In a word: size. Both are abnormal veins caused by faulty valves that allow blood to pool and enlarge the veins.

By definition, spider veins are less than one millimeter in diameter, and varicose veins are greater than three millimeters in diameter. Spider veins appear as thin red, blue or purple lines. They are frequently found in clusters and they usually do not bulge. Their length can be ½ inch to several inches long. Most spider veins do not cause any symptoms. In some cases they can cause itching, burning, or bleeding. They are not associated with increased risk of blood clots. Spider veins are treated with sclerotherapy and surface laser therapy.

Varicose veins usually bulge and can be as short as an inch or up to 3 feet in length. They can appear the color of normal skin, or they can appear bluish, greenish, or have a purple tint. They can have no symptoms, or they can be painful, achy, itch, burn, or bleed. Varicose veins can develop blood clots, a condition called superficial vein thrombosis or phlebitis. This condition can be quite painful and the area becomes tender, red, and swollen for a few weeks. It usually is treated conservatively and resolves on its own, although occasionally treatment is necessary that may include blood thinners, or removing the clotted off section of vein. Varicose veins are treated by ambulatory micro phlebectomy.

Spider Veins vs Varicose Veins