The Shelf Price Is Not the Lunchbox Price

A supermarket shelf tells you what a pack costs. It does not tell you what a lunchbox costs, and the two disagree often enough to be worth writing down.

I track 180 lunchbox ingredients. For each one I hold two things the shelf does not put together: the pack the shop actually sells, and how much of that pack goes into one lunchbox. The second number is the one that took the longest to get honestly, and it is the one that changes the answer.

The trolley and the lunchbox

Across those 180 ingredients, the median pack is $4.80 and the median lunchbox serve is $0.47. The median pack holds 10 serves.

Buying one of everything would come to $990.02. One lunchbox serve of everything comes to $121.83. That ratio is the whole point: most of what you carry out of the shop is not for today.

How many lunchboxes a pack is

Only 3 of the 180 are effectively single-serve. Those are the ones worth watching, and they are almost never the ones that look expensive.

The shelf ranking is about half right

Rank every ingredient by pack price, then rank it again by what one serve costs. The two orders agree, but loosely — the rank correlation is 0.59.

That number is doing real work in both directions. It is not near zero, so the shelf price is not noise: the 450g pack of chicken thigh at $17.90 really is expensive in a lunchbox too, at $3.58 a serve. But it is not near one either. 50 of the 180 ingredients sit on opposite sides of the median in the two rankings — cheap-looking and dear to use, or dear-looking and cheap to use.

Dear on the shelf, cheap in the box

These are the items people flinch at and shouldn't. Of the 81 ingredients whose pack costs more than $5.00, 21 come to under 50c a serve. A large bottle of oil is one of the most expensive single things in the shop and one of the cheapest things in the lunchbox.

Cheap on the shelf, dear in the box

The mechanism is the same every time: the pack is the serve, or close to it. The croissant row is the clearest case — $3.60, and one lunchbox uses the whole thing. It is the most expensive serve in the entire set while sitting in the bottom half of the shelf prices.

This is not an argument against buying any of them. It is an argument for knowing which line in the trolley is the one you are paying for repeatedly.

What the serve sizes are, and are not

They are our portions. Recording a cheese serve as 30g out of a 500g block is a choice, not a standard, and a family that cuts a thicker slice will get different numbers out of the same shelf.

What they are not is reverse-engineered. When these were written down, the serve size for each ingredient was recorded first — how much actually goes in a lunchbox — and the portion factors were corrected afterwards where the two disagreed. Five ingredients had to be fixed that way. Had the serve sizes been derived from the factors instead, every table above would be a circular argument dressed as a finding.

Pack sizes are not estimates either. Each one was read off the product listing at the chain that sells it, which is why a 220g hummus tub is recorded as 220g and not as the 200g I had originally written down.

What I would not claim

The same pack-size effect shows up sharply in the free-from aisle, where the alternative often comes in a smaller bag: what gluten-free and dairy-free swaps actually cost puts numbers on that.

These are one household's portions against one store's online prices, and Pak'nSave prices by store. Fresh produce sold by weight — bananas, potatoes, courgettes — moves week to week, so its position in either ranking is the least stable part of this.

And the 0.59 correlation is a snapshot. It is high enough that "expensive pack, expensive lunchbox" is a reasonable default; the useful part is knowing the 50 places where the default is wrong.

The full price table, with every row's reading date, is on the NZ Lunchbox Price Index page, published under CC BY. The chain-by-chain comparison of the same basket is in Where the supermarket gap actually is.

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