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TIPS and Breakeven Inflation

One-line definition

A Treasury Inflation-Protected Security (TIPS) pays a fixed real coupon rate on a principal that's adjusted for realized CPI inflation; the breakeven inflation rate is the market-implied average inflation rate that would make a TIPS and a same-maturity nominal Treasury pay the same total return.

Formula

breakeven_inflation = nominal_yield - real_yield

index_ratio = CPI_reference_now / CPI_reference_at_issuance

inflation_adjusted_principal = original_principal * index_ratio

tips_coupon_payment = real_coupon_rate * inflation_adjusted_principal / frequency

Why PMs care

Breakeven inflation is the cleanest market-based read on what investors expect inflation to average over a given horizon — it's quoted and watched the same way a credit spread is, as a single number summarizing a view the whole market is pricing. A rates PM with a view that inflation will come in above the current breakeven buys TIPS (or a breakeven-widener trade); a PM who thinks it'll come in below buys nominal Treasuries instead. Because TIPS coupons are paid on the inflation-adjusted principal, not the original face value, realized inflation flows through every coupon date, not just at maturity.

Worked example

A 10-year nominal Treasury yields 4.5%; a 10-year TIPS yields 2.0% (its real yield). breakeven = 0.045 - 0.02 = 2.5%. If realized CPI inflation averages more than 2.5% over the next 10 years, TIPS outperform; if it averages less, the nominal bond does. A TIPS issued with CPI_reference_at_issuance = 300 when the current reference CPI is 310 has index_ratio = 310/300 ≈ 1.0333 — its principal (and every coupon) is running about 3.33% above face value from realized inflation alone.

Common mistakes

  • treating the breakeven rate as a precise inflation forecast rather than a market price that also embeds a liquidity premium (TIPS typically trade less liquidly than nominal Treasuries) and an inflation risk premium — the breakeven is not a pure expectation
  • forgetting TIPS coupons scale with the inflation-adjusted principal, not the original face value — a real coupon rate applied to the wrong principal understates realized income in an inflationary period
  • comparing breakeven rates across different maturities without accounting for the term structure of inflation expectations — 5-year and 30-year breakevens can (and often do) diverge meaningfully

Limitations

tips_index_ratio and related functions here take CPI reference values as given inputs — actual TIPS indexation uses a specific lagged CPI-U reference schedule (roughly a 3-month lag) that this repo doesn't compute from a raw CPI release calendar.

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