RPIX (Retail Prices Index Excluding Mortgage Interest Payments)

RPIX is the United Kingdom’s Retail Prices Index excluding mortgage interest payments. It retains the broader RPI framework while removing the direct mortgage-interest-payment component, so it can differ materially from RPI when mortgage rates change. RPIX is not calculated by subtracting a cash payment or an inflation rate from the published RPI number.

Key Takeaways

• RPIX means RPI excluding mortgage interest payments, often abbreviated MIPs.
• It removes RPI’s direct response to mortgage interest rates but retains other RPI components and methods.
• RPIX, RPI, CPI, and CPIH are separate indexes; their levels and inflation rates are not interchangeable.
• The UK Office for National Statistics (ONS) continues to publish RPIX while identifying important shortcomings in the RPI framework.
• Existing contracts or historical analysis may refer to RPI or RPIX specifically, so substituting another index can change the economic result and may not be permitted.
• RPIX is a national aggregate, not a household-specific mortgage or living-cost measure.

Why Mortgage Interest Is Excluded

RPI includes a modelled mortgage interest payments component. Changes in mortgage rates can therefore affect RPI directly. This creates a feedback issue for some monetary-policy analysis: a policy-rate increase intended to restrain inflation can raise the measured RPI through mortgage interest before its wider effects appear.

RPIX removes that direct component. It does not remove:

• rent or every other housing-related item;
• indirect effects of interest rates on demand, exchange rates, wages, or business costs;
• other differences between RPI and newer consumer-price measures; or
• the impact of mortgage costs on actual borrowers’ finances.

The ONS RPI calculation methodology explains how the mortgage-interest component is modelled within RPI.

How RPIX Is Calculated

RPIX is aggregated from the applicable RPI components while excluding mortgage interest payments. The remaining component weights and official index-number methods determine the result.

It is incorrect to use:

Index points, expenditure weights, component price relatives, and percentage changes are different quantities. Removing a component requires recalculating the aggregate under the index methodology rather than subtracting a household’s cash expense or the MIP inflation rate.

Worked Example: Why RPI and RPIX Can Differ

Assume, solely for illustration, that mortgage interest has a 10% weight in a simple weighted inflation calculation and all other components have a 90% weight.

The simplified all-items result would be 5.2%, while the measure excluding mortgage interest would reflect the 3.0% change in the remaining basket after its weights are normalized.

This is not the official RPI or RPIX formula. It only shows why a large mortgage-interest movement can widen the gap between the two measures.

RPIX Compared With RPI, CPI, and CPIH

RPIX should not be described as CPI with housing removed. CPI and RPIX differ for reasons extending well beyond mortgage interest.

ONS has stated that RPI does not meet the required standard for designation as a National Statistic and has limited expansion of RPI-related outputs. Its publication clarification confirms continued publication of RPI and RPIX for existing needs. Users should check current ONS policy and series documentation because index governance can change.

Why RPIX Matters in Finance

• Historical policy analysis: RPIX was used in earlier UK inflation-targeting contexts and appears in historical reports and time series.
• Contracts and indexation: Some agreements may name RPIX, RPI, CPI, or CPIH. The exact wording, lag, interpolation, and fallback provisions matter.
• Bond and liability analysis: Inflation-linked cash flows must be evaluated using the contractually specified index rather than a convenient headline measure.
• Economic comparison: The RPI-RPIX gap can help isolate the direct effect of the mortgage-interest component within the RPI framework.
• Household analysis: RPIX can provide national context but does not measure an individual borrower’s refinancing schedule, loan rate, balance, or housing tenure.

Limitations and Common Mistakes

• Treating RPIX as a modern replacement for all UK consumer-price measures.
• Saying RPIX excludes housing; it excludes mortgage interest payments, not every housing-related component.
• Subtracting a mortgage-rate change or mortgage payment directly from RPI.
• Assuming RPIX is always lower than RPI; the MIP component can rise or fall.
• Comparing raw index levels with CPI or CPIH without accounting for different base periods and methods.
• Using RPIX in a contract that specifies RPI, CPI, CPIH, or another exact series.
• Ignoring the ONS’s stated concerns about the wider RPI methodology.
• Treating a national RPIX rate as a personalized household cost change.

Authoritative Sources

• ONS: Calculating the Retail Prices Index
• ONS: Inflation and price indices
• ONS: Publication arrangements for RPI and related indexes

Related Terms

• Consumer Price Index: General consumer-price-index concept, implemented differently across jurisdictions.
• Headline Inflation: All-items inflation in the specifically named index.
• Price Index: Statistical framework for combining price changes.
• Index-Linked: Cash flow or value adjusted under a specified index rule.
• Inflation Rate: Percentage change in a defined price index over a stated period.

FAQs

This article provides general economic education, not legal interpretation of an index-linked contract, a UK inflation forecast, or personalized mortgage or investment advice. Check current ONS data and governing documents for actual decisions.