Why Aren’t Thermos Flasks Spherical?

Do you want a Thermos flask that keeps hot liquids hot or cold liquids cold longer?

If so:

You should minimise the surface area, because heat escapes through the surface. A sphere is the optimal shape for that.

We do need a flat base to prevent it from tipping over if your hand accidentally hits it. This area of the base in this design has been chosen to match the stability of a traditional Thermos flask — in terms of the force needed to tip it. Make it wider if you’ve faced problems with your Thermos flask tipping over and you want something more stable.

We also need a cylinder on top. If you had a pure sphere with just a hole at the top, the flask would bump into your nose when you try to drink from it.

Since this shape is less grabbable than a traditional cylindrical Thermos, I’ve added a strap as shown:

Even with the strap, it’s less comfortable to drink from than a traditional cylinder shape, which is naturally optimised for grip.

The other drawback of this design is that it doesn’t fit in bottle holders or seatback pockets in cars, buses or planes.

Design is all about tradeoffs — if you want something (maintaining temperature longer), you have to give up something else.