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Can you put glass in the microwave depends on the piece, not on the word glass. Is glass microwave safe is true for cookware the maker marked for that use. It is not true for every jar, tumbler, or rim with metal paint. The FDA says microwaves pass through glass, so the glass is not what absorbs the energy. The food does. The dish then heats up from the food. That is why a handle can burn you even though the microwave did not heat the glass directly.
The same FDA page says to use cookware manufactured for the microwave, and that the manual for your oven lists the containers it allows. It also says metal pans and aluminum foil generally should not be used, because metal reflects microwaves, food heats unevenly, and the oven can be damaged. A gold or silver band on a glass is metal. Leave that glass out.
Marked glass versus a glass you hope is fine
| Piece | Microwave? | Why |
|---|---|---|
| Dish marked microwave-safe, no chips | Yes, as the maker describes | That mark is the permission |
| Pyrex glass bakeware, following Pyrex's rules | Yes | Pyrex says its glassware can be used in a microwave. The thermal-shock rules below still apply |
| Unmarked drinking glass or jar | No | No mark, and it was not sold as cookware |
| Glass with a metal rim or metal paint | No | FDA's general metal warning |
| Chipped, cracked, or deeply scratched glass | No | Pyrex says not to use damaged glassware |
| Empty glass, or a dish of oil with no food | No | Pyrex says not to heat empty glass or overheat oil or butter. FDA says some ovens should not run empty |
Pyrex is one brand, with written limits
Pyrex's safety page says Pyrex glassware can be used to cook, bake, warm, and reheat in a microwave and in a preheated conventional oven. It also says sudden temperature changes can break glass. Do not add liquid to hot glass, set hot glass on a wet or cold counter, handle it with a wet cloth, or move it from the freezer straight into a hot oven. Let it cool on a dry rack before it goes into the refrigerator or the sink. Do not use it on a burner, under a broiler, or in a toaster oven. Do not heat it empty, do not microwave popcorn in it, and do not overheat oil or butter. If a plastic cover is on the dish, Pyrex says the cover has to sit unsealed and off-center so steam can escape.
Pyrex also says it makes both soda-lime and borosilicate glass, that both are microwave-safe when the safety rules are followed, and that borosilicate resists extreme temperature change a bit more. That is not permission to ignore the shock rules on either kind. Other brands do not inherit Pyrex's page. If your dish is not Pyrex, the mark on that dish is the only yes.
What to do with a normal leftover
Confirm the mark and the rim
Microwave-safe, no metal band, no cracks. If any of those fail, use a different dish.
Add food, not an empty bowl
Leftovers should cover the bottom. An empty bowl is on the do-not list.
Vent the cover
Set a marked cover ajar. A sealed lid traps steam.
Stop when it is hot, and stir
Use the shorter time if you are unsure, stir, and continue. Do not walk away from oil or butter.
Set it on a dry board
Not a wet counter, not a cold stone sill, not the sink. Let it cool before the refrigerator.
Foam containers are a different material with a different rule: can you microwave Styrofoam. Wipe the cavity with how to clean a microwave. If the oven runs and the dish stays cold, use microwave not heating and stop. Do not pull the waveguide cover.
More small-appliance limits are under microwaves and small appliances.
Questions readers ask
Can I put glass in the microwave if the glass is thick?
Can I put glass in the microwave only when that piece says microwave-safe and has no metal trim. Thickness is not the mark. A thick unmarked tumbler stays out.
The dish went from the freezer to the microwave. Why does that matter?
Pyrex warns that a jump from freezer to high heat can break glass. Let frozen glass warm, or transfer the food into a room-temperature dish that is marked.
Are all Pyrex lids microwave-safe?
Pyrex says a cover can go in if it is unsealed and sitting off-center so steam escapes. Follow the cover that came with that dish. Do not snap it shut.
Can I reheat in a random jelly jar?
Not from this page. A jar with no microwave-safe mark is not cookware. Move the food to a marked dish.