How do you estimate the ARV?

[Vincent Zuber]

I've been using zillow for a week so far and I'm just wondering when I actually meet with a seller and inspect the property how do I figure out the after repair value? If a house is on zillow with a zestimate of 193,000 but the house is old and needs a lot of work almost a full rehab for example and has over 6.92 acres how do I figure out the price per acre and the after repair value?