Home price index rebasing sets each series to 100 in a chosen period, giving a chart a common starting point for comparing appreciation. It changes the displayed levels while preserving percentage changes. That shared scale cannot reveal which city has higher dollar prices or make differently constructed datasets equivalent.

What an index value of 100 means

A price index normalized to 100 uses that number as its reference-period value. A later reading of 125 means the measured price level is 25% above that reference; 90 means it is 10% below. Neither reading is a dollar price.

The reference is a convention. The March 1996 HPI technical description, published by FHFA’s predecessor agency, explains how its estimated price changes are expressed on a scale with the base period equal to 100.

Read the full base label. “January 2020 = 100” identifies one month; “2020 average = 100” identifies an annual average. The base year of a house price index need not be the year its observations begin.

How home price index rebasing works

For a chosen base month, use:

Rebased index = 100 × original index ÷ original index in the base month

Apply the formula separately to every series. Each needs its own denominator, taken from the same calendar month. The Bureau of Labor Statistics’ rebasing guide, last modified February 9, 2023, explains the underlying principle: changing the reference base leaves rates of change intact.

In a spreadsheet, suppose column B contains the original index and B2 contains the chosen base-month observation. Enter =100*B2/$B$2 in the first output row and copy downward. For a second series in column C, use =100*C2/$C$2. Lock the denominator so it does not move as you copy.

Choose a month with an observed, nonzero value in every series. If one lacks that month, choose a shared date rather than silently substituting a nearby observation. Preserve the downloaded precision and round only the display.

A worked example: different levels, identical total growth

These are hypothetical monthly indexes for two markets. Assume identical methods and coverage rules; only the markets differ. The table shows selected January observations, rebased to January 2020.

For A, the final calculation is 100 × 264 ÷ 200 = 132. For B, it is 100 × 198 ÷ 150 = 132. Both gained 32% from January 2020, even though their original ending levels differ.

Percentage change between any two observations is:

Percentage change = 100 × (later index ÷ earlier index − 1)

That is algebraically equivalent to the BLS percentage-change formula. Rebasing multiplies both observations by the same constant, which cancels in their ratio.

The paths still matter. From January 2023 to January 2026, A rises from 120 to 132: 10%, not 12%. B rises from 110 to 132: 20%. Those are three-year changes. Equal ending values establish equal cumulative growth only from the common base.

If you instead select January 2023 as the base, the January 2026 readings become 110 for A and 120 for B. The chart now answers a question about a different interval; neither series’ underlying growth rates changed.

Why index levels cannot rank dollar prices

A’s original reading of 264 does not establish that its homes cost more than B’s, with its reading of 198. Nor do the equal rebased readings of 132 establish equal home prices: each market has been scaled against its own starting point.

For a dollar-price comparison, obtain a separately defined price measure for comparable geography, property types and dates. A median sale price can describe the homes sold, but differences in the sales mix still matter. FHFA’s September 2010 research on house-price statistics explains how the composition of transactions can distort price-level comparisons.

Choose the measure to match the question before making the chart. Our home-price measures coverage provides context for that choice.

What a common base cannot fix

Dividing a series by a constant changes its scale, not the observations or estimation choices behind it. Check these distinctions before interpreting a gap between lines:

• Geography: A city boundary and a metropolitan area are different populations. Record the geographic definition, including its version.
• Transaction coverage: Purchase prices and refinance appraisals are different inputs. FHFA’s November 2018 methodology FAQs distinguish its purchase-only sample from the all-transactions sample, which adds refinance appraisals.
• Seasonal adjustment: Match seasonally adjusted series with adjusted series when comparing monthly momentum. Rebasing an unadjusted series does not remove seasonal patterns.
• Methodology: Rescaling cannot change property filters, weights or the estimation method. The 1996 technical description distinguishes arithmetic and geometric repeat-sales approaches; sharing a base does not erase that distinction.
• Timing and inflation treatment: A quarterly observation is not automatically a quarter-end monthly value. Putting nominal and inflation-adjusted indexes at 100 also leaves their different treatments of purchasing power intact.

You can plot differently constructed measures together to investigate disagreement. Explain that purpose and the differences. A common base alone does not justify treating the gap as a difference between otherwise equivalent markets.

What belongs beside the chart

Give readers enough information to reproduce the comparison:

• Producer and exact series name or identifier.
• Geography and property coverage.
• Frequency, seasonal adjustment and inflation treatment.
• Original and chosen reference bases, plus the observation range.
• Release date or data vintage, plus retrieval date.

A vintage is the version of the data available in a particular release. FHFA’s November 2018 explanation of revisions describes how additional transactions can revise historical estimates. Save the full source file: using a revised numerator with an older base-month denominator can introduce an artificial change.

For a chart presented as known on October 3, 2026, use a release available by that date. An older observation retrieved from a later release may contain information unavailable then. Our price data and reports page is a starting point for source selection; retain the exact release used.

Label this example: “Hypothetical home-price indexes; January 2020 = 100; selected monthly observations through January 2026; calculations by The Housing Current.” For a real chart, replace that label with the actual series and vintage details.