I don't know there exact algorithm but I had a client prove to me that Zestimates are not always accurate by altering the sqf. of his home on Zillow and then dropping it back down a week or so later. The price of the home doubled almost because he doubled the sqf. but then when he deducted it it didn't drop as quickly. Puzzled me and made me realize that Zestimates are not as accurate as an appraisal. Its just a round about price. :) hope that helps.