Price per wash, per cup, per kilometre

The points below about cost per use are ordered by how much difference they make, not by how often they get repeated.

Sold in one unit, used in another

Detergent is sold by volume and used by dose, coffee by weight and used by cup, fuel by volume and used by distance. Comparing sale units therefore compares the wrong thing whenever concentration, dosing or efficiency differ between products.

A concentrated product with a smaller dose can be cheaper per use while looking more expensive on the shelf. The conversion is a single division and it changes the ranking often enough to be worth doing. This is the household version of the same arithmetic that applies to devices and their refills.

Where stated yields come from

Manufacturer yield figures are based on a defined usage assumption, such as a standard load, a standard cup or a standard test cycle. Those assumptions are reasonable and are frequently not what happens in a real household.

Run the arithmetic and overdosing is common because packaging and habit both encourage it, and it silently raises cost per use. Measuring rather than pouring is the single change that brings actual usage closer to the stated figure. It also usually improves results, since most products are formulated for the stated dose rather than for more.

Building your own figure

Record how long a pack lasts under your own usage rather than relying on the stated yield. Divide the pack price by the number of uses you actually achieved to get a personal cost per use.

At the till, repeat once for an alternative product and you have a comparison that is specific to your household. This is more accurate than any published figure because it includes your habits, your equipment and your conditions. Two data points are usually enough to settle a choice you will repeat for years.

Where the difference is largest

Categories with wide variation in concentration, such as cleaning products, show the biggest gaps between shelf price and cost per use. Anything where a machine determines the dose, such as capsule systems, has a cost per use fixed by the format rather than by you.

Run the arithmetic and products where a portion is unusable, such as those that cannot be fully dispensed, carry a hidden premium. Energy-consuming products add running cost to consumable cost, and both belong in the same figure. The wider the variation in a category, the more the arithmetic pays.

What not to conclude

A lower cost per use says nothing about whether a product performs adequately for your purposes, which is a separate question entirely. Comparing on cost alone can lead to buying more of something cheaper and ending up no better off.

The honest approach is to establish which products do the job acceptably and then compare those on cost per use. This article is about the arithmetic rather than about which products work, which belongs elsewhere. Cost per use is a tie-breaker among acceptable options, not a substitute for finding them.

Prices, promotions and terms move constantly, so check the current number rather than this one.

Making it a habit

Convert to cost per use for the six or seven consumables your household buys most often, once. Write the figures somewhere you will see them, since the point is to recognise a good price later. Re-check when a pack size or formulation changes, because either can move the figure without moving the price.

Tracked over a quarter, ignore the arithmetic entirely for anything you buy rarely, where the effort exceeds the benefit. Frequency decides where this pays, exactly as it does everywhere else in household spending.

Everything above, in order of what to do first

1. Sold in one unit, used in another. Detergent is sold by volume and used by dose, coffee by weight and used by cup, fuel by volume and used by distance.
2. Where stated yields come from. Manufacturer yield figures are based on a defined usage assumption, such as a standard load, a standard cup or a standard test cycle.
3. Building your own figure. Record how long a pack lasts under your own usage rather than relying on the stated yield.
4. Where the difference is largest. Categories with wide variation in concentration, such as cleaning products, show the biggest gaps between shelf price and cost per use.
5. What not to conclude. A lower cost per use says nothing about whether a product performs adequately for your purposes, which is a separate question entirely.
6. Making it a habit. Convert to cost per use for the six or seven consumables your household buys most often, once.

The takeaway

Convert your most frequent consumables to cost per use once, and let that figure tell you when a price is good.

Decide what you would pay before you look at what they are asking.