Unit Price Comparator

Which pack size is really cheaper per unit, once the surplus you will never use is taken into account.

The larger pack almost always has the lower price per gram, and that is not the same thing as being cheaper for you. This calculator compares up to three pack sizes on price per unit, then asks how much you actually need and how much of the surplus you will genuinely use. If the leftover is going to sit in a cupboard, the effective price of the material you use is the whole pack price spread over the amount you needed, and the big pack becomes the expensive choice.

At a glance

What you enter: Size of pack A, Price of pack A, Size of pack B, Price of pack B, Size of pack C, if there is one, Price of pack C, Material the project needs, Share of the surplus you will really use
What you get: Pack A price per unit, Pack B price per unit, Pack C price per unit, Effective price per unit from A, Effective price per unit from B, Effective price per unit from C, Cheapest pack on the shelf price, Cheapest pack on the effective price, The two comparisons disagree, Saving per unit between the best and worst option, Saving as a share of the most expensive option, Surplus of A you must use for A to win, Surplus of B you must use for B to win, Cost of buying A, Cost of buying B, Cost of buying C
Formula shown: Yes, every step, right below your results.
Cost and privacy: Free, no account needed, and nothing you type is stored.

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Size of pack A units

Grams, metres, sheets, millilitres. Whatever unit the two sizes share.

Price of pack A

Size of pack B units

Price of pack B

Size of pack C, if there is one units

Leave at zero to compare only two packs.

Price of pack C

Material the project needs units

In the same unit as the pack sizes. This is what turns a shelf price into an effective price.

Share of the surplus you will really use %

Be honest. A surplus of a material you use every month is stock; a surplus of resin or adhesive is waste.

Reset

Results are estimates for general information only, always verify with a professional or official source before acting on them.

How to use the Unit Price Comparator

1 Enter Size of pack A (units), Grams, metres, sheets, millilitres. Whatever unit the two sizes share.
2 Enter Price of pack A
3 Enter Size of pack B (units)
4 Enter Price of pack B
5 Enter Size of pack C, if there is one (units), Leave at zero to compare only two packs.
6 Enter Price of pack C
7 Enter Material the project needs (units), In the same unit as the pack sizes. This is what turns a shelf price into an effective price.
8 Enter Share of the surplus you will really use (%), Be honest. A surplus of a material you use every month is stock; a surplus of resin or adhesive is waste.
9 Press Calculate. You get Pack A price per unit, Pack B price per unit, Pack C price per unit, Effective price per unit from A, Effective price per unit from B, Effective price per unit from C, Cheapest pack on the shelf price, Cheapest pack on the effective price, The two comparisons disagree, Saving per unit between the best and worst option, Saving as a share of the most expensive option, Surplus of A you must use for A to win, Surplus of B you must use for B to win, Cost of buying A, Cost of buying B, Cost of buying C, and every intermediate step stays visible under "How is this calculated?"
10 Change one number at a time to see what moves the result, that is usually the most useful thing a calculator can tell you.

Worked examples

Real numbers run through this calculator. Open any example to load it into the form above and change it.

Five hundred grams against two hundred

A project that needs 200 grams, with no realistic use for the surplus.

You enter

Size of pack A: 500 units
Price of pack A: $12.00
Size of pack B: 200 units
Price of pack B: $5.50
Size of pack C, if there is one: 0 units
Price of pack C: $0.00
Material the project needs: 200 units
Share of the surplus you will really use: 0%

You get

Pack A price per unit: $0.02
Pack B price per unit: $0.03
Pack C price per unit: $0.00
Effective price per unit from A: $0.06
Effective price per unit from B: $0.03
Effective price per unit from C: $0.00
Cheapest pack on the shelf price: 1
Cheapest pack on the effective price: 2
The two comparisons disagree: 1
Saving per unit between the best and worst option: $0.03
Saving as a share of the most expensive option: 54.2%
Surplus of A you must use for A to win: 90.3%
Surplus of B you must use for B to win: 0.0%
Cost of buying A: $12.00
Cost of buying B: $5.50
Cost of buying C: $0.00

Pack A is cheaper per gram at 0.024 against 0.0275, but with the surplus unused the 200 grams you use costs the whole 12.00, which is 0.06 a gram: more than twice the effective price of pack B. The break-even is 90 percent of the 300 gram surplus.

Open this example in the calculator →

A big pack you will actually get through

A project that needs 900 grams, with the surplus kept for the next run.

You enter

Size of pack A: 500 units
Price of pack A: $12.00
Size of pack B: 200 units
Price of pack B: $5.50
Size of pack C, if there is one: 0 units
Price of pack C: $0.00
Material the project needs: 900 units
Share of the surplus you will really use: 100%

You get

Pack A price per unit: $0.02
Pack B price per unit: $0.03
Pack C price per unit: $0.00
Effective price per unit from A: $0.02
Effective price per unit from B: $0.03
Effective price per unit from C: $0.00
Cheapest pack on the shelf price: 1
Cheapest pack on the effective price: 1
The two comparisons disagree: 0
Saving per unit between the best and worst option: $0.00
Saving as a share of the most expensive option: 12.7%
Surplus of A you must use for A to win: -31.3%
Surplus of B you must use for B to win: 214.5%
Cost of buying A: $24.00
Cost of buying B: $27.50
Cost of buying C: $0.00

Two packs of A cost 24.00 and leave 100 grams, all of which is kept, so the effective price is 0.024 a gram. Five packs of B cost 27.50 and also leave 100 grams, so 0.0275 a gram: the larger pack wins by about 13 percent, and the break-even surplus figure is negative because it wins with none of the leftover used.

Open this example in the calculator →

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Frequently asked questions

Is the bigger pack always cheaper?

It is almost always cheaper per unit and it is often more expensive for the work it does. Five hundred grams at 12.00 is cheaper per gram than two hundred grams at 5.50, but if the project needs 200 grams and the rest of the big pack is never used, the 200 grams you used cost you 12.00, which is 6 pence per gram against 2.75 pence. The big pack wins only if you get through enough of the surplus.

How do I compare two pack sizes properly?

Divide each price by its size to get the price per unit, then divide the total cost of the purchase by the amount your project actually needs. The first figure tells you which pack is better value on the shelf and the second tells you which purchase is better value for this project. When they disagree, the surplus reuse figure is what decides it, so be honest about whether the leftover will be used.

How much surplus is worth keeping?

The break-even percentage is the answer for the packs you are comparing, and it is often uncomfortably high. In the example above the larger pack needs about 90 percent of the surplus to be used before it beats the smaller one, which means the leftover has to be almost entirely consumed by this project and the next. If the break-even figure comes out above about 70 percent, buy the smaller pack unless you have a specific plan for the rest.

What if I need more than one pack?

Then the surplus is only the rounding remainder, which is usually small, and the shelf unit price is close to the effective price. Two packs of 500 grams for a project needing 900 grams leaves 100 grams of surplus, so the effective price is within about 11 percent of the unit price. This is the case where buying the large pack is straightforwardly correct, and it is worth noticing that it is a different situation from needing 200 grams of a 500 gram pack.