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How to Compare Prices Properly Using Unit Price
Bigger packs are usually cheaper per unit, but not always. Here is how to calculate unit price quickly, handle mismatched units, and factor in what you will actually use.
Unit price is total price divided by quantity. It converts packages of different sizes into one comparable number, which is the only reliable way to tell whether a larger pack is genuinely cheaper or just larger.
The formula
The choice of unit does not matter as long as it is the same for everything you are comparing. Per 100g is convenient for groceries because it keeps most answers in a readable range rather than producing long decimals.
Unit price = Total price / Quantity Express quantity in a consistent unit: per 100g, per litre, per item, per 100ml
Worked example: the bigger pack that isn't cheaper
Coffee, two options. A 250g bag at 6.49 and a 500g bag at 13.60.
Small bag: 6.49 / 250 = 0.02596 per gram, or 2.60 per 100g.
Large bag: 13.60 / 500 = 0.0272 per gram, or 2.72 per 100g.
The larger bag costs about 4.6% more per gram. Buying two small bags costs 12.98 against 13.60 for the same weight — a saving of 62 on an ordinary purchase, reached by dividing twice.
This happens more often than people expect, particularly on promoted sizes and on packs positioned as the family or value option.
Handling mismatched units
The concentrate case is the one that catches most people. A 500ml concentrate that dilutes 1:4 makes 2.5 litres of usable product. Compared against a 2-litre ready-to-use bottle, the concentrate might be the cheaper option by a wide margin even if the shelf price is higher, and shelf-edge unit pricing rarely accounts for dilution.
Multi-packs with no stated weight are the other awkward case. If a 12-pack does not list total weight, you can only compare per item, which is meaningful for identical products and misleading across different brands.
| Listed as | Convert to | Method |
|---|---|---|
| 500ml vs 1.5L | ml | Multiply litres by 1,000 |
| 750g vs 1.2kg | g | Multiply kg by 1,000 |
| 12 pack vs 500g | per item or per gram | Need the per-item weight to compare directly |
| Per roll vs per sheet | sheets | Multiply rolls by sheets per roll |
| Concentrate vs ready-to-use | prepared volume | Multiply concentrate by its dilution ratio |
When the lower unit price is the wrong choice
The honest version of the calculation uses the amount you will consume, not the amount in the pack. If you reliably use 200g of something a month, comparing a 250g and a 1kg pack on unit price alone assumes you will get through all 1kg before it expires.
- Perishables you will not finish. A 3kg bag at a better unit price is worse value if a third goes to waste; calculate on what you will actually use.
- Storage you do not have. Bulk only saves money if the items remain usable and findable.
- Quality differences. Unit price compares quantity, not what is in it, and a cheaper per-gram option may contain less of what you are buying it for.
- Cash flow. A larger outlay now for a lower unit price is still a larger outlay now.
- Shipping thresholds and delivery fees, which can reverse an online comparison entirely.
Doing it quickly in a shop
- Check the shelf label first. Many retailers are required to display unit price, though not always in the same unit across brands.
- If the units differ between two labels, convert one rather than eyeballing it.
- Round aggressively for a first pass. 6.49 for 250g is roughly 2.60 per 100g; that is enough to rank options.
- Only calculate precisely when two options are close, which is when the decision actually depends on the arithmetic.
- Re-check on promoted items. A multi-buy offer changes the unit price, and the shelf label often shows the pre-offer figure.
When the lowest unit price is misleading
- Assuming larger is always cheaper per unit. It usually is, and the exceptions are common enough to be worth two divisions.
- Comparing across different units without converting, especially grams against millilitres, which are not interchangeable.
- Using drained weight for one product and total weight for another on tinned goods.
- Forgetting that a multi-buy requires buying the full quantity. Three-for-two is not a discount if you only need one.
- Ignoring packaging that is included in the stated weight, which varies between brands and makes comparison less exact than it looks.
Assumptions and limitations
Unit price assumes the products are genuinely comparable. Two items at the same price per 100g may differ substantially in concentration, water content or ingredient quality, and the calculation cannot see any of that. It answers 'which is cheaper per unit', not 'which is better value'.
It assumes the quantity on the label is accurate and measured the same way. Net weight, drained weight and packaged weight are different conventions, and mixing them produces a comparison that looks precise and is not.
And it assumes full consumption. For anything perishable, the practical unit price is the total price divided by the quantity you will actually use, which can be very different from the figure on the shelf edge. That adjustment turns unit price from an arithmetic exercise into a genuinely useful one.
Unit price is only useful when the units are comparable
Before comparing two unit prices, convert both products to the same measure and make sure the package quantities are genuinely comparable. A lower price per kilogram does not settle the decision if one product has materially different quality, waste, usable yield or shelf life. Unit price is a cost comparison tool, not a complete value judgment.
Frequently asked questions
Do shops have to show unit prices?
Many jurisdictions require it for most packaged goods, though the rules and the required unit vary, and promoted or multi-buy items are sometimes handled inconsistently.
How do I compare a multi-buy offer?
Calculate the total cost of the required quantity and divide by the total amount received. Three items for the price of two on a 4.00 item is 8.00 for three, so 2.67 each.
Should I compare per item or per weight?
Per weight or volume whenever it is available, since item sizes differ between brands. Per item only works when the items are genuinely equivalent.
Does unit price work for services?
Yes, with the right unit — per hour, per user per month, per gigabyte. The same logic exposes which subscription tier is actually cheaper for the volume you use.
What if two products use different package sizes?
Convert both to the same unit, such as cost per 100 grams or cost per litre, before comparing.
Can the cheapest unit price be the worse deal?
Yes. Differences in usable quantity, quality, waste, durability or expiry can make a higher unit price more appropriate for the actual need.
Conclusion
Unit price is one division that removes most of the guesswork from size comparisons, and the assumption worth dropping is that the bigger pack is automatically better value. Convert everything to the same unit, do the division, and then apply the correction that matters most: divide by what you will actually use rather than what is in the package. That adjustment changes the answer more often than the arithmetic does.
Figures in this article are illustrative. Results depend on your own rates, fees, taxes and circumstances, and are for educational and planning purposes rather than financial advice.