Written by Lara Yehia. Lara researches home and kitchen products for Cupindy. · Published October 2026 · Last reviewed October 3, 2026
You flip the bowl over, squint at the bottom, and find a few wavy lines, a tiny number in a triangle, maybe a glass and a fork. Some of those marks are about the microwave. Most are not. This guide shows each one as a picture you can hold your dish up against, explains what "reheat only" really means, and tells you what to do when there is no mark at all.
How we put this together: we have not run lab tests. We read the microwave guidance from the US Department of Agriculture (USDA) and the Food and Drug Administration (FDA), the EU food-contact law, and the wording makers print on their own containers. We drew the symbols ourselves from the designs most commonly used, so they are clean enough to compare against. Sources are listed at the end.
The short answer
The microwave-safe symbol is a set of wavy lines: on their own, inside a little microwave, or above a dish. Some makers simply print the words "microwave safe". The same picture with a line or an X through it means keep it out of the microwave.
There is no single official design in the US. The mark is the maker's promise about that item, so a missing mark is not proof of danger either. It just means nobody has promised you anything.
The microwave-safe symbols, with pictures
These are the six marks you are most likely to find on a plate, bowl, mug or container. The first four all make the same claim.
Wavy lines
Microwave safe. The most common mark on plastic containers.
A microwave with waves
Microwave safe. Common on lids and boxes.
A dish with waves
Microwave safe. Often on ceramic and glass.
The words
"Microwave safe" or "microwavable". Same promise, in plain text.
Reheat only
Fine for warming cooked food for a short time. Not for cooking.
Crossed out
Not microwave safe. On dishes it is usually the same color as the rest of the mark.
Does the number of wavy lines mean anything? Not that we could find. Two, three or four lines make the same claim. Makers draw the symbol their own way, which is why yours may not match any picture exactly.
Can't see a mark? On plastic it is often molded into the bottom in the same color as the container. Tilt it under a bright light, or run a fingertip over the base. On ceramic it is usually printed near the maker's name.
What "reheat only" means
"Microwave safe, reheat only" is the maker telling you two things at once. The container can take a short trip through the microwave to warm food that is already cooked. It was not built to cook in.
Reheat-only containers are fine for: warming leftovers, soup or a packed lunch in short bursts, with the lid loosened or the vent open.
Keep them out of the microwave for: cooking raw food, long runs, boiling water, popcorn, and very oily or sugary food. Oil and sugar can heat well past the boiling point of water, and that is when plastic warps or scorches.
You will see this wording on real products. Rubbermaid's Brilliance glass containers, for example, are described by the maker as "microwave-safe for reheating only with lids unlatched". The glass base and the plastic lid have different limits, and the lid is the reason for the "reheat only".
So, does "reheat only" mean microwave safe? Yes, within those limits. If you want to cook in the microwave, or reheat something greasy for several minutes, move it into plain glass or ceramic first. Our guide to reheating food properly covers times and temperatures.
Symbols that look similar but are not microwave ratings
These sit right next to the wavy lines on many containers, and people often read them as microwave marks. None of them tells you anything about the microwave.
Glass and fork
Made to touch food. It comes from EU law and says nothing about heat.
Number in a triangle
Which plastic it is. #5 is polypropylene (PP). Not a microwave promise.
Plates and water drops
Dishwasher safe. Says nothing about the microwave.
Snowflake
Freezer safe.
An oven
Oven safe, a separate promise. Often comes with a top temperature.
The glass and fork is the symbol EU law allows on materials meant for food contact (Regulation (EC) No 1935/2004, Annex II). It means the item was made to touch food. The same regulation defines no microwave symbol at all.
The number in a triangle is a resin code. It names the plastic so it can be sorted for recycling. Newer items show it in a plain triangle, older ones inside chasing arrows. #5 (PP) is the plastic most often used for microwave containers, but the number alone is not the promise. A thin takeout tub can be #5 too. Our guide to which plastic grades are meant for food goes through all seven.
