How to Compare Unit Prices: Per Kg, Per Litre, Per Piece

The price on a pack tells you what the pack costs. It does not tell you whether it is good value. For that you need the unit price: what you pay per kilogram, per litre or per piece. It is the only fair way to compare a 400 g jar with a 750 g jar, or a six-pack with a single bottle.

The basic formula

Unit price is the price divided by the quantity, expressed in a base unit:

• per kg for things sold by weight: rice, cheese, coffee, washing powder,
• per litre for liquids: milk, oil, juice, detergent,
• per piece for things counted: eggs, nappies, rolls of kitchen paper, tea bags.

Convert grams to kilograms and millilitres to litres first. A 250 g pack is 0.25 kg; a 750 ml bottle is 0.75 l.

A worked example: three pack sizes

Say you need coffee and the shelf has three options:

Pack	Price	Calculation	Per kg
250 g	$4.49	4.49 ÷ 0.25	$17.96
500 g	$8.29	8.29 ÷ 0.5	$16.58
1 kg	$17.49	17.49 ÷ 1	$17.49

The 500 g pack is the cheapest per kilogram. The 1 kg pack, which most people would assume is the best deal, costs more per kilogram than the 500 g one. This is common: larger packs are often cheaper per unit, but not always, and the only way to know is to do the division.

Liquids: watch the bottle size

Bottles make this harder because sizes are close and easy to misread. Compare olive oil:

• 750 ml for €8.99 → 8.99 ÷ 0.75 = €11.99 per litre
• 1 l for €11.49 → €11.49 per litre
• 500 ml for €6.29 → 6.29 ÷ 0.5 = €12.58 per litre

The litre bottle wins here, by about 50 cents per litre over the 750 ml one. On a product you buy every month, that is a few euros a year from a single habit.

Per piece: count what you actually use

For items sold by count, divide by the number of pieces. Eggs:

• 6 eggs for $2.70 → $0.45 per egg
• 10 eggs for $4.20 → $0.42 per egg
• 30 eggs for $13.20 → $0.44 per egg

Again, the middle pack wins. For per-piece products, also check the piece is the same thing. Kitchen rolls with more sheets per roll, or tea bags with less tea in each, are not comparable just by count.

Multi-buy offers

“Buy 2 for $5” or “3 for 2” offers change the unit price only if you actually buy the full set and use all of it. Work out the effective unit price:

• Yoghurt 500 g at $2.80, or 2 for $5.00.
• Single: 2.80 ÷ 0.5 = $5.60 per kg.
• Pair: 5.00 ÷ 1.0 = $5.00 per kg.

That is a real saving of about 11 % per kilogram, as long as the second pot does not go off before you eat it. If half of it ends up in the bin, the single pot was cheaper.

On receipts, these offers often appear as a separate discount or campaign line rather than a lower price on the item line. That means the price printed next to the product is the shelf price, and the price you really paid is lower. We cover how to account for that in shelf price vs paid price.

Pitfalls to watch for

• Different units on the shelf label. One tag shows per 100 g, the next per kg. Convert before you compare.
• Drained weight. For tinned food, the net weight includes the liquid. If you only use the solid part, compare drained weight where it is printed.
• Concentration. A concentrated detergent may wash twice as many loads per litre. Here the useful unit is per wash, not per litre.
• Pack size changes. If the pack shrinks and the price stays the same, the unit price has risen. See shrinkflation.

Unit price over time

Comparing unit prices in the aisle helps you pick a pack today. Tracking the unit price of the same product over months tells you something different: whether it is getting more expensive, even when the pack price looks steady. That is the case for keeping your own price history.

How Tasarruf helps

Tasarruf reads the quantities and prices from your receipt photos and normalises them to per kg, per litre or per piece for you, so you can compare packs without a calculator. It keeps the shelf unit price and the paid unit price separately, once discounts are spread over the lines, and shows how a product’s unit price moved over time, including when a pack got smaller at the same price. See how Tasarruf works.