Why use this calculator
Supermarket pricing is designed to make comparison hard. Pack sizes are chosen so the arithmetic is awkward, promotional tickets shout about the discount rather than the rate, and the assumption that bigger is cheaper is wrong often enough to be worth checking. Unit pricing exists precisely to solve this and is required on shelf tickets in Australia, but the print is small, the units are sometimes inconsistent between competing products, and a multibuy offer usually does not show one at all.
This calculator does the division for you across three options at once and tells you what the difference actually costs over a year of buying the wrong one.
How to use it
- Enter the price you would actually pay for each option, after any discount, and the quantity in the pack. Use the same unit for every option.
- Comparing a pack listed in grams against one in kilograms without converting is the single most common error.
- Choose the unit type so the result is expressed sensibly: weights and volumes are shown per 100 grams or per 100 millilitres, which is the supermarket convention, while items, sheets and washes are shown per single unit.
- The third option is optional and can be left at zero.
What each result means
- Cheapest option
- The option with the lowest price per unit, which is the only fair basis for comparing different pack sizes.
- Option A unit price
- What option A costs per 100 grams, per 100 millilitres or per item, depending on the unit selected.
- Option B unit price
- The same figure for option B, calculated on the identical reference amount so the two are directly comparable.
- Option C unit price
- The same figure for the optional third option, shown only when you enter a price and size for it.
- Saving versus the worst option
- The percentage gap between the best and worst unit price. How much of the price is pack-size penalty rather than product.
- Extra cost if you buy the worst
- What choosing the most expensive option costs you for an equivalent quantity of product.
The formula
Unit price is simply the pack price divided by the pack quantity, then scaled to a common reference amount: price ÷ size × 100 for grams and millilitres, or price ÷ size for countable items. Expressing every option at the same reference amount is what makes them comparable. The percentage difference between two options is the gap in unit price divided by the cheaper unit price, which is the figure that tells you how much more you are paying rather than just that you are paying more.
How this number is used in practice
Bigger is not reliably cheaper. Retailers know that shoppers assume it is, which makes the large pack a good place to hide a higher unit price, and studies of supermarket shelves routinely find a meaningful share of products where the smaller pack wins. The pattern is strongest on promotional lines, where a discounted small pack can undercut a full-price large one substantially.
Multibuy offers deserve particular scrutiny. Two for a fixed price, three for the price of two, and bulk bundles frequently carry no unit price on the ticket, which is precisely the situation where one would be most useful. Work out the per-unit cost of the offer and compare it against the single-unit price before assuming the deal is a deal.
The saving only counts if you use the product. A cheaper unit price on a perishable item is a false economy if a third of it is thrown away, and storage space and cash flow are real constraints. Buying six months of something at a two per cent saving ties up money for a trivial return. The honest calculation is unit price on the quantity you will actually consume.
Worked examples
$4.50 for 500g against $7.80 for 1kg
90 cents per 100g against 78 cents. The kilogram pack is about 13 per cent cheaper per unit.
$3.00 for 250ml against $9.50 for 1 litre
$1.20 per 100ml against 95 cents. The larger bottle wins by about 21 per cent.
$6.00 for 12 items against $9.00 for 16 items
50 cents each against 56 cents. Here the smaller pack is the better buy, against the usual assumption.
When people use it
- Comparing two pack sizes of the same product on the shelf
- Checking whether a bulk or warehouse pack is actually cheaper
- Working out whether a multibuy offer beats the single price
- Comparing a promotional small pack against a full-price large one
- Costing out subscription or delivery pricing against retail
Tips
- Convert to the same unit before comparing. Grams against kilograms without converting is the most common error.
- Bigger is not reliably cheaper. A meaningful share of supermarket lines price the larger pack higher per unit.
- Multibuy tickets often carry no unit price. Work it out yourself before assuming the offer is good value.
- Use the price you will actually pay, including any discount, not the ticketed full price.
- A cheaper unit price on a perishable is a false economy if you throw part of it out.
- Concentrates and refills should be compared on made-up volume, not on bottle size.
What it assumes, and where it stops being accurate
- The products being compared are genuinely equivalent in quality, concentration and usable quantity.
- All sizes are entered in the same unit; the calculator cannot detect a mismatch between grams and kilograms.
- Prices are what you will actually pay, including discounts but excluding loyalty points and cashback.
- Wastage, spoilage and storage costs are not modelled and can outweigh a small unit-price saving.
- Packaging weight is excluded. The size entered is assumed to be product, not gross pack weight.
Terms used on this page
- Unit price
- The cost per standard measure, per 100 grams, per litre or per item, allowing different pack sizes to be compared.
- Multibuy
- An offer priced for several units together, which often carries no unit price on the ticket.
- Reference amount
- The common quantity every option is expressed against so the comparison is fair.
- Shrinkflation
- Reducing pack size while holding the price, which raises the unit price without changing the shelf ticket.
Questions people ask
- Is the bigger pack always cheaper?
- No, and assuming so is exactly what pricing strategy relies on. Surveys of supermarket shelves regularly find a meaningful share of products where the smaller pack has the lower unit price, particularly when the small pack is on promotion and the large one is not. Checking takes seconds and is worth doing on anything you buy regularly.
- How do I compare different units like grams and kilograms?
- Convert everything to the same unit before entering it: 1 kilogram is 1,000 grams and 1 litre is 1,000 millilitres. The calculator then expresses both at a common reference, normally per 100 grams or per 100 millilitres. Failing to convert first is the single most common source of a wrong answer.
- What is unit pricing and is it required by law?
- Unit pricing shows the cost per standard measure, per 100 grams, per litre, per item, so different pack sizes can be compared directly. In Australia, large grocery retailers are required to display it under an industry code, though the print is small and it is often missing from multibuy tickets, which is where it would be most useful.
- Should I always buy the lowest unit price?
- Only if you will use it. A cheaper unit price is a false economy on something perishable you throw away, and bulk buying ties up cash and storage space for what is often a small percentage saving. Calculate the unit price for the quantity you will genuinely consume before the product goes off or goes stale.
- How do I compare a concentrate against a ready-to-use product?
- Compare on made-up volume rather than bottle size. A 500 millilitre concentrate that dilutes to five litres should be entered as 5,000 millilitres, otherwise the ready-to-use product will look far cheaper than it is. The same applies to laundry products sold by washes rather than volume.
Use this with
Unit Price Comparison Calculator answers one part of the question. These pick up where it stops: Discount Calculator, Percentage Calculator, Meal Cost Calculator and Budget Planner Calculator.
Where these figures come from
This calculation is jurisdiction-neutral arithmetic. No country-specific rules are applied.
- ACCC: unit pricing for grocery retailers. The code requiring unit prices to be displayed
- CHOICE: how to use unit pricing. Independent Australian consumer guidance on supermarket pricing
Unit Price Comparison Calculator produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.