UK Online Shelf-Price Index

It measures posted online prices. Each week the full online catalogue of four UK supermarkets — Tesco, Sainsbury's, Asda and Aldi — is read as any visitor sees it. Products present in both this week and last week are matched by the retailer's own product id, and the index is the weighted mean of their price ratios, chained week on week from a base of 100 at ISO week 2026-W39. Shelf price only: loyalty-card prices and multibuy promotions are recorded separately and excluded here.

It is not CPI, and it is not a measure of prices paid. The ONS consumer prices index uses scanner data with transaction quantities and includes discounts actually taken; this index has no quantities and no transactions. Treat it as a fast, product-level indicator of what the four retailers are asking online, published days after the week it describes.

Known limits. Online catalogues only, so in-store-only lines are absent; prices are national online shelf prices except where a retailer exposes a default store; Aldi publishes no barcodes, so cross-retailer joins there use name and pack size. Category rows can rest on fewer than four retailers where one retailer's cell is too thin. The four-week figure is empty until four weeks exist. There is no year-on-year figure until a year exists. Suspect moves (over 200% in a week) are excluded and listed in the weekly note.

Coverage. About 70,000 products a week across the four retailers; matched counts are shown on every row. Weeks with a missing retailer are published with a coverage warning in the metadata and never backfilled or interpolated. Morrisons joins from November 2026, and the index continues on the four-retailer basis with Morrisons reported separately until a full year of five-retailer data exists.

Methodology and due diligence for the underlying data: data.yappman.com. Weekly "what moved" notes: UK Grocery Price Moves. The product-level data behind the index is licensed separately.