Days of cover divides every barrel the US holds — commercial crude stocks plus the Strategic Petroleum Reserve — by one day of refinery runs. It answers the question a closed strait raises: how long can refineries keep running on barrels already bought, before the missing supply reaches the pump.
All three inputs come from the EIA's Weekly Petroleum Status Report, so the series updates weekly: commercial crude stocks held by refiners and traders — the barrels the market can actually buy — the government-held Strategic Petroleum Reserve, and the crude US refineries actually put through per day. The page recalculates the ratio from the live data and shows nothing cached, estimated or substituted.
Two of the inputs are choices rather than facts. A government can release the SPR to buy weeks — and refilling it later becomes new demand — while refiners can cut runs to stretch what is left, which lowers the denominator and flatters the ratio while producing less fuel.
The two directions mean opposite things for US equities. A falling cover figure is the buffer draining — the day the missing barrels reach the pump getting closer, and with it the move in the oil price, the inflation print and the rate path that stock valuations are priced off. A rising figure is the cushion rebuilding — but read why before reading relief into it: cover lifted by an SPR release is borrowed time, and cover lifted by refinery run cuts is less fuel being made.
Watch the direction and the rate, not the level alone. A falling cover figure with a steady weekly drain is arithmetic on today's numbers, not a forecast — the rate changes every week. When the cover runs out, the shortfall shows up in the Brent price, and from there in inflation, rates and US equity valuations.