Core RPI YoY

Core RPI YoY means RPIX, the UK Retail Prices Index without mortgage interest payments, as a 12 month % change. ONS never says Core RPI, and publishes the series every month. It tracks the average monthly change in consumer goods and services bought by private UK households only. It leaves out the top 4% by income and pensioners mostly on state support. It uses about 760 items and 180,000 monthly quotes from 20,000 outlets at 150 sites across 12 regions.

Price estimates run from 1947 on a January 1987 base of 100, and RPIX was the 2.5% target gauge from May 1997 to December 2003. Arabic outlets use only general terms: UK consumer prices and the UK inflation rate.

Prices are mostly watched on index day, the 2nd or 3rd Tuesday of the reference month. Fuel prices reflect the monthly average, and fruit and vegetables get an extra Friday collection. Full data, weights and the briefing note land together, with dates fixed 18 to 30 months ahead.

The series appears in the inflation bulletin at 7:00am, usually on the 2nd or 3rd Wednesday after the reference month. That falls between the 13th and 21st, about 4 to 5 weeks after index day. Dates sit 18 to 30 months ahead, and the release has never been missed. RPI and RPIX are never revised, guarding index contracts, though ONS flags errors that shift a headline rate by over 0.1pp.

RPIX is the full RPI basket minus mortgage interest, weight 963 of 1,000 in 2024.

• Food and catering: groceries and eating out, from bread to restaurant meals.
• Alcohol and tobacco: drink and tobacco prices with duty effects.
• Housing, fuel and light: rent, owner costs, council tax and energy bills, but no mortgage interest.
• Household goods and services: furniture, appliances, post, phones and subscriptions.
• Clothing, personal goods and services: apparel, footwear, pharmacy goods and personal care.
• Motoring, travel and leisure: car costs, petrol, fares, holidays and recreation.

The April 2024 release of 22 May 2024 put headline RPI at 3.3% (385.0), down from 4.3% (383.0) in March. RPIX was 2.3% (380.1), down from 3.3% (378.1) in March, with the next release on 19 June 2024. That compares with 372.8 for RPI and 371.4 for RPIX in April 2023, and 367.2 with 366.1 in March 2023. The full 1.0pp gap between the two is the mortgage interest that RPIX strips out.

ONS blamed the fall on housing and household services after the Ofgem cap cut gas and electricity. Alcohol and tobacco, food, recreation and communication also pulled down. Restaurants, hotels and miscellaneous items partly offset the fall.

1. Elementary averages: RPI blends quotes with Carli (average of price ratios) and Dutot (ratio of average prices). CPI uses Jevons (geometric mean) instead, and weak Carli widens the gauge gap.
2. Weights: each item carries a weight in parts per 1,000, mainly from the Living Costs and Food Survey. National accounts feed the higher levels, weights are whole numbers, and the set resets each January.
3. Higher aggregation: items join a fixed Laspeyres basket, chain linked each January at 100. RPIX runs the same math with zero weight for mortgage interest, on the January 1987 series at 100.
4. Annual rate: the YoY % is 100 times the gap to a year earlier, over that level. It prints to 1 decimal from rounded indices, so rework can shift the last digit. For example, RPIX 380.1 in April 2024 over 371.4 in April 2023 is 100 times 8.7 over 371.4, about 2.342% to 2.3%.
5. Seasonal adjustment: monthly shifts carry seasons and sticky prices, so the 12 month change beats 1 to 2 month moves.