T-Bill Calculator
For informational purposes only. I bond interest calculations are estimates. Verify current rates at TreasuryDirect.gov.

I Bond Calculator

Estimate the value and yield of a Series I Savings Bond using the official TreasuryDirect composite rate formula. Rates below are pre-filled from the current announcement and are editable.

Verified rate Current composite rate: 4.26%  (fixed 0.9% + semiannual inflation 1.67%). Effective May 1, 2026 – October 31, 2026. Source: TreasuryDirect.gov announcement. Rates announced May 1 and November 1 each year.

Rate Inputs (editable — update when new rates are announced)

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Composite Rate Formula (TreasuryDirect):
Composite = fixed + (2 × semiannual inflation) + (fixed × semiannual inflation)
Computing…

Bond Inputs

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Max $10,000/year per TreasuryDirect.

Composite Rate
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Estimated Value
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Total Interest
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Period Months Held Estimated Value Interest Earned Note

This table assumes the composite rate stays constant for the entire hold period — in reality the inflation component will change every 6 months. Treat this as a rough projection.

Related: T-Bill Calculator  |  T-Bill Yields by Term  |  T-Bill vs CD  |  How T-Bills Work

Frequently Asked Questions

What is the I bond composite rate formula?

The composite rate = fixed rate + (2 × semiannual inflation rate) + (fixed rate × semiannual inflation rate). The semiannual inflation rate is half the annualized CPI-U change. This formula is published by TreasuryDirect.

How often does the I bond rate change?

Rates are announced every May 1 and November 1. The fixed rate you receive when you buy a bond is locked in for the life of the bond (up to 30 years). The inflation component adjusts every six months from your purchase date.

Is there a penalty for redeeming I bonds early?

Yes. If you redeem before five years, you forfeit the last three months of interest. Bonds cannot be redeemed at all in the first 12 months after purchase.

What is the annual purchase limit for I bonds?

You can buy up to $10,000 in electronic I bonds per calendar year via TreasuryDirect. An additional $5,000 in paper I bonds can be purchased with a federal tax refund.

How are I bonds taxed?

I bond interest is subject to federal income tax but exempt from state and local income tax. You can defer federal tax until redemption (or 30 years, whichever comes first), or report it annually. Interest used for qualified education expenses may be tax-free under certain conditions.