Does this fit inside a mitten or glove? Do I need to purchase 2 of these to wear with my mittens or gloves?

Dear Heather, the size of the hand warmer is 2.32*4.01*1.06 in(D*L*H), so it doesn't fit inside a mitten or glove, you can buy one and warm your hands(it provides heat both sides), hope it helps you.