Why the shelf price alone can mislead you
A 500 g bag for $4.50 and a 900 g bag for $7.20 cannot be compared by looking at the totals, because you are not buying the same amount. The fix is to divide each price by the quantity you actually receive and express both on one scale, such as price per 100 g, per 100 ml or per item. That number is theunit price. This calculator works it out for two products at once, accepts metric and US units in the same comparison, understands multipacks, and adds a delivery fee when there is one.
The formula
Unit price = (price + delivery fee) ÷ total quantity × reference amount
The total quantity is the size of one item multiplied by the pack count, converted to a base unit (grams for weight, millilitres for volume, or plain items). The reference amount is 100 for weight and volume and 1 for items. A pack of six 12 fl oz cans therefore has a total of 72 fl oz, or about 2,129 ml, and its price is divided by that total, not by 12.
The calculator compares weight with weight, volume with volume and count with count. It refuses to compare a weight against a volume, because 1 g and 1 ml are the same only for water; a gram of honey and a gram of olive oil occupy different volumes. If one label lists grams and the other millilitres, look for a density or a servings figure instead of guessing.
Conversion factors the calculator uses
| Unit | Equals |
|---|---|
| 1 kg | 1,000 g |
| 1 lb | 453.59237 g |
| 1 oz | 28.349523125 g (1 lb = 16 oz) |
| 1 L | 1,000 ml |
| 1 US fl oz | 29.5735295625 ml |
| 1 UK (imperial) fl oz | 28.4130625 ml |
Fluid ounces deserve a warning: a US fluid ounce is 29.5735 ml, while a UK imperial fluid ounce is 28.4131 ml, about 4% smaller. The calculator uses the US value because that is what American labels print. For British products, convert the label to millilitres yourself, which is usually printed beside the ounces. Also remember that "oz" on a food label means weight (ounces avoirdupois) while "fl oz" means volume, and the two are not interchangeable. For other conversions, use the unit converter.
Worked examples
All prices below are made-up numbers in US dollars to illustrate the arithmetic, not real products.
Example 1: two sizes of the same item. Bag A is 500 g for $4.50 and bag B is 900 g for $7.20. Per 100 g, A costs $0.90 and B costs $0.80. B is about 11.1% cheaper per gram, and A costs about 12.5% more than B. Both percentages are correct; they just use different baselines, which is why the calculator reports the first and this page explains the second.
Example 2: multipack versus a large bottle. A pack of 6 cans of 12 fl oz each costs $6.60. The total is 72 fl oz, which is 2,129.3 ml, so the pack costs $0.310 per 100 ml. A 2 L bottle at $2.10 costs $0.105 per 100 ml. In this made-up case the cans cost about 195% more per millilitre than the bottle, even though a multipack looks like the thrifty choice.
Example 3: delivery fee included. Shop A sells a 1 kg item for $18.00 plus $4.99 delivery. Shop B sells the same 1 kg for $21.50 with free delivery. The listed prices favor A, but A really costs $22.99, or $2.299 per 100 g, against $2.150 for B. Free delivery wins by $0.149 per 100 g.
Example 4: pounds against kilograms. 2 lb for $9.98 is 907.2 g, so $1.100 per 100 g. One kilogram at $11.00 is $1.100 per 100 g. The two are essentially tied, and you could not see that without converting.
Shrinkflation: same price, less inside
When a manufacturer keeps the price but reduces the contents, the effective price rises byold quantity ÷ new quantity − 1. A 10% smaller package is an 11.1% price increase, not 10%, because you now pay the same money for less.
| Contents reduced by | Old 100 becomes | Effective price increase |
|---|---|---|
| 5% | 95 | about 5.3% |
| 10% | 90 | about 11.1% |
| 15% | 85 | about 17.6% |
| 20% | 80 | about 25.0% |
| 30% | 70 | about 42.9% |
For example, a snack that stays at $4.00 while going from 16 oz to 14 oz moves from $0.2500 to $0.2857 per ounce, an effective increase of 14.3%. Entering the old and the new size as Products A and B shows the change immediately. If you buy something every week, glance at the net contents on the label whenever the package looks different.
