Grocery savings live in swaps, not in sales

These are listed in the order worth acting on, which with substituting within a category is not the order they are usually presented in.

Why swapping beats waiting

A promotion applies to selected items in selected weeks, whereas a substitution applies to every shop you do from now on. The gap between tiers within a category is frequently wider than the reduction any ordinary promotion delivers. Substitution also requires no timing, no planning and no attention once the decision has been made once.

That combination of size and permanence is why it moves a household total more than promotion-chasing does. It is also the least interesting way to save money, which is roughly why it gets written about so rarely.

Where tiers come from

Most categories are built as a ladder, with an entry level, one or two middle tiers and a premium option at the top. The differences between rungs are real but they vary enormously, from substantial to almost nothing depending on the product. For some products the specification genuinely changes between tiers, and for others the difference is mainly presentation.

Which is which cannot be determined from the packaging, and it differs by category rather than by any general rule. That is precisely why testing beats theorising, because the answer is specific to the product and to your household.

Testing a swap properly

Change one product at a time so that any reaction can be attributed to the thing that actually changed. Test on an ordinary day rather than on an occasion, because an occasion sets an unfair standard for the comparison. Give it more than one use where the product is something people get used to, since first impressions overweight novelty.

The number underneath says something else: keep the swap if nobody notices and abandon it if they do, without treating either outcome as a verdict on anybody. A household that swaps ten products and keeps seven has done considerably better than one that argues about all ten.

Where substitution is a false economy

Products that get used up faster because they work less well save nothing, and the unit rate conceals this entirely. Anything that gets thrown away uncooked or unfinished has a cost far above whatever was saved on the purchase.

Items tied to a specific dietary or medical need are not candidates at all and should be left alone. The correct measure is cost per satisfactory use rather than cost per unit, which sometimes reverses the answer. That measure is easy to apply once and impossible to guess, which again argues for testing rather than assuming.

The categories worth starting with

Start with the staples bought every week, because the effect of any change there repeats itself indefinitely. Cleaning products, basic dry goods and household consumables are usually the easiest and least contentious swaps. Leave anything with strong household preferences attached to it until later, since early failures discourage the whole exercise.

Run the arithmetic and products used as ingredients rather than eaten directly are generally more forgiving than those served on their own. The aim is to build a run of successes first, because the habit matters more than any individual swap.

Making the change stick

Write the swaps you kept onto your list so that the decision does not have to be made again in an aisle. Revisit the list occasionally, because ranges change and a product that failed once may have been reformulated since.

Across a sale weekend, do not measure the result in a single shop, since basket totals vary for reasons that have nothing to do with tiers. Compare a month against a month, which is long enough for the effect to be visible above the ordinary noise. The saving is undramatic and continuous, which is the only kind that survives a busy year.

Everything above, in order of what to do first

1. Why swapping beats waiting. A promotion applies to selected items in selected weeks, whereas a substitution applies to every shop you do from now on.
2. Where tiers come from. Most categories are built as a ladder, with an entry level, one or two middle tiers and a premium option at the top.
3. Testing a swap properly. Change one product at a time so that any reaction can be attributed to the thing that actually changed.
4. Where substitution is a false economy. Products that get used up faster because they work less well save nothing, and the unit rate conceals this entirely.
5. The categories worth starting with. Start with the staples bought every week, because the effect of any change there repeats itself indefinitely.
6. Making the change stick. Write the swaps you kept onto your list so that the decision does not have to be made again in an aisle.

The takeaway

Move between tiers rather than between weeks, because a swap applies to every shop and a promotion applies to one.

The cheapest purchase is still the one you did not make.