Unit price: how to tell if the bigger pack is really cheaper
The bigger pack is cheaper only if its price per unit is lower, and the price on the shelf does not tell you that. Divide the price by the amount in the same unit each time, compare the results, and you will know which pack gives more for your money. Shrinkflation, where the pack shrinks but the price stays the same, is the most common reason the answer changes without warning.
Unit priceWhich pack is cheaper?OpenPrice per unit, step by step
Unit price is the price divided by the quantity, usually expressed per kilogram, per litre or per item. The steps are the same every time:
- Write down the price and the quantity on the label, in the same units.
- Convert the quantity to a common unit, such as grams or millilitres.
- Divide the price by the quantity, then scale to one kilogram, one litre or one item.
- Compare the results, not the prices.
A worked example. Pack A is 500 g for $2.50. Pack B is 400 g for $2.40.
- Pack A: $2.50 ÷ 500 g = $0.005 per gram, which is $5.00 per kilogram.
- Pack B: $2.40 ÷ 400 g = $0.006 per gram, which is $6.00 per kilogram.
Pack B is cheaper to buy, but Pack A is the better value per kilogram, by a dollar. The shelf price suggests the opposite.
Conversions you will use
Labels mix metric and imperial units, so you need a few conversions. These are the common values:
| From | To | Multiply by |
|---|---|---|
| 1 ounce (oz) | grams | 28.35 |
| 1 pound (lb) | grams | 453.6 |
| 1 US fluid ounce | millilitres | 29.57 |
| 1 US gallon | litres | 3.785 |
| 1 UK gallon | litres | 4.546 |
A US fluid ounce is not the same as an imperial one, so check which system the product uses. A 12 US fluid ounce can holds about 355 millilitres, since 12 × 29.57 = 354.8. A pound is 453.6 grams, so a 1 lb bag is a little under half a kilogram, and the price per kilogram is that price divided by 0.4536. The unit converter does these conversions for any pair of units, including US and UK gallons.
When the bigger pack is not cheaper
Bigger packs are often cheaper per unit, but not always. These are the usual reasons they are not:
- Shrinkflation. The price stays the same, but the contents fall. A pack that was 500 g and is now 450 g at the same price costs 2.50 ÷ 0.45 = $5.56 per kilogram, instead of $5.00. That is a unit price rise of 11.1%, even though the shelf price has not moved.
- Promotions. A "buy two, get one free" offer can make the larger pack cheaper, or not, depending on the unit price of the standard pack.
- Different types. A concentrate, a refill or a multipack may have a different unit than the single item, so compare like with like.
- Waste. A large pack you cannot finish may cost more than a smaller one, once the spoiled part is counted.
Shrinkflation is a change in the pack, not a price increase, so it is easy to miss. Keep a note of the size of a product you buy regularly, and check the unit price when the packaging changes.
Percentage changes in unit price
To describe a change in unit price, use percent change: (new unit price − old unit price) ÷ old unit price × 100. In the example above, the change from $5.00 to $5.56 per kilogram is (5.56 − 5.00) ÷ 5.00 × 100, which is about 11%. The percentage calculator gives the same result if you enter the two unit prices.
Matching units and rounding
Compare unit prices only when the units match. A price per 100 g and a price per kilogram differ by a factor of ten, so convert before you compare. A price per item and a price per kilogram do not compare at all, unless you know the weight of each item. Round the unit price to the cent for the comparison, but keep more decimal places during the calculation, so that rounding does not decide the winner.
Steps that work at the shelf
- Read the total quantity on the label, not just the number of items.
- Work out the price per kilogram or litre, or per item if the items are all the same.
- Compare the two unit prices, and note any offer that changes the price.
- Check the net weight, since packaging can be heavy or light for its size.
- Record the figure if you buy this product often, so that you notice changes later.
Edge cases
- Drained weight. Canned food and jars often show a drained weight as well as a net weight. Use the one that matches what you are comparing.
- Liquid and solid. A litre of liquid and a kilogram of solid do not compare unless you know the density. Compare within a type of product.
- Loyalty prices. A member price may change the answer. Compare the price you actually pay.
- Price per item with differing sizes. Two packs of eight may hold items of different sizes. Compare by weight or volume, not by the count.
Quick answers
What is the quickest way to compare two pack sizes?
Divide each price by its quantity in the same unit, such as grams or millilitres, and compare the two results. The lower one is the better value.
What is shrinkflation?
It is when a product gets smaller while the price stays the same, or rises only a little. The unit price goes up, even though the shelf price does not change.
How many grams are in an ounce?
An ounce is 28.35 grams, and a pound is 453.6 grams. Use these to convert imperial labels before comparing with metric ones.
