How to use the cost per unit calculator
Enter the price of the pack and how much it contains, and you get the price per unit. It is the only figure that makes two different pack sizes comparable - and the one that decides which item is genuinely cheaper.
What the defaults produce
Here is what the preset values give:
- Price: $89.50
- Quantity: 1.50
A worked example
A pack costing 89.50 containing 1.5 kg works out at 59.67 per kilo. Another pack at 65 for 1 kg costs 65 per kilo - so the more expensive pack was in fact the cheaper product.
Shelf labels are regulated
Unit pricing rules in many markets oblige retailers to show the price per standard unit next to the selling price, precisely so shoppers can compare across pack sizes. The calculator earns its keep when the label is missing, unreadable or when you are pricing an offer yourself.
Bigger is not always cheaper
The assumption that the large pack always wins is wrong surprisingly often. Promotions on smaller packs, different suppliers and plain pricing mistakes mean the unit price sometimes points the other way - so check rather than assume.
Recalculate the promotions
Offers like three for two or bulk discounts change the unit price. Take the total you actually pay and the total quantity you take home - then the promotion becomes comparable with the regular price, and you can see whether it is a genuine deal or clever signage. Promotions are designed to be read quickly rather than carefully, which is exactly why the arithmetic pays.
Same unit on both sides
Never compare price per kilo with price per litre, or per item with per hundred grams. Convert to the same unit first - obvious in principle, but competing products often differ precisely to make the comparison harder.
Drained weight matters
Tins and frozen goods may state both gross weight and drained weight. Calculate on what you actually eat rather than on brine or ice - otherwise a water-heavy product looks far cheaper than it really is.
Cheap per kilo is expensive if wasted
The real unit price is based on what you use, not what you buy. Half a bulk pack in the bin doubles the cost per unit consumed. For perishables in a small household the smaller pack is therefore often the economical choice.
Useful far beyond the supermarket
The same arithmetic reveals cost per wash in detergent, per serving in protein powder, per metre in cable or per kilowatt-hour in a battery. Wherever pack sizes differ, unit price is the only honest comparison.
Common mistakes
Comparing different units, calculating on gross pack weight, and trusting that the large pack is cheaper. All three cost small amounts every week - and small amounts every week become a meaningful sum over a year. Calculate once and you save every week afterwards, without giving anything up.
Make it a habit
Work out the unit price for the ten items you buy most often, once, and note which pack size wins. After that you never need to repeat the exercise - the habit takes care of itself at the shelf. Ten calculations done once give you a reference that lasts for years, because pack sizes change far less often than prices do.