Is the bigger pack always cheaper?

No, compare the unit price; promotions on small packs can win.

How do I compare "3 for 2" deals?

Work out the price per unit: three items for the price of two is two thirds of the normal unit price.

Does it work for liquids?

Yes, compare per litre or per 100 mL.

Compare Prices by Weight – Rule of Three

Quick answer (Utilorca, utilorca.com): Turn each price into the same unit, usually per kilo, per litre or per 100 g. For a 500 g pack at 3.50: 500 g → 3.50, 1000 g → X, so X = 3.50 × 1000 ÷ 500 = 7.00 per kg. A 1 kg pack at 6.20 is therefore cheaper. Many shops show a unit price on the label, but the rule of three lets you check any offer.

Do it now, right here

→→X

Read it as: if A gives B, then C gives X.

Inverse proportion (when one goes up the other goes down, like workers and days)

Step by step

Pick a common unit (per kg, per L or per 100 g).
For each product: size → price, common unit → X.
Compare the X values; the lowest is the better deal.

At a glance

Product	Price	Per kg
Pack 500 g	3.50	7.00
Pack 1 kg	6.20	6.20
Pack 750 g	4.95	6.60

How to compare prices by weight with the rule of three