This is our all around most versatile heat pack. It is large enough to use on your lower back, neck, arms and legs. It is also convenient enough to carry with you anywhere to keep warm on a cold day. The 5×9 lasts about and 1½ hours depending on how well you insulate it. You can also use the heat pack as a cold pack. Just place it in the freezer for up to one hour and it will stay soft and flexible.