Which Index Governs What
There is no such thing as “the” inflation rate. The BLS and BEA publish several indices, they diverge persistently, and — this is the part that costs money — different ones are hardwired into different obligations you already hold. Knowing which index runs which contract is worth more than any forecast.
| Index | Population / method | What it governs |
| CPI-U | All urban consumers; fixed basket, Laspeyres | TIPS principal accretion (NSA, 3-month lag); Series I bond inflation component; most private cost-of-living escalators in leases and alimony |
| CPI-W | Urban wage earners and clerical workers; narrower, more weight on transportation | Social Security COLA, via the Q3-over-Q3 average under 42 U.S.C. §415(i) (section “Cost-Of-Living Adjustments”) |
| C-CPI-U | All urban consumers; chained, allows substitution | Federal tax bracket thresholds, standard deduction, and most dollar limits, under IRC §1(f)(3), “Cost-of-living adjustments” |
| PCE | Broader scope incl. employer-paid healthcare; chained | The Federal Reserve’s 2% target (core PCE is the operational gauge) |
The chained-CPI tax increase you are already paying. Look again at row three. Before the TCJA, tax brackets were indexed to CPI-U. The TCJA switched the indexing to the chained C-CPI-U, and the OBBBA made that permanent. Chained CPI accounts for substitution — when steak gets expensive it assumes you buy chicken — and therefore runs roughly 0.25 to 0.30 percentage points per year below CPI-U.
That gap does not sound like much. It compounds:
At a 0.28-point annual wedge, your bracket thresholds fall about 2.8% behind true consumer prices in a decade and 8% behind over thirty years. Every year, a slightly larger slice of a raise that merely kept pace with your actual cost of living gets taxed at the next rate up. This is bracket creep, it is legislated, it is permanent, and nobody has to vote for a tax increase to make it happen. Plan for your effective rate to drift upward across your career even if statutory rates never change — which is one more argument for Roth conversions early and for harvesting gains in years you control.
And the COLA that does not match your costs. Social Security is indexed to CPI-W — an index built on the spending of wage earners, who drive more and consume less medical care than retirees. Retirees’ actual basket is heavier in healthcare and housing, the two categories that have outrun the average for decades. The BLS even publishes an experimental elderly index, CPI-E, which has historically run above CPI-W. It has never been adopted, because adopting it would cost the trust fund money. The practical consequence: assume your Social Security benefit loses real purchasing power gradually throughout retirement, and do not model it as fully inflation-proof.