The price-to-rent ratio helps determine if buying or renting is more affordable in a city by dividing median home price by annual rent. Ratios below 18 favor buying; above 18 favor renting. Cities like San Jose and San Francisco have high ratios (above 35), indicating renting is cheaper, while cities like Detroit and Memphis have low ratios (below 15), favoring buying. Local factors and market shifts impact these ratios.