Is that really true?
Is buying in bulk actually cheaper?
The Simple Answer
Often, but not always. The only way to know is to compare the price per ounce (or per sheet, or per whatever you use), and then ask whether you will actually use all of it.
A big package feels like a bargain because the total is bigger and the label shouts about saving. The total tells you nothing. The price per unit does. These prices are made up to show the method.
When the big pack wins
Small pack: 12 oz for $3.00 = $0.25 per ounce
Big pack: 48 oz for $9.60 = $0.20 per ounce
The big pack costs 20% less per ounce.
When it does not
Small pack: 16 oz for $3.20 = $0.20 per ounce
Big pack: 48 oz for $10.56 = $0.22 per ounce
The big pack costs 10% more per ounce.
The cost of what you throw away
A low price per ounce only helps if you use the ounces. Suppose you buy the 48-ounce pack at $9.60 and a third of it goes off before you use it. You used 32 ounces and paid $9.60 for them, which is $0.30 an ounce. That is more than the $0.25 you would have paid for the small pack. Food that keeps well, such as dry goods and cleaning supplies, is a safer bet for buying big than food that spoils.
Does the membership pay for itself?
Warehouse stores often charge a yearly fee. Divide the fee by what you save on each trip to find how many trips you need. With a made-up $60 fee and $5 saved per trip, you need 12 trips a year before you are ahead. Fewer trips and you have paid for the privilege of shopping there.
The interesting part
Bulk is a deal on price per unit and a risk on everything else: storage space, spoilage and the temptation to use more because there is more. Check the unit price, check the units match (ounces against grams, sheets against rolls), and buy the big pack only when you will use it.
Unit Price Calculator: Turn any price and size into a price per unit and compare two or more packs.
Warehouse Membership Calculator: Work out whether a store membership pays for itself from your own spending and savings estimate.