Unit price calculator: compare up to four pack sizes by price per kg, litre, 100 g, ounce or item to see which is cheapest and by how much.
By AdminDrafted with AI writing toolsUpdated Checked examples: 5
Item 2 is cheapest at 5.99 per kg, 14.18% less than the most expensive option.
| Item | Pack | Price | Per kg | Dearer by |
|---|---|---|---|---|
| 1 | 500 g | $3.49 | 6.98 | 16.53 % |
| 2 | 1 kg | $5.99 | 5.99 | 0 % |
The result is a price per kg. 500 g = 0.5 kg.
The result is a price per kg. 1 kg = 1 kg.
In the formula, P_i is a pack price, Q_i its quantity expressed in the comparison basis, u_i its unit price, and S the percentage saving against the highest unit price.
Unit price is the price divided by the size, after every pack is converted to the same unit: per kg or litre, per 100 g or 100 mL, per ounce or fluid ounce, or per item for goods sold by count. For the same product, the pack with the lowest unit price is the cheapest, and the saving is measured against the dearest pack.
With the defaults, 500 g for 3.49 costs 6.98 per kg and 1 kg for 5.99 costs 5.99 per kg, so the larger pack is 14.18% cheaper per kilogram. Packs can be entered in different units: grams, kilograms, ounces and pounds are converted with 1 oz = 28.349523125 g exactly.
Only packs sold by the same kind of measure can be compared, since weight cannot be turned into volume without the product's density. A lower unit price saves money only if you use the larger pack before it spoils.
Divide each price by its quantity, with every pack in the same unit. A 1.5 L bottle for 2.40 costs 2.40 ÷ 1.5 = 1.60 per liter; a 500 mL bottle for 0.75 costs 0.75 ÷ 0.5 = 1.50. The small bottle wins: three of them hold 1.5 L for 2.25.
Put every pack in the same unit before dividing. These factors are exact:
Take 16 oz for 4.50 against 500 g for 3.20. Sixteen ounces is 453.59237 g, so the first pack costs 4.50 ÷ 0.45359237 = 9.92 per kg. The second costs 3.20 ÷ 0.5 = 6.40 per kg, which is 35.49% cheaper.
The saving divides the gap by the higher unit price. With 500 g for 3.49 (6.98 per kg) against 1 kg for 5.99 (5.99 per kg), the 1 kg pack saves (6.98 − 5.99) ÷ 6.98 = 14.18%.
Divide the same gap by the lower price and you get the premium: the 500 g pack costs (6.98 − 5.99) ÷ 5.99 = 16.53% more per kilogram. Both figures are right; they answer different questions.
Use the total quantity the price buys. Six 250 mL cans for 3.00 hold 1,500 mL, so they cost 2.00 per liter. Dividing the six-pack price by one can's 250 mL gives 12.00 per liter, six times too high.
Price a multibuy as the whole deal. Three 400 g packs at 2.70 each, sold as 3 for 2, are 1.2 kg for 5.40: 4.50 per kg instead of 6.75.
| Scenario | Item 1 unit price | Item 2 unit price | Saving, % |
|---|---|---|---|
| Default | 6.980000 | 5.990000 | 14.183381 |
| Ounce basis | 0.197880 | 0.169814 | 14.183381 |
| Mixed mass packs | 9.920802 | 6.400000 | 35.489085 |
| Egg counts | 0.250000 | 0.230000 | 8.000000 |
| Equal liquids | 2.000000 | 2.000000 | 0.000000 |
Default is the 500 g against 1 kg pair above, priced per kg, and ounce basis prices the same pair per ounce. Mixed mass packs is the 16 oz against 500 g example, per kg.
Egg counts is 12 for 3.00 against 30 for 6.90, per egg. Equal liquids is 1 L for 2.00 against 500 mL for 1.00, a tie at 2.00 per liter. The first two rows show that changing the unit rescales both prices but leaves the saving at 14.18%.
The ACCC's unit-pricing guidance explains how a standard measure helps Australian shoppers compare similar products.
In the US, the Uniform Unit Pricing Regulation in NIST Handbook 130 is a model regulation that states can adopt. Where it applies, stores that show unit prices must use the same unit for every size of a product.
Use the discount calculator to work out a sale price before comparing packs, and the ounces to grams converter for a single size conversion. The recipe scaler shows how much of an ingredient a bigger batch needs.
Checked against: 3.49/0.5 = 6.98 per kg; (6.98 − 5.99)/6.98 = 14.18%
Checked against: With 1 oz = 28.349523125 g (NIST): 3.49/(500/28.349523125)
Checked against: 4.50/(16 × 0.028349523125 kg) = 9.92080…
Checked against: Hand calculation: 3/12 = 0.25, 6.90/30 = 0.23; 0.02/0.25 = 8%
Divide the price by the quantity in the unit you want to compare. A 500 g pack at 3.49 is 3.49 ÷ 0.5 = 6.98 per kg, or 3.49 ÷ 5 = 0.698 per 100 g. Convert every pack to the same unit first; setting a price per ounce against a price per 100 g gives the wrong answer.
No. Promotions on small sizes and multipacks can reverse the usual bulk discount, so check the unit price each time. In the eggs example, 30 for 6.90 is 0.23 an egg against 0.25 for a dozen at 3.00, an 8% saving; a half-price offer on the dozen would make it the better buy at 0.125 an egg.
In the US, the Uniform Unit Pricing Regulation in NIST Handbook 130, which states can adopt, requires stores that show unit prices to use one unit for every size of a product: per kilogram, 100 g, pound or ounce for goods sold by weight; per litre, 100 mL, gallon, quart, pint or fluid ounce for liquids; per item for counted goods. EU Directive 98/6/EC sets the unit price, including VAT, per kilogram, litre, metre, square metre or cubic metre.
28.349523125 grams exactly, because the avoirdupois ounce is 1/16 of the international pound of 0.45359237 kg; NIST SP 811 lists the factor. A US fluid ounce measures volume, 29.5735295625 mL, so a price per ounce and a price per fluid ounce can only be compared if you know the product's density.
This calculator includes 5 worked examples with independently sourced answers. They run in the test suite, and you can run them here too.