Japan's CPI base-year revision may raise first-half 2026 inflation but lower the subsequent inflation outlook

Overview

This report discusses the impact of Japan's CPI base-year revision in 2026. Japan's Consumer Price Index is calculated as a weighted average of individual price indices based on average consumption patterns. Because consumption behavior changes over time, the weights are updated every five years. This revision will switch the CPI base year from 2020 to 2025, with a retrospective revision of CPI data for January-June 2026 scheduled for August 7, 2026.

Core views

Goldman Sachs expects core CPI and new core CPI inflation rates for January-June 2026 to be revised slightly upward under the new base year. The main reasons include lower weights for education- and childcare-related items, which will reduce the negative contribution from free policies, and a substantial increase in the rice weight, which will magnify the positive contribution of rising rice prices to inflation from 2025 through early 2026. However, the report also believes that the 2025 base-year revision may weigh on the future inflation outlook because the negative contribution from falling rice prices will increase with the higher weight, while the lower mobile phone weight will weaken the positive contribution from potential mobile phone price increases.

Analysis framework

The report compares Japan's 2025 base-year consumption weights with those under the current 2020 base year, estimates the differences in core CPI and new core CPI inflation rates for January-June 2026, and further analyzes the directional impact of changes in the weights of major items on inflation forecasts for FY2026-27.

Methodology notes

• Inflation statisticsCPI base-year revision

  Updating CPI item weights to reflect a new average consumption structure.

  CPI is a weighted average of price indices for various goods and services. A base-year revision changes item weights and may also change the estimation methods for some price indices, thereby affecting historical comparability and future forecasts.

• Inflation decompositionWeight contribution analysis

  The impact of price changes on overall CPI depends on both the magnitude of price movements and item weights.

  The report focuses on changes in the weights of rice, mobile phones, high school tuition, school lunches, and childcare fees, and uses them to assess upward or downward pressure on core and new core CPI.

Asset mapping & comparison

Structured mapping from thesis to named assets (strengths, weaknesses, peers, risks).

• Japanese interest rates

  The CPI revision affects inflation expectations and policy rate pricing.

  Strengths

  The short-term upward revision to inflation data may reinforce market focus on the persistence of inflation.

  Weaknesses

  Pressure on future inflation forecasts may weaken expectations for continued rate hikes.

  Comparison

  There is a directional divergence between the upward revision to short-term historical data and the downward revision to medium-term forecasts.

  Risks

  If rice or mobile phone price trends differ from the report's assumptions, interest-rate pricing may be adjusted.

• Japanese macro inflation expectations

  Changes in base-year weights directly affect the statistical readings of core CPI and new core CPI.

  Strengths

  The new base year more closely reflects the consumption structure in 2025.

  Weaknesses

  Some price-index estimation methods will also change, leaving uncertainty around current estimates.

  Comparison

  Compared with the 2020 base year, the 2025 base year increases the rice weight and reduces the weights of mobile phones and certain education and childcare items.

  Risks

  The official revision results may differ from Goldman Sachs' rough estimates.

Key data

• Base-year revision date2026-08-07Japan's CPI will switch to the 2025 base year, with data for January-June 2026 revised retrospectively.
• Retrospective revision scopeJanuary-June 2026Inflation rates through December 2025 will not be revised.
• Rice weight0.62% → 1.01%The rice weight under the new base year will nearly double, magnifying the contribution of rising or falling rice prices to CPI.
• Mobile phone weight0.90% → 0.58%The lower mobile phone weight may weaken the positive contribution of potential future price increases to inflation.
• Direction of impactSlight upward revision for January-June 2026, with downward pressure for FY2026-27The direction of short-term retrospective data revisions may differ from that of future forecasts.

Impact & implications

For investors, the CPI base-year revision may make Japan's already published inflation data for the first half of 2026 appear slightly higher, but the more important impact is that the future inflation path may be revised lower. If rice prices decline in the future, their drag on inflation will be stronger under the new weight; meanwhile, the lower mobile phone weight will reduce the inflationary boost from potential price increases. This may affect market assessments of the persistence of Japan's inflation, the pace of monetary policy normalization, and the interest-rate path.

Risks

• The base-year revision involves not only changes in weights but also changes in the estimation methods for some price indices, creating uncertainty around current estimates.
• Some price indices have not yet been released, which may affect the final magnitude of the revision.
• Rice prices have shifted to a negative contribution since April 2026, making the future price path a significant influence on inflation forecasts.
• The timing and magnitude of potential mobile phone price increases remain uncertain.

What to watch

• The official results of Japan's CPI revision to the 2025 base year on August 7, 2026.
• The magnitude of upward revisions to core CPI and new core CPI for January-June 2026.
• Whether the negative contribution of rice prices to future CPI expands under the new weight.
• Actual mobile phone price increases from the second half of 2026 through the first half of 2027.
• The impact of changes in Japan's inflation forecasts on monetary policy and interest-rate market pricing.