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I bond composite rate calculator

By Jude WallisPublished August 18, 2026

A Series I bond composite rate is the fixed rate plus twice the semiannual inflation rate plus their product. A 1.2 percent fixed rate and 1.5 percent semiannual inflation produce 4.218 percent.

I bond composite rate

4.218%

Fixed 1.20 percent plus twice 1.50 percent inflation, plus the product.

Fixed rate: 1.20%
Semiannual inflation: 1.50%
Composite rate: 4.218%

Fixed rate

The annual fixed rate attached at purchase. 1.2 here means 0.012.

Semiannual inflation rate

The six-month inflation component. 1.5 here means 0.015.

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The formula

c = f + 2i + fi

$f$ is the annual fixed rate as a decimal. $i$ is the six-month inflation rate as a decimal. $c$ is the annualized composite rate, floored at zero.

Two rates, one composite

The fixed rate stays with the bond. The inflation component resets every six months from CPI-U. With $f = 0.012$ and $i = 0.015$ , $0.012 + 2(0.015) + 0.012(0.015) = 0.04218$ , or 4.218 percent.

How Series I bonds work is the explainer. How TIPS work is the marketable cousin.

A zero fixed rate still earns inflation

Set the fixed rate to 0 and keep semiannual inflation at 0.015. The product term is 0. The composite is 0.03, or 3 percent. Leaving out the product on the first example would have printed 4.2 percent instead of 4.218 percent.

Inflation and purchasing power is the index path underneath $i$ .

A floor, not a traded price

The composite rate has a floor of zero. A Series I bond does not trade at a real yield. Nominal rate is the stated rate. Real rate is growth in purchasing power. The fixed component is the closest piece of this formula to a real rate before tax.

What this page is not doing

It does not apply the 12-month lock, the three-month early-redemption interest penalty, or a purchase limit. Treat the output as the announced composite from $f$ and $i$ . This is educational material, not financial advice.

Worked examples

1.2 percent fixed and 1.5 percent inflation

A Series I bond has a fixed rate of 0.012, or 1.2 percent. Its semiannual inflation rate is 0.015, or 1.5 percent. What is the composite rate?

Double the semiannual inflation rate: $2 \times 0.015 = 0.03$ .
Compute the interaction term: $0.012 \times 0.015 = 0.00018$ .
Add the fixed rate, annualized inflation and interaction: $0.012 + 0.03 + 0.00018 = 0.04218$ .
Convert the decimal composite 0.04218 to a percentage: 4.218 percent.

A fixed rate of 0.012 and semiannual inflation of 0.015 produce a composite of 0.04218, which is 4.218 percent.

Zero fixed rate and 1.5 percent inflation

A Series I bond has a fixed rate of 0 and a semiannual inflation rate of 0.015, or 1.5 percent. What is the composite rate?

Double the semiannual inflation rate: $2 \times 0.015 = 0.03$ .
The interaction term is $0 \times 0.015 = 0$ .
Add the terms: $0 + 0.03 + 0 = 0.03$ .
Convert the decimal composite 0.03 to a percentage: 3 percent.

A fixed rate of 0 and semiannual inflation of 0.015 produce a composite of 0.03, which is 3 percent.

Dropping the product term

Writing $f + 2i$ and stopping prints 4.2 percent on the first example. The Treasury formula also earns the fixed component on the inflation-adjusted value, which is the $fi$ term.

Common questions

Is 4.218 percent a holding-period return?

It is the announced composite rate for that pair of inputs. Accrual, compounding and any early-redemption penalty still apply to a real holding.

Can the composite go negative?

The formula on this page floors at zero. A negative inflation component can offset a positive fixed rate down to that floor.

Is the output financial advice?

No. Educational material. It combines the two rates you typed.

Keep reading

This page is educational material, not financial advice. The figures come from the formula shown and assume the inputs you enter hold for the whole term. Your own rate, fees, taxes and timing will differ, so treat the output as arithmetic to check a decision against, not as a recommendation.