Week 2 — The Unit Price

Is the larger pack actually the better buy? Unit price, usable quantity, and why "cheaper per kilo" can still be a worse purchase.

3 August 2026

Core question#

Is the larger or cheaper-looking package actually better value?

Week 1 showed that one price can hide several different numbers. This week does the same thing with quantity. Supermarkets already show unit price: the small print under the tag, usually price divided by weight or volume. That division helps, but it is not the whole story.

Usable quantity#

Unit price answers “which pack is cheaper per unit”. It does not answer “which pack is cheaper per unit I will actually use”. A 2 kg bag of spinach is only a better deal if most of it gets eaten before it turns to sludge in the fridge. The useful divisor is not the amount you bought; it is the amount you use.

Try it below with any two packs — the calculator separates “price per unit in the pack” from “price per unit you’ll actually use”, and the two numbers can disagree.

Bulk purchasing and shelf life#

Bulk discounts can be real, but they come with waste risk, and waste risk is not printed on the shelf label. A good bulk purchase needs an honest estimate of how much you will use before it spoils, expires, or stops being useful.

Lowest unit price is only useful when the quantity is actually used.

The $249 headphones#

The headphones don’t have a unit-price problem in the literal sense — you can’t buy 70% of a pair of headphones — but the same logic applies to bundles. A $249 pair sold alone and a $279 “bundle” with a carry case and a spare cable are only comparable once you price the case and cable at what they’re actually worth to you, not at whatever number makes the bundle look generous.

The $279 bundle is invented for this example, as is its contents. No real bundle is being described.

Learning outcomes#

By the end of this week, you should be able to:

• define unit price as total price divided by quantity
• distinguish “quantity in the pack” from “quantity you will actually use”
• compute both readings for a two-pack comparison using the calculator above
• explain why a bulk discount can still be the worse purchase
• connect this week’s usable-quantity idea to a bundle price, such as headphones sold with accessories

Before class#

Bring the price and size of two competing pack sizes of something you actually buy — two coffee bags, two boxes of the same cereal, two bottle sizes of the same shampoo. Real numbers, not estimates.

In class#

Using the pack sizes you brought, compute the unit price for both packs in the calculator, then estimate honestly how much of the larger pack you’d actually use before it spoiled, expired, or you lost interest. Does the “cheaper per unit” pack survive that second calculation?

What you should leave knowing#

Lowest price per unit only helps if the units get used. Your honest usage estimate, not the shelf sticker, is what turns a unit price into a real saving.

Where this goes#

Week 3 asks where the reference number on a discount tag comes from in the first place — the same reference-price question from Week 1, now with a name.