Series I Bonds vs TIPS: Inflation Protection Compared

Two Ways the Government Protects Purchasing Power

When U.S. CPI-U inflation peaked at 9.1% in June 2022, Series I savings bonds suddenly paid a composite rate of 9.62% — and demand for them overwhelmed TreasuryDirect servers, crashing the website in the final hours before the rate reset. More than $7 billion in I bonds sold in October 2022 alone. That surge illustrated both the appeal and the limitation of I bonds: extraordinary inflation protection, with a $10,000 annual purchase ceiling that makes them useful for individuals but impractical for large portfolios.

Treasury Inflation-Protected Securities (TIPS) have no purchase limit, trade freely on secondary markets, and can be held in IRAs — making them the institutional standard for inflation protection. Both instruments track the same inflation index, but they do so in fundamentally different ways.

The I Bond Composite Rate Formula

The I bond interest rate resets every six months (May 1 and November 1) based on a two-component formula:

Composite Rate = Fixed Rate + (2 × Semiannual CPI Rate) + (Fixed Rate × Semiannual CPI Rate)

The fixed rate is set by the Treasury at each reset and remains constant for the life of the bond. The semiannual CPI rate reflects the six-month change in the Consumer Price Index for All Urban Consumers (CPI-U). Each I bond earns its composite rate for six months, then resets based on the new CPI data.

• Fixed rate (May 2024): 1.30% — the highest fixed rate since 2007
• Semiannual CPI-U (Nov 2023 to May 2024): 1.48%
• Composite rate (May 2024): 4.28%
• Previous peak: 9.62% (May 2022) during height of 2021–2022 inflation surge

The fixed component is the most durable part. A bond bought in May 2024 at a 1.30% fixed rate will earn 1.30% above inflation for its entire 30-year life — a real yield of 1.30%, which in historical context is attractive.

TIPS: Real Yield Through Principal Adjustment

TIPS work differently. Instead of adjusting the interest rate, TIPS adjust the bond's principal value by CPI. A $10,000 TIPS at 2% real yield, after a year of 5% inflation, adjusts its principal to $10,500. The 2% coupon then applies to $10,500, paying $210 instead of $200. At maturity, the investor receives the inflation-adjusted principal — $10,500 in this case (or the original $10,000 if deflation occurred and the adjusted principal fell below par).

Deflation Floors: Comparing the Protections

Both instruments have deflation protections, but they work differently. I bonds have a strong floor: the composite rate cannot go negative. In a deflationary period, the rate simply falls to zero (or the fixed rate, if positive) — the principal never shrinks. An I bond purchased for $10,000 will always be worth at least $10,000.

TIPS have a weaker floor for intermediate holders. TIPS principal falls during deflation (tracking CPI downward). The floor guarantee — that investors receive at least original par at maturity — only protects those who hold to maturity. Anyone who bought a 10-year TIPS four years ago and sells today receives market price, which may reflect cumulative deflation adjustments. The 2008–2009 period briefly sent TIPS prices below par on secondary markets.

The Phantom Income Problem with TIPS

TIPS create a significant tax complication for taxable accounts: "phantom income." Each year, the IRS taxes the inflation adjustment to TIPS principal as ordinary income — even though the investor receives no cash from that adjustment until maturity or sale. In a 5% inflation year, the owner of $100,000 in TIPS owes federal tax on $5,000 of phantom income. For this reason, TIPS belong in tax-advantaged accounts (IRAs, 401ks) wherever possible. I bonds, by contrast, allow tax deferral on all interest until redemption — a meaningful advantage in taxable accounts.

• Best for taxable accounts: I bonds (tax deferral, no phantom income)
• Best for IRAs/401ks: TIPS (phantom income issue disappears; no annual purchase limit)
• Best for large portfolios: TIPS funds (e.g., Vanguard Inflation-Protected Securities Fund, Schwab US TIPS ETF) — I bond limits cap investment too severely
• Best for emergency funds: I bonds (after 12-month lockup), especially when real rates are positive

The $10,000 I bond limit restricts annual investment for high earners to less than 1% of a $1 million portfolio. For substantial inflation protection — say, a 10%–20% TIPS allocation — TIPS funds are the only practical option. I bonds and TIPS are complements, not substitutes.

This article is for informational purposes only and does not constitute financial advice.