Although there are markings in form of symbols to tell us if a container is microwave safe or not, I have some plastic containers which don't have any symbols but are microwave safe. Is there any method or rule of thumb to find if a material is safe to put in the microwave?

  • How did you determine that the unmarked containers are microwave safe?
    – Stephie
    Commented Apr 5, 2019 at 16:38
  • They were very cheap containers, which I usually get when I order food in bulk quantity. I have a lot of them so it didn't bother me to take a risk and try heating food in one of those.
    – baba
    Commented Apr 5, 2019 at 17:19
  • I was just wondering, because you wrote “are microwave safe”.
    – Stephie
    Commented Apr 5, 2019 at 17:21

3 Answers 3


The standard method when I was young, and microwave ovens were new to the consumer market (i.e. had only recently started costing less than a used car), the standard test of any cookware was to fill a glass with water, and put the glass and the piece in question in the oven for a minute. If the water got hot, and the cookware didn't, it was microwave safe. If the cookware got hot, it wasn't microwave safe.

Later, there was a gradation -- some cookware would get hot in the microwave, either intentionally (to provide browning, make popcorn pop, etc.), or otherwise, but was still safe to use, providing the cook knew it would get hot. Many ceramics are in this category; they absorb some energy and get hot, but aren't damaged by it.

Generally, if you have an item that is likely to be damaged by excessive heat (like most plastics), you'll want to check it using the glass of water method (be careful, the water will be hot, whether the item being checked is or not). If it's not prone to heat damage, like ceramic, you can just proceed to cook with it, being cautious of the potentially hot surface.

It is worth noting here (thanks for the prompt, @EdGrimm) that many plastic storage containers may soften in the microwave even if they test as "safe" on the basis of direct heating -- and even if they're safe with boiling water, as microwaves can superheat steam or water well above boiling. Beyond that, there is considerable evidence that heating any food or drink, by any method, in a plastic container of any sort is inadvisable due to the presence of chemicals like bis-phenyl A (BPA) and other plasticizers or manufacturing aids.

  • Microwave popcorn poppers have a metal plate molded into the base of the container which provides the heat to pop the corn kernels.
    – Stan
    Commented Apr 5, 2019 at 19:57
  • This feels like it's missing the step where you test if the container that didn't get hot from the microwave will get damaged from the heat of containing food that does get hot in the microwave. This is the test where most cheap plastic containers fail for me. A lot of cheap plastic starts getting soft not long after 100 degrees Celsius, and while conventional cooking won't raise the water temperature past boiling while it's still liquid, microwaving super-heat steam before it leaves the container.
    – Ed Grimm
    Commented Apr 26, 2019 at 2:54
  • @EdGrimm Back in the day, it was assumed that plastic wasn't microwave safe. Any plastic. This test was for glass and ceramic (the latter of which might or might not be safe, depending on the type of clay and the glaze used).
    – Zeiss Ikon
    Commented Apr 26, 2019 at 11:02
  • That was then, this is now. We know better. I feel it would be good if there was an answer that actually addressed the point, but I feel like your answer is close enough that I'd rather not just put up my small piece. It felt like reading your answer that you understand the situation, but aren't spelling it out, and you'd likely fix your answer if I pointed out this point also should be addressed.
    – Ed Grimm
    Commented Apr 26, 2019 at 11:40
  • I'll edit to make this clearer. For that matter, with today's knowledge, I probably wouldn't suggest using any thermoplastic container in the microwave due to BPA and similar contaminants/unlabeled chemicals.
    – Zeiss Ikon
    Commented Apr 26, 2019 at 11:46

Polypropylene plastic (abbreviated as PP) and designated as a type "5" (shown embossed in the container mold as a "5" inside a recycle triangle) is classified as a microwave oven "safe" plastic.


Very simple...After 10 seconds in a microwave oven if it melts or deforms in any way try another test victim. If it is hot to the touch do not use.

A suitable container needs to be transparent to radio frequency energy. if it is relatively cool to the touch that would work. Some plastics have impurities that could leach into your food as well as causing hot spots that could cause a fire or arcing inside the oven (especially if the impurity is metallic in nature).

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