A Stanley, Yeti, Hydro Flask or any vacuum insulated stainless steel cup is metal. It reflects microwaves and arcs, and it will not warm what is inside.
The steel walls block microwaves entirely, so the drink does not heat even if nothing goes wrong. What can go wrong is arcing at the lid, the handle and any metal edge, which can damage the oven or start a fire. Plastic lids and handles do not make the cup safe.
| Short answer | No |
|---|---|
| Why | Stainless steel reflects microwaves and arcs |
| Does the drink heat | No, the steel shields it |
| Applies to | Insulated tumblers, travel mugs and metal water bottles |
| Alternative | Pour it into a ceramic mug |
How to warm a drink from an insulated cup
- Pour the drink into a microwave safe ceramic or glass mug.
- Heat in 20 to 30 second bursts and stir between them.
- Pour it back, since the insulation will keep it warm.
- If it is only lukewarm, warm the empty cup with hot tap water first.
- Wash the cup and lid as normal.
What happens when you get it wrong
- Sparks and noise. Arcing happens quickly, often within seconds.
- Damage to the cup. Heat and arcing can scorch the exterior coating and pit the steel.
- Damage to the microwave. Repeated arcing can scorch the interior and damage components.
A ceramic mug for reheating.
A kettle or stove for a larger amount, then pour it into the insulated cup.
Frequently asked questions
Can you microwave a Stanley cup with the lid off?
No. The metal body is still the problem, not just the lid.
Can you microwave a plastic Stanley?
Check the product. Plastic versions may be marked microwave safe, but steel ones never are.
Why did my cup spark?
Microwaves induce charge on the metal edges, which arcs. It is the same as a fork.
Can you wash a Stanley cup in the dishwasher?
Many are listed as dishwasher safe. See the Stanley dishwasher page.