An index-linked bond is a bond whose principal, coupon, redemption value, or interest calculation is tied to a reference index. The most familiar examples are inflation-linked government bonds such as U.S. Treasury Inflation-Protected Securities, but the exact mechanics vary by issuer and document.
Key Takeaways
- Index-linked bonds can link cash flows to inflation, consumer prices, commodity prices, or another stated index.
- The index formula matters as much as the label: check the base index, index lag, interpolation method, floor, cap, and redemption rule.
- Inflation-linked bonds can help preserve purchasing power, but they still carry interest-rate, liquidity, tax, and issuer-specific risk.
- U.S. TIPS have Treasury-specific rules, including principal adjustment and a maturity floor; other index-linked bonds may not work the same way.
Basic Mechanics
$$
\text{Index Ratio} = \frac{\text{Current Reference Index}}{\text{Base Reference Index}}
$$
$$
\text{Adjusted Principal} = \text{Original Principal} \times \text{Index Ratio}
$$
If a bond has 1,000 of original principal and the index ratio rises to 1.04, the inflation-adjusted principal becomes 1,040. A coupon rate applied to the adjusted principal then produces a larger nominal coupon payment than it would have produced on the original principal.
Index-Linked Bond vs. Nominal Bond
| Feature | Index-Linked Bond | Nominal Bond |
|---|
| Principal | May adjust with an index. | Usually fixed at stated face value. |
| Coupon base | May apply to adjusted principal or an indexed amount. | Usually applies to fixed principal. |
| Main return driver | Real yield plus index adjustment. | Nominal yield and price change. |
| Inflation exposure | Often reduced, depending on formula. | Investor bears more inflation risk. |
| Main review point | Index rules, lag, floor, tax, issuer, and liquidity. | Yield, duration, credit, call, and liquidity. |
Practical Example
A pension plan comparing a nominal government bond with an index-linked bond would not simply compare coupon rates. It would compare real yield, expected inflation, index lag, maturity, liquidity, and tax effects. If realized inflation differs from expectations, the index-linked bond and nominal bond can deliver very different purchasing-power outcomes.
Common Mistakes
- Assuming every index-linked bond has the same maturity floor as U.S. TIPS.
- Confusing inflation protection with protection from market price losses before maturity.
- Ignoring index lag, seasonal adjustment rules, and which version of CPI or another index is used.
- Comparing coupon rates instead of real yields and inflation-adjusted cash flows.
- Forgetting that taxable accounts may owe tax before inflation-adjusted principal is received in cash.
Public Source Checks
- TreasuryDirect TIPS explains U.S. TIPS principal adjustment, maturities, and the maturity floor.
- BLS CPI FAQ explains what the Consumer Price Index measures and its limitations.
- TreasuryDirect I bonds is useful when distinguishing marketable TIPS from retail Series I savings bonds.
FAQs
Can an index-linked bond lose money?
Yes. Even if the index adjustment is favorable, the bond can fall in market value before maturity because of real-rate changes, liquidity conditions, credit risk, or unfavorable document terms.
Is every index-linked bond tied to CPI?
No. Many inflation-linked bonds use a consumer price index, but the exact index and adjustment method come from the bond documents.