Guides · 4 min read
How long does a thermos keep drinks cold and hot?
What the numbers mean, what makes the difference — and how to get the most out of your bottle.
Fjella · 2 October 2026
When an insulated bottle promises “cold for 24 hours”, that’s a good starting point — but the time you actually get depends on how you use it. Here’s what happens inside the bottle, and how to get the most out of it.
How an insulated bottle works
An insulated bottle has two steel walls with a vacuum in between. Heat travels very poorly through a vacuum, so your drink neither loses nor gains much heat through the sides. Most of the heat exchange happens through the lid and the opening.
Our times
| Cold | Hot | |
|---|---|---|
| Blåvand 750 ml | approx. 24 hours | approx. 12 hours |
| Classic 500 ml | up to 12 hours | up to 6 hours |
| Tromsø travel mug 340 ml | approx. 8 hours | approx. 6 hours |
The times are a guide. A large bottle holds its temperature longer than a small one because there’s more drink relative to the surface — and a mug with a drinking opening in the lid gives off heat a little faster than a bottle.
Five things that make the difference
1. Pre-chill or pre-heat
Rinse the bottle with cold water — or boiling water for coffee and tea — for a minute before filling it. That way your drink doesn’t spend its own temperature cooling or warming the steel.
2. Fill it to the top
Air at the top is where the temperature escapes. A full bottle lasts longest.
3. Add ice cubes
A few ice cubes give you extra cold in reserve, so the water stays cold even longer.
4. Close the lid straight away — and keep it closed
Every time you open it, a little cold or heat escapes. Take one good sip rather than many small ones.
5. Think about the surroundings
A bottle in a hot car or in direct sun works harder than one in the shade or in your bag. The insulation does most of the work — but not all of it.
Which bottle should you choose?
- All day outdoors: Blåvand with 750 ml, cold for about a day.
- Bag, school and office: Classic — light and 500 ml.
- Coffee on the go: Tromsø travel mug with a one-handed click lid.
All of them can be engraved with a name — so there’s never any doubt whose bottle it is.
