Is it a wart or callus on your foot? Both occur on the bottom of the foot but they are different. Plantar warts are caused by a virus that enters the foot through a break in the skin. A callus is a thickening of the skin due to pressure. How can you tell the difference? Warts often have black dots which are capillaries along with tiny nerves that grow into the warts. Also, pinch it. If it is painful it's likely to be a wart. If not, it's probably a callus.