Price Per Unit Calculator
The bigger pack is not always cheaper per unit, and shops rely on people assuming it is. This is the only reliable check.
Option B is cheaper by
7.9%
- Option A per 100
- $0.70
- Option B per 100
- $0.64
- Option A per unit
- 0.0070
- Option B per unit
- 0.0064
- Option B buys this much more
- 1.09x per unit of money
- Option B works out at $0.64 per 100 against $0.70 — 7.9% less.
- The bigger pack is not always cheaper per unit, and shops rely on people assuming it is. Comparing the unit price is the only reliable check.
- Use the same unit for both — grams against grams, or millilitres against millilitres. Comparing a price per kilogram against one per pound is how this goes wrong.
- Cheaper per unit is only cheaper if you use it. On anything perishable, the large pack you throw half of away is the expensive one.
About comparing unit prices
A 500g pack at $3.50 works out at $0.70 per 100g. A 900g pack at $5.80 is $0.64 per 100g — about 8% cheaper. That is the answer, and it is not visible from the shelf prices without doing the division.
Sometimes the larger pack is dearer per unit. It happens often enough to be worth checking every time rather than assuming, particularly on promotions where a multibuy on the small size beats the family pack. Shops know that shoppers assume bigger is better value, and pricing follows what shoppers assume.
Per 100 units is the sensible basis for comparison, which is why shelf labels use it. A price per single gram or millilitre is fractions of a penny and rounds to nothing useful.
Two things will break the comparison. Using different units for the two options — a price per kilogram against one per pound — gives a meaningless answer, so convert first. And cheaper per unit is only cheaper if you use it: on anything perishable, the large pack you throw half of away is the expensive one.
What it works out
- Price per unit and per 100 units for two options
- Which is cheaper and by what percentage
- Any unit — grams, millilitres, sheets, items
The formula
Unit price = Price ÷ Size Compare the two
$3.50 divided by 500 is $0.007 a gram — $0.70 per 100g. $5.80 divided by 900 is $0.00644 a gram, or $0.64 per 100g.
Option B is cheaper by 7.9%. Put another way, the same money buys 1.09 times as much.
Per 100 is the useful basis because a per-gram price is fractions of a penny. That is exactly why shelf labels quote per 100g or per 100ml — a number you can compare at a glance, which is the whole purpose of unit pricing regulation.
The failure mode is unit mismatch. A price per kilogram compared against one per pound produces a number that looks like an answer and is not. Convert both to the same unit before dividing, every time.
- Price
- What the pack costs, after any discount you would actually get.
- Size
- The quantity in the pack. Both options must use the same unit.
A worked example
A 500g pack at $3.50 against a 900g pack at $5.80.
That works out to 7.9 %.
- Option A per 100
- $0.70
- Option B per 100
- $0.64
- Option A per unit
- 0.0070
- Option B per unit
- 0.0064
- Option B buys this much more
- 1.09x per unit of money
Questions
How do I compare two pack sizes?
Divide each price by its size to get a price per unit, then compare. A 500g pack at $3.50 is $0.70 per 100g; a 900g at $5.80 is $0.64 — about 8% cheaper.
Is the bigger pack always cheaper per unit?
No, and it is worth checking every time. Promotions on smaller sizes frequently beat the family pack, and pricing follows the assumption that shoppers will not check.
Why do shelf labels use price per 100g?
Because per gram is fractions of a penny and useless for comparison. Per 100 gives a number you can read at a glance, which is what unit pricing rules are for.
What if the units are different?
Convert first. Comparing a price per kilogram with one per pound gives a number that looks like an answer and is not. The unit converter handles it if you need it.
Should I always buy the cheapest per unit?
Only if you will use it. On anything perishable, the bulk pack you throw half away is the expensive one. Storage space and the cash tied up count too.
How do I compare items with different unit types?
You cannot meaningfully — a price per sheet and a price per gram are not comparable. Pick the unit that reflects what you actually use, and compare on that for both.