Can You Heat Up a Thermos In A Microwave

‘Thermos’ is a household name, known for its array of insulated food and beverage containers. For over a century, the thermos LLC has developed countless food and beverage receptacles, including the world’s first stainless steel vacuum bottle. Other thermos products like vacuum flasks, containers, jugs, mugs, food flasks, lunch bags, etc are suitably designed, and extensively used by individuals of every age group.

Every day, the question: “can you heat up a thermos in the microwave” is being asked; on the internet, in the minds of people skimming through the user’s manual, and definitely on yours now. This is because, though a thermos can keep beverage, soup or any other content warm for 7 hours and more, the need to always have your nutriment – hot, whenever you are ready to gulp it down can’t be overlooked.


Keeping a hot food warm, and a frozen one cold, are a few of the things a thermos is popular for. However, with time & temperature change, the need to heat up a thermos, and its content could arise, usually to defrost frozen meals, or make previously hot edibles warm again. If a thermos must be heated up, then the first place to look would be the microwave.


Despite the popular opinion that putting metal in the microwave would lead to incessant sparks, fire or even an explosion, the thermos can be safely placed in a microwave till the content heats up. However, certain measures must be observed.


The thermos is always made of a metal case, unfortunately, microwaves can’t pass through metals; so to heat up the thermos and it’s content in the microwave, the following must be done:

  • Always open the lid of your thermos to warm it’s interior and content up. This is to allow the microwaves get in through the opening.
  • Endeavor to start the heat up process early because though a microwave can heat up a thermos to 35°C or more, it takes a lot of time.
  • Always endeavor to gently jiggle the the thermos occasionally to allow the warmth which is always concentrated at the top spread to other parts of the container. The thermos should be first taken out of the microwave before jiggling.


As mentioned earlier, the popular opinion is that metals shouldn’t be taken anywhere close to a microwave. There have been records of explosions and fires. However, the thermos is largely an exception because of the following factors:

  • THE THICKNESS OF THE METAL: the thickness of the metal coating of every thermos container is one of the reasons why it could be placed in a microwave without much concern. When you heat up a metal in the microwave, it reflects the waves, or absorb it. Some metals are too thin, hence they readily heat up, and start a fire in the microwave; however, the thermos is made up of a thicker metal which hardly heats up enough to warm up it’s interiors, let alone, catch fire.
  • THE SMOOTH AND PLUMP SURFACE: this is another reason why heating up a thermos in a microwave is relatively safe. Metals with bumps and sharp edges can easily cause sparks, however, all thermos receptacles have one thing in common; that is a smooth and plump surface. This surface design reduces the chances of sparks, flames and an explosion.


While it’s true that you can heat up a thermos in a microwave, the defects are quite enormous ( which is why some experts advise that you find an alternative way if heating up your thermos, besides using the microwave). Some of these shortcomings include:

  • INEFFICIENCY: the degree of inefficiency is the first thing to consider before putting a thermos in a microwave. It is very inefficient to place a thermos in a microwave, and in most cases, the length of the container is longer than the appliance.
  • SLOW HEATING PROCESS: compared to microwave suitable containers, the heating process if the thermos is slow. It takes 3-5 times more than plastics ceramics and other suitable containers to heat up. Also, using a thermos only allows microwaves to enter from the opened top, and not from every part of the container, making the heat up process less even.
  • IT REQUIRES CONSTANT ATTENTION: though the thermos metal is thick and plump, it is however, also a metal. Heating up a thermos in the microwave requires constant attention, to ensure that their is nothing like a rough edge to trigger sparks, and in a situation where it is already set off, the microwave should be turned off immediately. Again, because the heat is only concentrated at the top, their is need for constant jiggling. This is inconvenient, requires constant attention, and can’t always be kept up with.
  • MICROWAVE DAMAGE: anything that goes wrong in the course of heating up a thermos in a microwave could damage the microwave and the thermos as well. The metallic surface also weakens the microwave transmitters, which in turn, enfeebled the microwave.


Similar to plastic, glass & ceramic containers, the thermos can be heated up in the microwave. Nonetheless, it will require extra patience, attention, and a readiness to turn off your microwave at any sight of danger because though the metal is uniquely shaped and designed, it is still metal.


i) Are all thermos made of metal? Although some thermos beverage bottles, water jugs food jars, etc are made of plastic cases, thermos containers are generally made of metal.

ii) WHAT IS THE BEST WAY TO HEAT UP A THERMOS ?: you can easily heat up your empty thermos by filling it with boiled water. First, pour in the water cover the thermos lid and leave for some minutes, then pour the water out to get a warm thermos. This is only used to heat up an empty thermos.

iii) ARE THERMOS FLASKS MICROWAVEABLE ?: This is the crux of this article. It is not an ideal microwave container however, with extra attention, you can microwave

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