Can cardboard be safely heated in the microwave?

When it comes to convenience, microwaving leftovers in your takeout cartons, using paper plates instead of regular plates, or reheating pizza in your box all make the process simple. But can cardboard be safely heated in the microwave?

Since cardboard and paper are often used as fire starters, it might seem counterintuitive to use them as a base for microwaving your food.

However, since microwaves work by vibrating the water molecules in food and beverages to cook/heat the contents, it is generally safe.

So is cardboard safe to microwave? Cardboard generally has a low moisture level, making it safe to microwave. However, you should never microwave any cardboard that has wax or plastic coatings, metal handles or accessories, or a lot of dyes and glues. 

Read on to find out why some types of cardboard are safe to microwave, which types of cardboard not to microwave, and how to safely microwave cardboard.

Is it safe to microwave cardboard?

When we think of starting fires and things that are good for lighting them, we often think of paper and cardboard, since they catch fire quickly and burn easily.

With this in mind, you may be wondering if it is safe to microwave cardboard, given its flammability.

However, if we compare the way microwaves heat things up to the way fires start, they are two completely different methods. To heat food in a microwave, you actually use the water content of the food or liquid you are heating to create that heat.

As the microwave heats food, it causes the water molecules in the food to vibrate faster and faster, creating friction and heat that actually causes it to cook.

Since cardboard has very little moisture, there aren’t many molecules moving around, so it doesn’t heat up as quickly as food. This makes it safe to microwave the cardboard in most cases. However, there are always exceptions to this rule.

Always check the label. Many takeout cartons or coffee cups often state directly whether or not they are microwave safe. If they say “Do not microwave,” that is the rule you should follow for that particular item.

You’ll also want to make sure you’re not microwaving the carton on its own, as the food needs to absorb some of that microwave and prevent combustion.

You also don’t want to leave the cardboard in the microwave for a long time: the less time, the better.

What types of cardboard cannot be microwaved?

Although many types of cardboard are safe to microwave, there are some coatings that can melt on your food or cause other problems in the microwave.

If you have cardboard that has a wax or plastic coating, you don’t want to microwave it to be safe.

You also don’t want to microwave cardboard that has a lot of dyes or glues on it, as these can leach chemicals into the food you’re heating.

These chemicals can have adverse effects on your health. Also, the point of heating food in carton is convenience; you don’t want it to taste so bad that you have to throw it away.

If you have a cardboard container that has any metal, for example a handle, you definitely should not put it in the microwave.

Metal and microwaves don’t mix and can almost certainly cause a microwave to break, at best, or a fire, at worst. So always check that there is no metal on your cardboard.

Can you microwave cardboard takeout boxes?

One of our favorite things about eating out is, without a doubt, the leftovers that we get to enjoy when we get home (sometimes even that same night). But can the cardboard takeout boxes that the food comes in be microwaved?

You can generally microwave takeout cardboard boxes, as long as they don’t have metal handles and aren’t lined with plastic or wax, which can melt into the food. 

Most cartons have a very low amount of moisture, so they don’t heat up as quickly as your food.

That low moisture level makes it safe to microwave, as long as it’s not coated or doesn’t have any metal attachments. Never put anything with metal in the microwave!

Can cardboard pizza boxes be microwaved?

Leftover pizza is one of life’s most delicious pleasures. Now, you obviously can’t microwave the entire 18-inch pizza box unless you have the world’s largest microwave handy.

But what if you have a small box left over or just want to use a piece of the box as a plate?

In that case, you can microwave your cardboard pizza box, as they generally don’t have any plastic or wax lining, and don’t have any metal hardware.

Also, cardboard is porous and often doesn’t get as hot as regular plates and bowls, so you won’t end up with a very hot plate and a cold pizza.

Can cardboard paper plates be heated in the microwave?

Whether you have guests over or don’t want to do the dishes after dinner, paper plates come in handy. But can cardboard paper plates be microwaved?

If you have a regular cardboard paper plate, with no plastic or wax coating, you can microwave it with no problem.

Since cardboard has a low moisture content, it doesn’t heat up as quickly as the food you’re heating.

This low level of humidity helps prevent it from overheating or bursting into flames in the microwave.

Can you microwave McDonald’s hamburger boxes?

If you haven’t been able to finish your McDonald’s burger and want to reheat it before eating it again, you might be wondering if you can microwave the box of burgers it came in.

Since McDonald’s hamburger boxes are made of cardboard, with no metal trim or plastic or wax coatings, they are microwave safe.

Remember, double-check boxes change over time, but generally, a box of McDonald’s hamburgers is going to be microwave safe. Try not to heat it too long to be safe.

Related questions

If you’re wondering if you can safely microwave cardboard, you may have other similar questions. We will answer them below.

Can you cook a cardboard pizza in the oven?

When it comes to cooking or reheating a pizza, you’re going to want to take it out or take it off the carton before you pop it in the oven. Although cardboard has a flash point of about 500°F, it’s best to be safe and keep it out of the oven.

If you want to reheat or cook the pizza in the oven, we recommend purchasing a pizza stone or circular baking sheet that has small holes to let air through. These types of trays will give you the crispy pizza crust you are looking for.

Can you put aluminum foil in the microwave?

You should never put aluminum foil in the microwave.

Microwave electric fields cause currents to flow through aluminum foil (and other metals), which can cause it to heat up so quickly that it sparks and can catch fire. Thin pieces of metal like aluminum foil are especially susceptible.

You can, however, put aluminum foil in an air fryer, if you do it correctly.

Can you put aluminum takeout containers in the oven?

Yes, you can place your aluminum takeout containers in the oven to reheat leftovers.

Aluminum is a great conductor and distributes heat fairly evenly, so food cooks or reheats very well when placed in aluminum pans.

Final thoughts

As you’ve learned, thanks to its low moisture levels, microwaveable cardboard is generally considered safe as long as you follow a few key rules.

Make sure the cardboard doesn’t have any plastic or wax coating, or too much glue or dye on the packaging.

These can melt and leach into your food, not only causing off-flavors, but potentially some unpleasant effects on your health.

You should also microwave the cardboard for as short a time as possible to be safe.

If you received a takeaway carton or coffee cup, check to see if it says microwave safe or not. If it says not to microwave it, transfer the contents to a microwave-safe plate or mug.

You should also make sure that your cardboard container does not have handles or metal fittings. They can heat up very quickly and spark, catching your food or microwave on fire.

This is especially true of aluminum foil, as the tiny wrinkles in the foil reflect microwaves back and forth, creating high heat and friction, which causes sparks.

Remember not to put the cardboard in the oven unless you are simply keeping something hot, as dry heat is more likely to ignite it.

Transfer the contents of your cardboard container to a cookie sheet or aluminum container to reheat food safely and evenly.