Why the purchase price alone says little about the real cost

Two products that do the same job can cost very different amounts once you look not just at the purchase price but also at the supplies you need over the time you use them. The cost per use sums up exactly that: the total spent on buying the product and its consumables, divided by the number of times you actually use it. The product that is cheaper to buy is by no means automatically the cheaper one over time.

This calculation is especially worth doing for products that need a steady supply of consumables, such as printer cartridges, coffee pods, razor blades or laundry detergent, where the one-off purchase price is only a small part of the total cost.

How to work out the cost per use

The formula could hardly be simpler: total cost divided by the number of uses. The total cost is made up of the purchase price of the product itself and, where relevant, the cost of the consumables it needs over a set period. The number of uses is what you actually get for that price, such as the number of washes per bottle of detergent or the number of cups per pack of coffee pods.

A worked example with two laundry detergents

Say a 1-litre bottle of detergent costs 5.99 euros and, according to the dosage instructions, is enough for 20 washes. That gives you 5.99 euros divided by 20, or roughly 0.30 euros per wash. A second, more concentrated 1.5-litre bottle costs 7.49 euros but is enough for 33 washes. Here you get 7.49 euros divided by 33, or roughly 0.23 euros per wash.

Even though the second bottle costs 1.50 euros more to buy, you save about 7 cents on every single wash. For 20 washes, it works out at only about 4.54 euros instead of 5.99 euros, and you still have 13 washes left in the bottle.

When the comparison is not quite fair

You cannot always break two products down to the same kind of use. A larger pack may be cheaper per unit, but it only helps if you actually use it all up before it goes off or stops working properly. With detergent, cosmetics or food, it is therefore worth checking how long the product keeps before you reach for the big pack just because of a lower cost per use. If you only end up using half of it because the rest has passed its date in the cupboard, the supposedly cheaper choice was in fact more expensive.

When a higher purchase price pays off

A higher purchase price mainly pays off if the more expensive product either lasts much longer or needs far fewer consumables per use. If you only use a product rarely, on the other hand, the calculation often shifts in favour of the cheaper option, because a high purchase price hardly pays for itself over just a few uses. How often and for how long you actually use a product is therefore at least as important as simply comparing purchase prices.

Where this calculation matters most

With some product groups, consumables make up such a large share of the total cost over time that they deserve a closer look of their own. How this plays out for printers through the cost per page is described in the article Printers: the cost per page. With coffee machines, the running costs vary a great deal depending on whether you use pods, beans or filter coffee, as the article Coffee machines: pods, beans or filter? shows. And with vacuum cleaners, the battery and bags play a similar role, as you can read in the article Vacuum cleaners: battery, bags and running costs.

What is often left out of the calculation

• Maintenance costs, such as changing a filter or descaling, which cost money or extra materials in their own right.
• The actual lifespan of the main device, which often turns out shorter than expected and makes the cost per use worse after the fact.
• Accessories that only one of the products being compared needs on top, such as an adapter or attachment bought separately.
• The delivery costs for top-ups, if consumables have to be reordered separately on a regular basis and were not included in the original purchase price.

If you include these points in your calculation, you get closer to a realistic comparison than by looking only at the purchase price and consumables. The lifespan of the main device in particular is hard to predict exactly in advance, so it helps to estimate cautiously rather than optimistically and to work with a somewhat shorter period of use rather than the best possible one.

Using the cost per use when comparing prices

Once you have worked out the cost per use for two similar products, you can apply the figure to similar models too, without having to redo the whole calculation every time. A good addition to this is looking at the price per item, per kilo or per litre for individually packed goods, as explained in the article Working out unit prices per item, kilo and litre. What makes up the total cost of a single online purchase beyond the item price is described in the article Working out the total cost of an online purchase.

In practice, it is worth working out the cost per use once and noting it down for the things you buy regularly, instead of starting from scratch with every new offer. If the price of the consumables changes later on, you only need to put in that one new figure to see whether the ranking of the products you compared has shifted.

Frequently asked questions

Is the calculation worth it even when the price difference is small?

Yes. Especially with products you use very often, even small differences per use add up over time to a noticeable amount.

What do I do if a manufacturer gives no figures for the number of uses?

Then a rough estimate of your own, based on the pack size and the usual dosage or period of use, will help, even if it is less precise than a figure from the manufacturer.

Does the calculation also work for one-off purchases without consumables?

There, the calculation shifts more towards lifespan: the total cost divided by the number of times you use the product before it breaks down or is replaced.

Should the lowest purchase price always come second?

Not always. With products you rarely use, the purchase price often remains the more important factor, because a high price hardly pays for itself over only a few uses.