4 things you should always buy in bulk—and 2 you shouldn't

There are lots of ways to cut spending: buying store-brand skin products, renting clothes you're only going to wear once, and opting for frozen produce, for example. One of the less intuitive ways to save money, though, is to buy more.

Buying in bulk can save you money and time in the long run, but only if you purchase the right products. Here are four items to always buy in bulk, and two not to, according to experts.

Socks, underwear, and plain T-shirts are usually a better deal when you buy them in bulk, consumer-savings expert Andrea Woroch says. On Amazon Prime, five Fruit of the Loom white T-shirts cost $14.98 (about $2.99 per shirt), but 12 cost $28 ($2.33 per shirt).

More from Grow:
How to get Costco savings without buying the $60 membership
How much money you'd have now if you invested $500 in the Dow 10 years ago
What to buy from Costco for your holiday parties 

"Having these items in bulk can also make packing for a long trip much easier," says Kristin McGrath, a shopping expert at Offers.com. "If you already have two weeks' worth of socks, you won't have to run to the store and buy them last minute."

"If there's something that comes up on your grocery list every single week, consider buying it in bulk to save yourself money and shopping time in the long run," McGrath says. This is especially true for nonperishables like canned vegetables or pasta.

At Walmart, one can of Del Monte canned corn is $1.14, but if you buy an 8-pack for $7.18, the price of one can comes down to 0.89 cents. Pasta, which also has a long shelf life, is sold in bulk at Walmart. An 8-pack of Barilla penne and spaghetti will cost .07 cents per ounce, while one box of thin spaghetti is .08 cents per ounce.

"Because these items have no real expiration date, you can take as long as you need to finish them," McGrath says. "Assuming you have enough storage space, that is." At Target, a 6-pack of Charmin Ultra Soft Toilet Paper Mega rolls is $6.99, or $1.17 per roll. But if you buy the 24-pack, it's only $1 per roll. That's a 15% discount per roll.

The average American uses 141 rolls of toilet paper per year, according to a 2018 data from Statista. At $1.17 per roll, 141 rolls would set you back approximately $165, almost $25 more than if you were to only pay $1 per roll.

During the first three months of their life, a baby will go through 700 diapers, according to baby registry site Babylist. Woroch suggests stocking up.

A 198-pack of size-one Pampers for babies between 8 and 14 pounds is $47.52. That's 0.24 cents per diaper. But if you get the 32-count, you'll pay about 0.28 cents per diaper and you'll be running to the store more frequently.

Not everything is smart to buy in large quantities, though. Here are a couple of products experts say you shouldn't buy in bulk, even if the savings seem tempting.

"Buying food and condiments in bulk can be prudent if you're throwing a big party or event, but be careful about buying it for your home fridge or freezer for everyday use," McGrath says.

Mustard is good for about one year after opening and ketchup is good for two — their high vinegar content extends their shelf life. But mayonnaise only keeps for two-to-three months after its "best buy" date, and barbecue sauce is good for only four months after being opened.

If your family doesn't frequently use a condiment, a huge bottle will just take up space in your fridge and, ultimately, have to be thrown out.

Medicine has a long shelf life and expiration dates are generally conservative, but the longer after the expiration date you take a drug, the less effective it might be. And if a drug's effectiveness is degraded by 10%, it's unlikely to harm or help you, pharmacist Mike Fossler told NPR.

"In all likelihood, your family will only need to use cough and cold syrup or pills a few times a year and will barely make a dent in that huge bulk bottle before it expires," McGrath says.

If it's important to you that a drug is 100% potent, avoid buying it in bulk. "Save by opting for the generic version in a smaller container," Woroch says. "I usually buy the medium-size container to get a better price per unit but not overbuy."

The article 4 Products to Always Buy in Bulk—and 2 Not To originally appeared on Grow by Acorns + CNBC.