These work very well in keeping your feet warm in very cold conditions. I've used them in non-insulated leather work boots to just below freezing and they kept my feet warm. They do start to lose their warmth around 6 yrs. I like to place mine right under my toes, to keep them warm. The warmer is thin enough that you might forget your warning them.

The size of the warmer is about 8-inch long x 4-inch wide approx. There smaller then what I expected and it's tricky to get them fit just right with your sock and boot. But once you get them fitted on your sock and slide your foot in the boot its not too bad. I have only used them in temperatures as low as 9F. and they were great, they worked.