How to Compare Unit Prices

What you need

The price and the quantity of each option, in the same unit. The label gives you both: the price on the shelf edge and the quantity on the pack. If the two options use different units — grams against kilograms, millilitres against litres — convert one of them first, or the comparison is meaningless.

Step 1 — Get the numbers

Write down the price and the quantity for each option. For materials, the quantity is the coverage or the volume: litres of paint, square feet of tile, metres of timber. Check that the quantity is what you think it is — packs shrink, and a "new larger pack" is sometimes the old pack at a new price.

Step 2 — Divide price by quantity

Unit price is the price divided by the quantity. The unit price calculator does it for two products at once and tells you which is cheaper and by how much. The same division works for materials: the price of a tin of paint divided by its litres is the price per litre, and the price of a box of tiles divided by its square feet is the price per square foot.

Step 3 — Compare like for like

Compare the same product at the same quality: the same paint, the same tile, the same timber grade. A cheaper unit price on a thinner tile or a weaker paint is not a saving — it is a different product. Watch for different pack sizes hiding different quantities, and for quality differences the label does not shout about.

Step 4 — Check the saving

Work out the saving as a percentage: the difference between the two unit prices, divided by the higher one. A 16% saving on something you buy once is a nice bonus; the same saving on something you buy every week is real money. For area and volume, convert the units first with the area converter or the volume converter, then compare the unit prices.

Worked example

Product A costs $12 for 500 g: $12 ÷ 500 = $0.0240 per g. Product B costs $20 for 1,000 g: $20 ÷ 1,000 = $0.0200 per g. Product B is cheaper, and the saving is |0.0240 − 0.0200| ÷ 0.0240 = 16.67%. Those are exactly the numbers the unit price calculator gives you. The prices are the calculator's example figures — use your own prices when you compare real products.

Common mistakes

• Comparing different units. Grams against kilograms, millilitres against litres — the numbers look comparable and they are not. Convert first.
• Assuming bigger is cheaper. Sometimes it is; often it is not. The unit price is the only test — the pack size is marketing.
• Ignoring quality. The cheapest per unit is not the best buy if it does not last. Compare like for like, then choose.

Safety

Comparing prices is safe work — there is no hazardous step in this guide. The only risk is buying the wrong thing: check the unit (grams, litres, square feet) matches before you compare, keep the receipt, and check the returns policy before you open a bulk buy.

Try these calculators

• Unit Price Calculator — unit price for two products, and the saving.
• Area Converter — convert square feet, metres, yards and acres.
• Volume Converter — convert cubic feet, yards, metres, litres and gallons.