free I Bond vs T-Bill Calculator

Free I bond vs T-bill calculator. After-tax yields, $10,000 electronic cap, 12-month lock, 3-month penalty. Browser tool — nothing is uploaded.

Amount this calendar year ($)I bond composite %T-bill annualized yield %Federal marginal tax %Hold (years)Held 5+ years (no 3-month penalty)Qualified education exclusion (I bond)

Electronic I bonds cap at $10,000 per SSN per calendar year, plus $5,000 paper via a tax refund. Both are usually state-tax exempt; T-bill interest is federally taxed as it accrues. I bonds lock for 12 months. Composite rates reset every May and November — paste the current ones.

After-tax I bond %

3.02%

After-tax T-bill %

3.12%

Leads (future value)

T-bill

I bond future value

$11,607

T-bill future value

$11,658

Penalty months

Electronic cap left

Paper cap left

$5,000

Over $10k electronic?

How to use this tool

Paste this year's I bond composite and a T-bill yield.
Set federal tax, hold period, and whether you expect the education exclusion.
Read after-tax yields, future values, and leftover I bond caps.

Worked example

$10,000, 4% I bond composite, 4% T-bill, 24% federal, 5-year hold: after tax they match because both are federally taxed at redemption / accrual. Turn on the education exclusion and the I bond keeps the full 4%. Cash an I bond at year two and the 3-month penalty lets the bill win even at the same pre-tax rate.

Frequently asked questions

What limits does this respect?: $10,000 electronic I bonds per Social Security number per calendar year, plus $5,000 paper via a federal tax refund. T-bills have no $10k cap (TreasuryDirect noncompetitive is much larger).
Are both state-tax free?: Usually yes — they are Treasury obligations. Federal tax still applies unless an I bond qualifies for the education exclusion.
Why are rates inputs?: I bond composites reset every 1 May and 1 November. T-bill auction yields move constantly. A hardcoded rate would rot.
Liquidity?: I bonds lock for 12 months and lose the last 3 months of interest if cashed before 5 years. T-bills mature in weeks to a year and can be sold on the secondary market.
Uploaded?: No.