I-bond vs T-bill vs HYSA calculator

After-tax cash at your horizon. Cap, lock, and penalty from TreasuryDirect. Rates are yours — defaults are examples, not today’s auction or today’s bank. Read the cash-sleeve guide first.

Quick answer: This calculator compares I bonds, Treasury bills, and a savings account after federal and state tax, using rates you type. It applies the $10,000 I-bond cap, the 12-month lock, and the 3-month penalty the page already describes.

Cash to park ($) Horizon (months) Federal ordinary rate % State ordinary rate % Apply state tax to the HYSA (T-bills and I-bonds stay exempt)

31 U.S.C. § 3124 exempts Treasury interest from state income tax. Uncheck if you already live in a 0% state and typed 0 above.

I bond · example, not a live feed

Composite rate % May–Oct 2026 new-issue print was 4.26%. Type the rate for the bond you will buy. Apply 3-month penalty (cash before 5 years)

HYSA · example, not today’s bank

APY %

T-bill · example, not today’s auction

Discount rate % Tenor

After-tax at horizon

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I-bond interest compounds semiannually at the composite you typed (held constant — real composites reset every six months). T-bills roll at the same discount. HYSA compounds annually at the APY. Tax is modeled at the horizon on accrued interest. I-bond path clips at $10,000 per calendar year and parks the overflow in the HYSA. Paid sequence: park-the-cash playbook.

What this means for you

What this means for you: the winner is after-tax cash from the rates you typed, not a prediction of next period's inflation rate. I bonds are limited and locked. T-bills in this model are exempt from the state tax you type. A savings account is not.

How to use this calculator

Open the I bonds vs T-bills calculator and keep the sample numbers or replace them with your own. The result updates as you type.
Read the headline figure first, then the breakdown or chart under it.
Use Copy share link if you want those inputs in the URL. The page is an educational estimate, not a filing, approval, or recommendation.

FAQ

Why is the I-bond purchase capped?

The page uses the $10,000 annual electronic cap. It does not model the paper tax-refund purchase.

What is the 3-month penalty?

The page withholds the last three months of interest if you cash out after the lock and before five years, which is the Treasury rule it describes.

Are the rates live?

No. You type them. The page is not a TreasuryDirect scrape.

Educational, not financial advice. Page updated October 24, 2026 — no tax, limit, or rate figure was changed in this review.