A diabetic foot ulcer is easier to describe by what it does than by what it looks like. The most useful thing to know is what it often does not do: diabetic ulcers are often painless, because of decreased feeling in the feet. An ulcer that does not hurt is not a smaller problem than one that does. It is often the same problem, found later.
Where they form follows from the cause. Foot ulcers are partly caused by too much pressure on one part of your foot, so they appear where the foot takes weight or rubs. They may also start as something that looks minor. If you get a blister, you may not notice, and it may get worse.
Color is the one part of the appearance that is clear-cut. After dead tissue has been removed, the ulcer may seem bigger and deeper. It should be red or pink. Wounds that are pale, purple, or black are less likely to heal.
Separate from any sign of infection, contact your provider if your foot ulcer is very white, blue, or black.
Wounds on the same foot can have a different cause and a different look. Ischemic ulcers — wounds from poor blood flow — can be dark red, yellow, gray, or black sores, with raised edges around the wound (looks punched out), no bleeding, and a deep wound through which tendons may show. Those describe ischemic ulcers rather than diabetic ones, but they are worth knowing, because people with poor blood flow often also have nerve damage or foot ulcers from diabetes.
There’s no photo, size, or checklist that lets you identify one yourself. A wound on the foot of someone with diabetes is examined, measured, and checked for feeling and pulses — not recognized by eye at a distance. Knowing what to look for helps you decide to show someone, not decide what it is.