Details that change the answer
- Drained weight. For canned or jarred food packed in liquid, compare the drained weight when it is printed, because you are paying for the solids.
- Concentration and servings. Detergent, juice concentrate and supplements differ in strength. Compare cost per load, per serving or per capsule by entering the number of uses as "items".
- Sheets, not rolls. Paper products vary in sheet size and ply. Compare the total sheet count or total area if the label gives it.
- Coupons and points. Enter the price you will actually pay at checkout. Instant discounts belong in the price; points or cash back that arrive later usually do not.
- Taxes. If one shop shows tax-inclusive prices and another adds tax at the register, convert both to the same basis first. The sales tax calculator can help.
- Different reference units. Shelf labels may print price per ounce, per 100 g or per pound. Compare only numbers on the same basis.
When the lower unit price still loses
- You will not finish it. A large container that spoils half-full costs more per portion you actually eat than the small one.
- Storage. Freezer or pantry space is a real constraint, and bulk buying that forces you to clear space or buy a new container has a cost.
- Quality differences. A different grade, fat content or ingredient list means the products are not equivalent, so the unit price compares the wrong things.
- Convenience packaging. Individually wrapped portions cost more per gram but may prevent waste, and that can be a good trade.
Common mistakes and myths
- "Bigger is always cheaper." Often true, but not reliably; the premium for a large size is sometimes negative. Calculate rather than assume.
- "Buy one get one free is always half price." It is half price per item only if you want two, and only if the regular price is fair. Enter the total paid and the total quantity to check.
- Comparing different bases. A price per 100 g of 0.90 and a price per kg of 8.00 are not directly comparable by eye. 8.00 per kg is 0.80 per 100 g.
- Rounding too early. Compare unit prices to at least three decimals when the numbers are small, or the difference may vanish.
- Ignoring pack count. Typing 12 in the size box when the label says 6 items of 12 fl oz gives a result off by a factor of six. Use the pack count field.
How to use the result
The percentage in the verdict is how much cheaper the winner is per unit, which tells you whether the difference is worth acting on. A 2% gap on a one-off purchase is rarely worth a longer trip; the same 2% on something you buy every week is worth a thought. Use the result to pick a default brand or pack size for staples, then re-check when prices change or the package changes. For discount percentages, try the discount calculator, and for turning gaps into percentages, the percentage calculator.
Limits of this calculator
It does only arithmetic on the numbers you type. It does not look up live prices, check expiry dates, judge quality, or know whether a label rounds its contents. Everything runs in your browser and nothing you enter is sent anywhere. Comparing two products at a time keeps the inputs short; for three or more, compare the current best against each challenger in turn.
Frequently Asked Questions
Can I compare products labeled in different units?
Yes, as long as they measure the same kind of thing. You can mix g, kg, oz and lb for weight, and ml, L and US fl oz for volume. Weight and volume cannot be compared directly, and the calculator will say so instead of guessing.
How do I enter a multipack?
Put the size of one item in "Size of one item" and the number of items in "Pack count". Six 12 fl oz cans are entered as 12, fl oz, 6. For things counted rather than measured, such as 30 rolls, choose "items" with a size of 1 and a pack count of 30.
Where do coupons and loyalty points go?
The calculator uses exactly the price you type. Enter the price after instant coupons. Rewards that come back later, such as points or cash back, are better left out so that the comparison reflects what leaves your wallet today.
What currency should I use?
Any, as long as both products use the same one. The tool only divides, so dollars, euros or pounds all work. The result is shown as a number per 100 g, per 100 ml or per item, in whichever currency you entered.
How can I tell whether a package got smaller?
Compare the net contents printed on the label with an old receipt, photo or a previous package. Enter the old size and price as Product A and the new one as Product B; the unit price difference is the real change in cost.
Related tools
- Unit converter - convert lengths, weights and volumes before you compare
- Discount calculator - the price after a percentage off
- Percentage calculator - percentage change and difference
- Sales tax calculator - add or remove tax from a price