Tonic water price

Index, feed.

My local grocery store reports that it sells its brand’s canned tonic water at the following per-liter prices:

This is immediately surprising for two reasons:

• The smaller package, made of smaller cans, is reported as less expensive by volume than the larger package. Usually, larger purchases correspond to lower prices by amount of product.

• The prices are abnormally high: they would mean that the amount of soda in a 2L bottle would sell for about $40.

Let us compute the actual per-liter prices:

• Small cans: $\frac{4.29}{6 × 0.222} = 3.22$;

• larger cans: $\frac{8.29}{12 × 0.355} = 1.95$.

This confirms that the prices on the label were both incorrect, by about an order of magnitude.

Notes:

• I am surprised, but probably should not be, that this is not properly automated: I assumed that most products would be registered in a database with all the relevant details, including volume or mass of product, so that labels could be emitted automatically. Perhaps the inconsistent descriptions between the two labels was a telltale sign (“boisson gaz tonique diete” vs “soda tonique diete”).

• Reverse-engineering the numbers tells me that the per-liter price of the small cans, $19.30, was likely computed as $\frac{4.29}{0.222} \approx 19.32$ (dividing by $0.222222$ gets us closer to 19.30); and for the larger cans as $\frac{8.29}{0.355} \approx 23.35$.

  So the printed per-volume prices are as if the unit price were for a single can.

www.kurokatta.org

David Haguenauer <www@kurokatta.org>
$Id: tonic-water-price-per-liter.malblog,v 1.4 2026/03/29 19:03:19 homer Exp $