Unit Price Comparison Calculator
Compare several package sizes by price per unit to find which is genuinely the best value, even when the sizes use different units.
How to use this calculator
- 1List each package on its own line: name, price, size, unit.
- 2Use whatever units are on the label — mix grams and kilograms freely, since everything is normalised before comparison.
- 3Check the warning about pack size: the biggest package is usually but not always the cheapest per unit.
How the calculation works
Unit price = price ÷ (size × unit factor)- size × unit factor
- The package size converted to a single common base unit — grams for mass, millilitres for volume
- Unit price
- Cost of one base unit, which is the only figure that can be compared across package sizes
Everything is converted to one base unit before comparing, which is why a 1.5 kg bag can be ranked directly against a 750 g box. Comparing the raw prices, or the raw sizes, tells you nothing without that step.
Mass and volume cannot be compared against each other. A price per gram and a price per millilitre measure different things, and the calculator refuses rather than producing a number that looks meaningful.
Results are shown per kilogram or per litre rather than per gram or millilitre, purely so the figures stay readable — the ranking is identical either way.
Worked example
Four box sizes of the same product
- 1.Small box: $3.49 ÷ 250 g = $0.01396 per g, or $13.96 per kg.
- 2.Medium box: $5.99 ÷ 500 g = $0.01198 per g, or $11.98 per kg.
- 3.Family size: $8.99 ÷ 750 g = $0.011987 per g, or $11.99 per kg.
- 4.Bulk bag: $14.49 ÷ 1,500 g = $0.00966 per g, or $9.66 per kg.
- 5.The bulk bag wins at $9.66/kg — 30.8% cheaper per kilogram than the small box. Note the medium box very slightly beats the family size despite being smaller, which is the kind of thing shelf labels hide.
Result: Bulk bag at $9.66/kg — 30.8% cheaper than the worst option
Why package sizes are hard to compare on purpose
Retail package sizes are rarely round numbers, and rarely simple multiples of one another. A product line might run 250 g, 500 g, 750 g and 1.5 kg at prices that are similarly irregular. This is not accidental: comparison friction is a well-understood pricing lever, and irregular sizing makes the arithmetic just awkward enough that most shoppers fall back on heuristics like "bigger is cheaper" instead of actually checking.
Many jurisdictions now require unit pricing on shelf labels precisely because of this. The regulations help, but they are not a complete fix — labels within the same aisle frequently quote different bases (per 100 g on one item, per kg on the next), which reintroduces exactly the comparison problem the rule was meant to remove.
When bigger is not cheaper
The assumption that larger packages always cost less per unit is usually right and occasionally, profitably, wrong. Retailers sometimes price the largest size at a premium — betting that shoppers will assume it is the bargain and not check. The pattern shows up most often on promoted sizes, on "family" or "value" branded packs, and when a smaller size happens to be on temporary discount.
This is the single most useful thing a unit-price comparison surfaces, which is why the result flags it explicitly whenever the largest pack is not the winner.
What unit price does not account for
A lower price per unit is not automatically the better purchase.
- Spoilage — the cheapest per unit is worthless if half of it expires before use. For perishables the effective unit price is based on what you actually consume, not what you bought.
- Storage and cash — bulk buying ties up both shelf space and money. For a household that is an inconvenience; for a business it is working capital that could be deployed elsewhere.
- Concentration and formulation — a concentrated detergent at a higher price per litre may cost less per wash. Where products differ in strength, compare per use rather than per volume.
- Packaging weight — for products sold by gross weight, packaging can distort the comparison. Drained weight is the honest basis for tinned goods.
What this assumes, and where it stops
Assumptions
- The items being compared are genuinely equivalent products — same quality, concentration and formulation.
- Sizes are the actual product quantity, not gross weight including packaging.
- All items use the same kind of unit; mass and volume cannot be compared and the calculator rejects a mix.
Limitations
- Does not account for spoilage, storage cost, or the working capital tied up in bulk purchases.
- Cannot compare products of different concentration or strength — a concentrated product needs comparing per use, not per volume.
- Assumes the entire package gets used. For anything perishable, the real unit price is based on the portion actually consumed.
Common questions
Is the biggest package always the best value?
Usually, but not reliably. Retailers sometimes price the largest size at a premium on the assumption that shoppers will not check, and a temporary discount on a smaller size can easily flip the ranking. This calculator flags it explicitly whenever the largest pack is not the winner, because that is exactly the case worth catching.
Why can I not compare a price per gram against a price per millilitre?
Because they measure different physical quantities. Converting between them requires knowing the product's density, which varies by product and is not something a price comparison can assume. If a product is sold by weight in one package and volume in another, you need the density from the label to compare them properly.
Should I always buy the lowest unit price?
Only when you will actually use the whole package. For perishables, the effective unit price is based on what you consume rather than what you buy — throwing away a third of a bulk pack usually wipes out a 20-30% unit-price saving entirely.
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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