Definition
The average quantity of a part consumed per day; it is the common starting point for the kanban card count, the reorder point and safety stock, and nothing can be calculated without it.
In detail
Daily usage is an average, but the averaging window matters: how long the window is and how often it is recalculated must be decided together with the demand variation. Volume 12 (a third-party reprint) states plainly that you do not continue without a daily demand figure; the official Volume 14 gives only the formula at the same spot, without that statement. It is not takt time — takt is the rate at which the customer wants product, daily usage is how much of this part is consumed, and one finished product may pull several parts at different daily usages. Real consumption usually runs above the order quantity because of defects and scrap, so take the number from consumption rather than from shipments.
Example in practice
The team computes daily usage of 100 pieces over the past twelve weeks; with a 5 day lead time and one day of safety stock the kanban quantity comes to 600 pieces. When the season peaks, a new window puts daily usage at 140 and the card count is adjusted.