Price Per Unit Calculator

Price per unit is how two pack sizes get compared on equal terms. A 500 g box of cereal at 3.50 costs 0.70 per 100 g. A 1 kg box at 6.00 costs 0.60 per 100 g, which is 14% cheaper per unit even though the box costs 2.50 more. That is the whole calculation: total price divided by quantity, then scaled to a round number of units so the answer is readable.

The idea is old enough to be written into law. In the United States the model regulation that states adopt, published by NIST in Handbook 130 as the Uniform Unit Pricing Regulation, sets out how retailers display price per unit of measure. The Fair Packaging and Labeling rules require the net quantity of contents on every consumer package, and that quantity is the number this calculator divides by. Shelf labels do not always show the unit price clearly, and when the quantity on the pack changes while the price does not, the label is where the change hides. This calculator does the division for any unit: grams, litres or pieces.

Two outputs come back. The price per single unit is the right figure when the unit is large, such as a litre or a piece. The price per 100 units is the right figure when the unit is small: a per-gram price rounds to nothing useful, while a per-100 g price reads like a shelf label. A 5 litre bottle at 18 is 3.60 per litre and 360 per 100 litres, and both read cleanly; for a 1 kg bag at 6.00, the per-gram figure rounds to 0.01 and the per-100 g figure of 0.60 is the one to read.

Quick example

The defaults are a total price of 18 for a quantity of 5 litres, which gives 3.60 per litre and 360 per 100 litres. Switching the unit to grams or pieces leaves both figures where they are; only the wording of the label moves.

Which inputs matter most

Only two numbers drive the result: total price and quantity. Doubling the price doubles the unit cost, doubling the quantity halves it, and the unit type changes the label but not the arithmetic. The practical trap is the quantity field, which has to be in the same unit for both products being compared: a 1.5 litre bottle against a 330 ml can is 1,500 against 330 in millilitres, or 1.5 against 0.33 in litres, never one of each.

What's happening under the hood

Price per unit equals total price divided by quantity. Price per 100 units is that figure multiplied by 100. Nothing else is applied: no tax, no discount, and no rounding until the number is displayed.

Why run the calculation

Because pack sizes are not designed to be compared. A 340 g jar at 2.89 and a 500 g jar at 3.99 look like a small price difference for a lot more product; per 100 g they are 0.85 and 0.80, so the larger jar is cheaper, but by 6%, not the 47% more product the shelf suggests. Multipacks are the same problem the other way: three 330 ml cans at 4.50 are 0.45 per 100 ml, and one 1.5 litre bottle at 5.20 is 0.35 per 100 ml. And when a 200 g pack at 2.00 becomes a 180 g pack at 2.00, the price has not moved but the unit price has risen 11%, which is the change the shelf never announces.

Worked example

Imagine comparing two laundry detergent bottles:

• Brand A: 2 litres at 8.00, which is 4.00 per litre, or 400 per 100 litres
• Brand B: 5 litres at 18.50, which is 3.70 per litre, or 370 per 100 litres

Brand B costs 0.30 less per litre, 7.5% less. Over several refills the difference accumulates, and the calculator surfaces it without the mental division. Whether it is a saving in practice depends on whether all 5 litres get used, which is the point a Cornell study on measure-based versus usage-based unit pricing makes: a price per litre is not the same as a price per wash, and for concentrated products the two can rank differently.

Common scenarios

Unit pricing matters when:

• Shelf labels are missing, faded or quote different units for neighbouring products
• An own-brand and a branded product come in different pack sizes
• A larger pack is on offer and the question is whether it is cheaper per unit or just bigger
• A product's pack size has shrunk while its price has stayed put
• A multipack is set against a single larger container

What the result does and does not capture

It captures: the arithmetic relationship between total outlay and quantity, expressed per single unit and per 100 units.

It does not capture: quality, brand, shelf life, storage space, delivery costs or payment terms. Two products with the same unit price can differ a great deal in how long they last or how well they work, and a cheaper unit price on something that spoils before it is used is not a saving at all. The figure is a price-per-quantity metric for direct numerical comparison and nothing more.

Educational illustration

The outputs are for educational and planning purposes. Purchasing decisions rest on more than unit price, and the result here is one input into that judgement rather than a standalone signal.