How to clean a flask

FInd out how to clean a stainless-steel flask and keep it germ-free, fresh and functioning.


Thermos flask on kitchen table

Key steps

  • Check the manufacturer's advice.
  • Never submerge the flask in water.
  • Wash the flask and cup with a sponge, warm water and washing-up liquid.
  • Rinse under hot water.
  • Rinse the stopper in baking soda and water.
  • Air dry, towel dry and store without the stopper.
  • Use baking soda and hot water for stains.

As part of the wider movement towards sustainability, many of us are now using thermal flasks instead of single-use cups on a daily basis to get our hot drink fix on-the-go. This is great news for the planet, but it also means cleaning a flask needs to be done thoroughly and regularly to keep them fresh and sanitary. Read on for a simple guide to how to clean a Thermos flask or similar brands.

Want more advice on going green? Read our article for how to reuse old jam jars.

When cleaning a flask, make sure you always check the manufacturer’s advice first. If you no longer have the label, make sure you always use mild cleaning agents.

How to clean a flask: a step-by-step guide to cleaning Thermos flasks and other brands

With this simple guide to cleaning a Thermos flask or other brand, we’ll show you how to clean a stainless-steel flask with ease.

  1. Always make sure you check for any manufacturer’s guidelines. Keep any labels for future checks!
  2. To make sure it is safe and hygienic to use, always wash your stainless-steel flask prior to its first use.
  3. Make sure you clean your stainless-steel flask after every use.
  4. Follow these steps for cleaning a Thermos flask or other brand:
    • Use warm water and a mild washing-up liquid.
    • Dip a soft cloth or sponge into your soapy water and wash out the interior of your flask.
    • Avoid submerging your flask in water to prevent damage.
    • Rinse thoroughly to remove all traces of washing-up liquid. It is best to do this beneath hot, running water.
  5. Rinse your flask stopper in a solution made up of ¼ cup baking soda and 1l of water. Use a sponge or cotton bud to wipe in all the crevices of the stopper.
  6. Don’t forget to wash the cup out, too! Use the same steps for cleaning your flask found above.
  7. Allow your flask, stopper, and cup to drain and air-dry.
  8. Use a clean towel to dry off any excess liquid.

How to keep your flask working at its best

Now you know how to clean your flask, there are some important dos and don’ts you should be aware of to keep it in the best condition.

  • DO pre-fill your flask with either hot or cold water (depending on what you want to use it to store!) for around 3 minutes before use.
  • DO store your flask with the stopper off.
  • DO use 1 tsp baking soda inside your flask filled with hot water to remove stubborn stains. For the best result allow this to soak overnight.
  • DON’T overfill your flask. This could lead to leaks and damage to the stopper.
  • DON’T use a dishwasher to clean your flask.
  • DON’T use any abrasive, solvent or bleach-based cleaners.

With these simple steps, you now know how to clean a Thermos flasks or other stainless-steel flask with ease. For more cleaning advice, check out our guide for how to clean bottles.

Originally published