Cost per piece is the standard way to compare pack sizes and it is only half of the calculation. The other half is what proportion you will actually use.

The Arithmetic Everyone Does

A hundred at nine pence each is cheaper than fifty at twelve pence each. That is correct and it is the comparison most listings invite.

It assumes you use all hundred, which is where it breaks down.

The Arithmetic That Matters

Divide the pack price by the number you will actually consume before the item is discarded, outgrown, degraded or forgotten.

A hundred-pack at nine pounds, of which you use sixty, cost fifteen pence per used piece. The fifty-pack at six pounds, fully used, cost twelve.

The larger pack was cheaper per piece and more expensive per piece used.

Where the Larger Pack Genuinely Wins

• Anything with a high and predictable consumption rate
• Items that do not degrade, so leftovers keep indefinitely
• Situations where running out has a real cost
• Anything where reordering is a nuisance
• Where storage is genuinely free rather than notionally free

Catalogues that print both the pack price and the per-piece figure, of the kind at juvale.net, make the first comparison easy. The second one only you can do, because it depends on your consumption.

Delivery Changes the Sums

A cheaper small pack ordered four times may cost more in total than one large pack delivered once, and it certainly costs more in time.

Where shipping is charged per order, that is a straightforward addition. Where it is free above a threshold, buying to the threshold is frequently cheaper than buying below it.

Storage Has a Cost

Rarely quantified and genuinely real, particularly in a small home or a shared office.

A cupboard shelf occupied for a year by supplies is a shelf not used for something else. That does not make bulk buying wrong; it makes it a trade rather than a pure saving.

The Change-of-Circumstances Risk

The factor that catches people out. A five-year supply assumes five years of the same habits, the same equipment and the same household.

Appliances get replaced, children grow out of things, businesses change what they sell. Anything tied to a specific setup carries that risk, and shorter horizons are safer.

A Reasonable Rule

Buy the quantity you will use within a year for consumables, and within the expected life of the associated equipment for anything specific to it.

That captures most of the unit price saving without most of the risk.

Track and Correct

Noting when a pack is opened and finished gives a real consumption rate after two cycles.

Every order after that is a calculation rather than a guess, which is worth more than any comparison of unit prices.