Although the house finch and the purple finch are similar in size, shape and coloring, they do have distinct differences in each of those criteria that you can discern if you are willing to learn them and look for them. Paying attention to the little details is key when you're trying to identify the differences between these two types of finches.
Shape of the Bill
The bill of the purple finch is cone-shaped. It is straight, round and pointed, and it is smaller than that of the house finch. The bill of the house finch is curved in a downward arc, similar to the upper beak of a parrot. Both birds will eat seeds, the purple finch being quite fond of sunflower seeds. The house finch, however, is omnivorous, eating insects as well as plant foods.
The Males of Both Types
The male purple finch has a red head with two pink stripes alongside each eye. The male house finch has an all-red head with a single brown stripe stretching from the edge of his eye to the back of his head. The male purple finch has more red on his wings and body; the male house finch has more brown all over, his two prominent colors being red and brown.
The Females of Both Types
The female purple finch has a brown head with two white stripes around her eyes and a brown-streaked breast and flank, with an all-white belly. The female purple finch has rose-colored wing stripes while the female house finch has all-white wing stripes. The female house finch has a solid brown face with brown and white striping all down her breast, flank and belly areas. She is more predominantly brown than the female purple finch.
Sizes and Shapes
The purple finch is big and husky, measuring around 5 1/2 to 6 1/2 inches long. The house finch is slightly smaller and thinner, at 5 to 6 inches long. The tail of the purple finch is shorter than that of the house finch and has a more prominent notch in the tip. The house finch has shorter wings than the purple finch, due to the bird's smaller size; when folded, the house finch's shorter wings make the tail appear longer.