Unit Price Comparator

Why the shelf price lies and unit price tells the truth

Supermarkets rarely make it easy to tell which package is the better deal. A "family size" box can cost less per gram than the small one — or more — and the only honest way to know is to reduce every option to the same yardstick: price per unit. That single number strips away package size, brand framing, and "bigger looks cheaper" instinct, leaving a figure you can rank. This comparator does exactly that math for you, normalising mismatched units so a 500 g jar and a 1 lb tin sit in the same column.

How the normalisation works

Every unit you pick is converted to one of three base units before anything is compared: weight to grams, volume to millilitres, and count to each. The conversion factors are exact: 1 kg = 1000 g, 1 lb ≈ 453.592 g, 1 oz ≈ 28.3495 g, 1 L = 1000 mL, and 1 fl oz ≈ 29.5735 mL. The tool then computes price ÷ (quantity × factor) to get the cost of a single base unit, and displays it scaled to a friendly figure: per 100 g for weight, per 100 mL for volume, and per item for count. Rows sort automatically from cheapest to most expensive, and the lowest gets a green "Best" badge.

One deliberate limit: the tool only compares like base types. Weight is never pitted against volume, because a gram of one product and a millilitre of another aren't interchangeable. If you mix a weight item and a volume item in the same list, each is still costed correctly, but ranking them against each other would be meaningless — so judge them within their own type.

A worked example

The tool opens with two real options. Brand A is $3.49 for 500 g; Brand B is $5.99 for 1 kg. Convert both to per-100 g:

• Brand A: 3.49 ÷ (500 × 1) = $0.00698 per gram → $0.698 per 100 g.
• Brand B: 5.99 ÷ (1 × 1000) = $0.00599 per gram → $0.599 per 100 g.

Brand B wins by about 14% per gram, even though its sticker price is higher. The bigger pack is genuinely cheaper here — but notice that this isn't always true. Swap Brand B's price to $7.49 and the per-100 g figure jumps to $0.749, making the smaller Brand A the better buy. The whole point is that you can't eyeball it; the arithmetic decides.

Practical use cases

• Bulk versus standard. Warehouse-club multipacks are usually cheaper per unit, but not always — verify before assuming.
• Brand versus store label. Drop both in and see how big the premium really is per 100 g, not per package.
• Sales math. A "2 for $5" promotion is just $2.50 each; enter it as one item at $5 for a quantity of 2 to compare it fairly against a single unit on regular price.
• Concentrates and refills. A concentrated detergent at a higher price per mL can still cost less per wash — but you'll only know once it's normalised.

Tips that make comparisons fair

• Enter the unit on the label, not a rounded one. If a tin says 411 g, type 411, not 400 — small rounding skews close calls.
• Use the right base type. A "6-pack" is a count item; the individual can size (in mL) is a volume item. Decide which question you're answering: cost per can, or cost per mL of drink.
• Watch for "as prepared" sizes. Concentrates and dried goods list a small package weight but yield far more once mixed; compare the prepared quantity if you can.
• Save recurring lists. Name and store a comparison to localStorage with the Save List button, then reload it on your next trip instead of re-entering everything.

Common mistakes

• Comparing total price instead of unit price. The whole reason this tool exists is to stop you doing exactly that.
• Mixing weight and volume. "$2 per 100 g" and "$2 per 100 mL" are not the same value, even though the numbers look identical.
• Ignoring how much you'll actually use. The cheapest per-unit option is a false economy if half of it spoils or sits unused. Best value per gram only matters for what you consume.
• Forgetting non-price factors. Quality, freshness, and storage space have real value that a per-unit figure can't capture — let the number inform the decision, not replace it.

Privacy

All conversion and ranking happens in your browser. Saved lists live only in your device's localStorage — nothing is uploaded, so your shopping habits stay on your machine.