1 Acre = 6 272 640 Square Inches
Acre is an imperial, US customary area unit and used mostly for measuring the land, agriculture and forestry. It's usage is mostly replaced globally by metric area unit hectare. The abbreviation is "ac".
Square inch is a very small area unit with both sides of the square are 1 inch in length. It's mostly used to measure the area of small objects. The abbreviation is "sq in".