For how long do vacuum flasks keep warm or cold?

Vacuum flask users commonly ask the classic question:
“How many hours does it keep warm?” or
“How many hours does it keep cold?”

Vacuum flasks are manufactured using double-layered steel and the air between these layers is vacuumed out to increase insulation. The thickness of the steel used, the type of steel, and even the size of the gap between the two steel pieces can affect the duration of temperature retention.

Almost all product sellers give a classic response to the questions mentioned above, saying “Keeps hot for 4-8 hours, cold for up to 12 hours.” This answer is given considering average durations.

So, how do you determine the actual duration your thermos can retain heat or cold? The answer to this question can only be clearly understood over time through use. If the quality of the steel used is 201 grade, it may retain heat for 1-2 hours, 304 grade for 4-6 hours, and for 316 grade, this duration can be even longer.

Most thermoses sold in the market use 304 grade steel. Information about the steel quality can usually be found on the inner body of the thermos.

304 Steel
Embossing indicating 304 Steel Quality (found inside the flask)

However, it has been observed that some unscrupulous sellers stamp 304 quality on products that are actually made of 201 quality steel.

When buying a vacuum flask, instead of focusing on the durations claimed by the seller, it would be useful to look at reviews made by users. As long as the importing companies import products made of high-quality steel and focus on quality to avoid damaging their brand value, the product you buy will likely meet your needs.

If you’re looking for a product for very professional use, then you might want to check out brands used by professional campers around the world.

In conclusion, most of the thermos models sold in the market for your daily use will likely meet your needs.