₩/g · Unit Price
Measure by price per unit, not by price — comparison only becomes valid the moment units match.
$6.00 for 500g, $15.60 for 1.2kg — is the big pack the bargain?
What are we trying to understand?
'Bulk is economical' is an intuition planted by marketing. Real shelves are full of 'inverted' products where the big pack costs more per gram — grab without computing and you lose.
The one rule of comparison is matching units. $6.00 and $15.60 can't be compared; 1.2¢/g and 1.3¢/g can, instantly. One division disarms the trick.
Add the time unit (subscriptions) and the waste rate (bulk you throw away) and you get the true unit cost — half of consumer judgment is unit-price sense.
Change one assumption at a time
The result follows the inputs and assumptions. Check the units and model before treating it as an actual outcome.
Predict first — nuts at $6.00 for 500g vs $15.60 for 1.2kg. Which is the better deal?
Duel 1 — compute the unit prices
Call the winner by gut, then reveal the per-gram (mL) prices.
Nuts 500g
$6.00
Nuts 1.2kg
$15.60
Read the calculation another way
The bigger pack is always cheaper per unit.
It flips constantly. Retailers know shoppers believe this, and sometimes premium-price the bulk size precisely because of it. There is no 'always' — only the division tells the truth.
The lowest unit price is always the best deal.
If you toss part of it, the waste rate inflates your effective cost. Throw away 20% of the 2.5L detergent and its per-mL price effectively rose 25%. The true unit cost divides by what you USE.
Why the calculation works, and when
The tool used in the price duel, formalized.
Unit price
Price per gram (mL, item, month). Comparison becomes valid only when denominators match.
The inversion test
Comparing two fractions — this inequality, not the sticker price, names the winner.
Effective unit cost (with waste)
w is the waste rate. Toss 20% and the denominator shrinks to 0.8× — the cost jumps 1.25×. Bulk's hidden fee.
Try the idea in another setting
Shelf tag unit prices
Most supermarkets print price-per-100g (or per-oz) in small type under the big price. Reading the small number replaces the calculator.
Subscription plans
$120/year vs $12/month — per month that's $10 vs $12. But if you'll only use it 6 months, annual loses. Watch the unit AND the horizon.
The bundle threshold
'Buy 2 get 1 free' cuts the unit price 33% — if you truly use all three. Once your waste rate passes 33%, the deal flips into a loss.
Fuel and electricity
Price per liter/gallon, cents per kWh — energy has always traded at unit prices. Unit pricing is consumption's native language.
Now explain it in your own words
Follow the mathematics behind the calculation
Related lab
Ratios
Unit price is the price:quantity ratio normalized to 1 — ratio sense in field conditions.
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You've read ratios between currencies; ratios between products are easier.
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You've measured value for one; now split money fairly among many — another ratio problem.
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