You know that feeling in Costco. The giant cart. The pallets of stuff. Everything seems like a deal because it is big. But bigger is not always cheaper. Sometimes the per-unit price in bulk is actually higher than the regular size at a grocery store. And even when it is cheaper, you have not saved money if half of it spoils before you use it.

I sat down with my last three Costco receipts and a calculator. Some things were great deals. Some things I had been wasting money on for years.

Check the unit price, not the sticker price

The price tag tells you the total. The unit price — dollars per ounce, per pound, per sheet — tells you the actual value. Most warehouse clubs list the unit price on the shelf tag in tiny print. Compare it to the unit price of the same product at your regular grocery store. I found that canned beans were cheaper per can at Aldi than in a Costco flat. Paper towels were cheaper at Costco by a wide margin. It depends entirely on the product.

What to always buy in bulk

• Paper products. Toilet paper, paper towels, tissues. They never expire and the per-unit savings are real — sometimes thirty to forty percent.
• Cleaning supplies. Laundry detergent, dish soap, all-purpose cleaner. Same logic. Does not spoil. Significant savings.
• Rice, pasta, dried beans. These last for years in sealed containers. The savings per pound are not huge — maybe twenty percent — but over a year of regular cooking it adds up.
• Frozen vegetables and fruit. Cheaper per pound than fresh, do not spoil, and are often flash-frozen at peak ripeness. Better nutrition than out-of-season fresh produce that traveled two thousand miles.

What to never buy in bulk

• Cooking oils and nuts. They go rancid. A gallon of olive oil seems like a deal until it tastes like crayons six months later.
• Spices. Ground spices lose potency after about six months. A giant jar of paprika you use once a month is just expensive red dust by the time you finish it.
• Fresh produce unless you have a plan and freezer space. A five-pound bag of avocados all ripening at the same time is a race you will lose.
• Anything you are buying for the first time. Do not commit to a year’s supply of a product you have never tried. Buy the small version first.

I once bought a two-pack of giant olive oil bottles because the per-ounce price was incredible. I use olive oil maybe twice a week. The second bottle sat in the pantry for eight months. When I finally opened it, it smelled like old play-dough. I had saved four dollars per bottle but thrown away an entire bottle. The math did not work.