No symbol? What it means, material by material
A lot of advice, including Google's own summary, says that anything without a mark should stay out of the microwave. That is the safe default for plastic. It is too strict for plain glass and ceramic.
The USDA's microwave guidance lists heatproof glass (it names Pyrex and Anchor Hocking) and glass-ceramic (it names Corning Ware) as fine to use, and says safe glass, ceramic and plastic "usually will be labeled for microwave oven use". Usually, not always. Here is how to read a dish with no mark:
Plain kitchen glass, glass-ceramic, plain glazed ceramic or stoneware
No metal trim, no cracks
Run the 30-second test below, then use it.
Plastic with no mark, takeout boxes, and tubs that held margarine, yogurt or cottage cheese
The USDA calls these cold-storage containers "not approved for cooking"
Do not microwave. Move the food to glass or ceramic.
Metal, foil trays, anything with gold or silver trim or metallic paint, foam cups and trays, brown paper bags
On the USDA and FDA lists of things to keep out
Never. Metal can spark, foam and paper can melt or scorch.
Old tub, takeout box or no mark at all? Replace it, don't test it
There is no home test that tells you whether a plastic is safe to heat food in. If a container has no mark, or the mark has worn off, the easy fix is a glass container that says in writing what is microwave safe and what is not. These three do, and each comes with a catch worth knowing.
As an Amazon Associate we earn from qualifying purchases. This does not change which products we recommend.
Start here
Rubbermaid Brilliance Glass, set of 4
What the maker says: glass bases are oven-safe up to 450°F; the vented lids are "microwave-safe for reheating only with lids unlatched". Sold and shipped by Amazon.
The catch: the lids are plastic and never go in the oven, and some buyers report lids cracking over time.
See the Rubbermaid set on Amazon
Pyrex Simply Store, 3 round containers
What the maker says: tempered glass; the BPA-free plastic lid is "freezer, microwave, and top-rack dishwasher safe".
The catch: some buyers report lids cracking, and like any tempered glass it does not like sudden temperature swings, so no freezer-to-hot-oven moves.
See the Pyrex set on Amazon
GENICOOK glass containers with glass lids, set of 5
What the maker says: borosilicate glass with a glass lid and steam vent, so no plastic sits over your food; microwave safe, and oven safe with the lid off.
The catch: reviews are mixed on the silicone seal, with some reporting it coming loose. Sold by a third-party seller, and it has far fewer ratings than the other two.
See the GENICOOK set on AmazonThe 30-second test for unmarked glass and ceramic
The FDA notes that the instructions that come with your microwave explain how to test a container. Most describe a version of this:
Put the empty dish in the microwave next to a cup of water, about two-thirds full. Never run the microwave with nothing in it.
Heat on high for 30 seconds.
Touch the dish carefully, from the edge.
Water hot and the dish cool or barely warm: fine to use. Dish hot: keep it for serving, not heating. Sparks or crackling: stop at once. That is metal in the decoration.
This test does not work for plastic. It shows whether a dish soaks up microwave energy. It cannot show whether a plastic releases chemicals into hot food, which is the real question with plastic. For the longer version of the test and what it can't tell you, see is porcelain microwave safe.
Is it microwave safe? Quick answers by material
Glass and Pyrex: Heatproof kitchen glass is fine, and the USDA names Pyrex. Keep out thin decorative glass and anything with metallic paint, and avoid sudden temperature changes. More in glass vs plastic food storage.
Ceramic and stoneware: Usually yes when it is plain, glazed and undamaged. Hand-made or crackled pieces can soak up water and get hot, so run the test.
Porcelain and china: Plain porcelain, yes. Gold, silver or platinum trim, no, because it sparks. Details in our porcelain guide.
Plastic: Only when the container itself carries a microwave mark. Even then, vent the lid and keep plastic wrap from touching the food. Why it matters: is microwaving plastic safe.
Takeout and grocery tubs: Treat them as cold storage unless they carry their own microwave mark. The USDA lists margarine tubs and yogurt and cottage cheese cartons as "not approved for cooking".
