The Rule of Palm is used when assessing the extent of burns in first aid. If you look at the victim's hands, each palm represent about 1% of the total body surface area. Now look at the burned areas and calculate how many palm-sized hands would cover the burns. This will give you an approximation of the area. The reason one uses the victim's hands and not their own is due to the differences in body sizes. An adult male with large hands would calculate a lower Rule of Palm number if they were assessing a toddler. If you use the toddler's palms as the measure, you can see that the burn area would be considerably higher.
For larger burns, use the Rule of Nines.