A Costco membership can save you money.

It can also convince you that you desperately need 48 granola bars, three pounds of cheese and enough paper towels to survive until 2030.

That’s the trick with warehouse clubs.

Buying in bulk isn’t automatically a bargain.

Sometimes the giant package really does offer a much better price. Other times, the grocery store sale down the street beats it — or you end up throwing away half of what you bought.

So what should you actually be looking for?

While prices vary by location and change over time, these nine categories are often worth comparing at Costco.

1. Gasoline

For many Costco members, the savings can start before they even walk into the store.

Costco gas is often competitively priced compared with nearby stations.

A difference of just 10 cents per gallon saves $1.50 on a 15-gallon fill-up.

That’s not enormous.

But if you’re filling up regularly throughout the year, those small differences add up.

There is one catch:

Don’t drive 20 minutes out of your way or sit in a long line just to save a dollar or two.

The best gas bargain is usually the one that’s already convenient.

2. Over-the-Counter Medications

This is one aisle where comparing unit prices can really pay off.

Common products such as:

• Pain relievers
• Allergy medicine
• Antacids
• Vitamins
• Cold remedies

may come in significantly larger quantities at Costco.

But don’t buy a 500-count bottle just because each pill is cheaper.

Check the expiration date and ask yourself whether you’ll realistically use it.

A bargain you eventually throw away isn’t a bargain.

3. Prescription Eyeglasses and Contact Lenses

If you wear glasses or contacts, Costco Optical is worth comparing with your current provider.

Frames, lenses and contacts can represent a significant expense, particularly when more than one person in your household needs them.

Your exact cost will depend on your prescription, lens options, insurance and location, so comparison shopping still matters.

But don’t automatically assume your eye doctor’s office is the only place to purchase your glasses.

Get the prescription.

Then compare the total price.

4. Rotisserie Chicken

There’s a reason Costco’s rotisserie chicken has such a following.

It can provide the foundation for several inexpensive meals without requiring you to cook an entire chicken yourself.

Dinner tonight.

Chicken sandwiches tomorrow.

Chicken salad for lunch.

Chicken tacos.

Soup.

The key is actually using it.

If you’re a smaller household, remove the remaining meat after your first meal and freeze portions you won’t eat within the next few days.

One inexpensive chicken can stretch surprisingly far when nothing goes to waste.

5. Cheese

Cheese can be a strong Costco buy, especially for households that use it regularly.

Cheddar, Parmesan, mozzarella and specialty cheeses are often sold in much larger packages than you’d find at a traditional grocery store.

Again, look at the price per pound, not just the price on the package.

A $12 block of cheese may seem expensive until you realize the smaller grocery-store package costs considerably more per pound.

If the Costco package is too large, some cheeses can also be portioned appropriately for later use.

The worst strategy?

Buying a giant package because it’s cheaper per ounce and then discovering a science experiment in the back of your refrigerator three weeks later.

6. Nuts

Walk through the nut aisle at a regular supermarket and you’ll quickly discover how expensive a small bag can be.

Almonds, walnuts, pecans, pistachios and mixed nuts are exactly the kind of products where larger packages can offer better value.

But nuts contain oils that can eventually turn rancid.

If you won’t finish a large bag quickly, consider storing them properly in the refrigerator or freezer.

That allows you to take advantage of a bulk price without rushing to eat three pounds of almonds.

7. Paper Products

Toilet paper and paper towels are classic warehouse-club purchases for a reason.

They don’t spoil.

You’re almost certainly going to use them eventually.

And larger packages can offer competitive per-unit pricing.

The important word is can.

Grocery stores, Target and other retailers regularly run sales, so Costco isn’t guaranteed to win every comparison.

Check the number of rolls, sheets per roll and total square footage when possible.

Two packages that look almost identical can contain very different amounts of paper.

And make sure you have somewhere to put it all.

Saving $5 isn’t especially useful if a year’s supply of toilet paper has to live in your dining room.

8. Batteries

Batteries are another product that can make sense in bulk because many households use them throughout the year.

Remote controls.

Flashlights.

Smoke detectors.

Toys.

Clocks.

Holiday decorations.

Costco’s larger packages can reduce the per-battery cost compared with buying a small pack every time something dies.

But don’t buy more than you’ll reasonably use.

Store batteries properly, keep them away from children, and check manufacturer guidance regarding shelf life.

9. Kirkland Signature Products

Sometimes the best Costco bargain isn’t simply buying more.

It’s buying Costco’s store brand.

Kirkland Signature covers everything from food and household products to clothing and personal-care items.

Store brands frequently cost less than comparable national brands, and switching even a handful of products you buy regularly can make a noticeable difference over a year.

But don’t buy based on the Kirkland name alone.

Compare:

Price ÷ quantity = unit price

Then compare that with the product you normally buy.

That’s how you find the real bargain.

The Costco Trap to Avoid

Here’s where Costco can get expensive.

You walk in for toilet paper and chicken.

You leave with a patio umbrella, new sweatpants, a 40-pack of cookies and something called an “executive massage chair.”

Total: $684.

The biggest danger at Costco isn’t necessarily overpaying for an individual product.

It’s buying things you weren’t planning to buy at all.

A $15 product you didn’t need isn’t a $5 savings.

It’s $15 you wouldn’t have spent otherwise.

Try shopping with a list and giving yourself a rule:

If it isn’t on the list, wait until the next trip.

If you still want it then, reconsider it.

One Number Matters More Than the Package Price

When comparing Costco with a grocery store, Walmart, Target or another retailer, don’t focus solely on the price printed in large type.

Look for the unit price.

One store might sell something for $9.99 and another for $15.99.

The $9.99 version looks cheaper — until you realize you’re getting twice as much product for $15.99.

Compare ounces to ounces, pounds to pounds or individual items to individual items.

That’s particularly important at Costco because the package sizes are so different.

Costco Isn’t Cheaper If You Throw Half of It Away

For retirees and smaller households, this may be the most important Costco rule of all.

A family of five can go through a huge package of strawberries pretty quickly.

A household of one or two may not.

That’s why the best bulk purchases tend to fall into one of three categories:

You use them frequently.

They have a long shelf life.

They can be frozen or stored safely for later.

If none of those applies, the supermarket’s smaller package may ultimately cost you less — even when its unit price is higher.

Costco can absolutely help stretch a household budget.

Just remember:

The biggest package isn’t always the biggest bargain.