Styrofoam (foam): The USDA lists foam-insulated cups, bowls, plates and trays among things not to microwave. If a foam container has no maker's microwave mark, move the food.
Paper plates and towels: The USDA lists most paper plates, towels and napkins as fine. Brown paper bags and newspaper are not. Use plain, uncoated paper for short warming.
Mason jars: Always take off the metal lid and band first. Whether the jar itself is meant for the microwave is the maker's call, so check the box or the maker's website.
Metal, foil and stainless steel: The FDA says metal pans and foil should generally not be used, because microwaves reflect off them. That includes stainless steel food containers.
Melamine: The FDA says food and drinks should not be heated on melamine dinnerware in the microwave. More in our food storage guide.
Silicone: Food-grade silicone lids and containers are often sold as microwave safe. Follow the maker's mark. More in is silicone safe for cooking.
Microwave safe is not oven safe (or air fryer safe)
Microwave safe, oven safe and dishwasher safe are three separate promises, and a container can make one without the others. Lids are where they split most often. Rubbermaid's glass bases go in the oven up to 450°F, but its lids never do. Pyrex lists its lids for the freezer, microwave and dishwasher, and not the oven.
An air fryer is a small oven that blows very hot air, so a plastic container that is fine in the microwave does not belong in it. For ovenware, see our guide to oven-safe cookware.
Questions people ask
What symbol means microwave safe?
Wavy lines. They may be on their own, inside a small drawing of a microwave, or above a dish. Some makers print the words "microwave safe" or "microwavable" instead. All of them make the same claim.
Is there an official microwave-safe symbol?
We could not find a US rule that sets what the symbol must look like, and the EU food-contact law defines no microwave symbol either. Each maker draws its own version, and the mark is that maker's claim about the item.
What does the microwave-safe symbol look like on plastic?
Usually three wavy lines, or a microwave with waves inside, molded into the bottom in the same color as the plastic. It often sits next to the recycling triangle, the snowflake and a dishwasher symbol. Tilt the container under a light to read it.
Is #5 plastic microwave safe?
#5 means polypropylene, the plastic most often used for microwave containers. But the number only names the plastic. Look for the wavy lines or the words "microwave safe" on the same item. A thin takeout tub can be #5 and still not be made for heating.
Does "reheat only" mean microwave safe?
Yes, for short warming of food that is already cooked, usually with the lid loosened. It is not meant for cooking raw food, long runs, boiling water or very oily or sugary food.
Does microwave safe mean heat-proof or oven safe?
No. Microwave safe is about the microwave only. Oven safe is a separate claim, often with a top temperature, and many microwave-safe lids must never go in an oven or air fryer.
Is it safe to microwave a dish with no symbol?
It depends on the material. Plain heatproof glass and plain glazed ceramic with no metal trim are usually fine; run the 30-second water test first. Plastic, takeout tubs, foam and anything metallic should stay out.
Why can't you microwave metal?
Microwaves bounce off metal instead of passing through it. The FDA says this makes food cook unevenly and can damage the oven, and metal trim on dishes can spark.
How can you tell if styrofoam is microwave safe?
Look for the maker's microwave mark on the container. Without one, don't microwave it. The USDA lists foam-insulated cups, bowls, plates and trays among things to keep out of the microwave.
Sources reviewed
- USDA Food Safety and Inspection Service: Cooking with Microwave Ovens
- FDA: Microwave Ovens (updated October 12, 2023)
- FDA: Melamine in Tableware, Questions and Answers
- Regulation (EC) No 1935/2004 on materials in contact with food, Article 15 and Annex II
- ASTM International: revisions to the Resin Identification Code standard
- Product wording quoted from the Rubbermaid, Pyrex and GENICOOK listings on Amazon.com, read October 3, 2026.
Related reading: non-toxic microwaves with no coating inside · choosing non-toxic dinnerware · best meal prep containers.
