How Long Does A Thermos Keep Hot?

Thermos  |     |  

How Long Does a Thermos Keep Drinks Hot?

Thinking about upgrading your lunch box with a thermos? Whether it’s for hot coffee, hearty soup, or a homemade meal, knowing how long a thermos can really keep food hot is key before you invest.

In winter especially, no one wants to open their lunch only to find it lukewarm. A good thermos solves that problem—keeping your food or drinks at the right temperature for hours.

 

The Short Answer

Most thermos flasks keep contents hot for 6 to 12 hours, depending on the model, size, and how you use it.

In this guide, we’ll explain how a thermos works, what affects heat retention, and which models perform best.

 

How it Works

A thermos is built to slow down heat loss. Most high-quality models, like those from The Oball, use double-walled stainless steel with a vacuum layer in between.

  • The airless gap between the two walls acts as insulation, reducing heat transfer.
  • The inner stainless steel wall reflects heat back into the contents, further preventing it from escaping.

This design means your drink or food can stay hot for many hours without reheating.

 

Real-World Heat Retention

A good thermos will keep liquids hot for at least 6 hours and warm for up to 12 hours. Performance varies by size:

  • Small bottles (300 ml) cool faster
  • Medium bottles (500 ml) hold heat longer
  • Large bottles (1 liter) can keep liquids above 70°C for at least 7 hours

Example test with a 0.5 L thermos:

  • Start: 94.6°C
  • After 2 hours: 90.1°C
  • After 4 hours: 86.6°C
  • After 6 hours: 83.2°C
  • After 24 hours: 65.1°C

 

How to Make Your Thermos Keep Heat Longer

  • Preheat Before Use – Pour boiling water in, wait 10 minutes, then empty before adding your drink or meal.
  • Fill It While Hot – The hotter the contents, the better the retention. Fill it up to the top to reduce air space.
  • Seal the Lid Tightly – Only open it when ready to drink or eat to prevent heat loss.
  • Use an Insulated Bag – Placing your thermos in a cooler bag can extend warmth up to 24 hours.

 

Our Top Pick for Long Heat Retention

The Oball Stainless Steel Thermos

  • Keeps drinks hot for up to 12 hours
  • Made with premium stainless steel
  • Features double-walled vacuum insulation
  • Ideal for coffee, tea, soup, or hot meals
  • Stylish, durable, and perfect for year-round use

If you want hot coffee in the morning and warm soup at lunch—this is the one to get.