For eight consecutive weeks, at the same time each week, we logged shelf prices for six categories at one supermarket. The rhythm of promotions turned out to be more regular than expected.

The most regular are paper goods and cleaning products: broadly a four-week cycle, with discounts steady at 20 to 30 per cent.

Fresh food runs on time of day, not week

Fresh food works differently. It follows the clock rather than the calendar: most stores mark down same-day fresh items in the last two hours before closing, with the discount deepening as the hour approaches.

The promotions area at a supermarket entrance.
The promotions area at a supermarket entrance.
  • Paper and cleaning: roughly a four-week promotional cycle
  • Fresh: marked down in the last two hours, deepening
  • Dairy: short-date discounts in the final third of shelf life
  • Large packs: not always a lower unit price

One counter-intuitive finding: larger packs are not always better value. Within a promotional cycle, a small pack with the discount applied can undercut the standard unit price of the large one.

When comparing, convert everything to a price per 100 grams or per item. Shelf labels usually carry it already — in very small type.