Basket size describes how much goes into a typical sale. Retailers use it in two ways, and it is worth knowing which one someone means. Most often it is the count of items, sometimes written as units per transaction: total items sold divided by the number of sales. Some owners use it to mean the dollar value of the sale instead, which is really the same thing as average transaction value. Both are worth watching, and together they show whether customers are buying more things, dearer things, or neither.
Basket size depends heavily on the kind of shop. A coffee window sees one or two items per sale. A greengrocer or a deli sees four, five or more. A bottle shop might see a single six-pack or a mixed dozen.
In practice
A greengrocer's till shows 4,200 items sold across 1,000 sales in a week, a basket size of 4.2 items. That number matters when choosing a stamp rule. If the card were one stamp per item, with ten stamps for $10 off, an ordinary customer would fill it in about two and a half visits, which turns the reward into a running discount. One stamp per visit over $10 means the same ten stamps take ten visits, which for a twice-weekly regular is about five weeks.
Basket size is also where a small shop can add value without anyone feeling sold to. A greengrocer who puts the lemons next to the fish, or the basil next to the tomatoes, often sees basket size creep up by a few tenths of an item. On 1,000 sales a week, an extra 0.3 items at an average of $2.50 each is $750 a week.
A falling basket size is a useful early signal. It can mean a competitor is taking the bigger weekly shop while you keep the top-ups, and that is worth knowing before the regulars go entirely.