400 acres to sq mi conversion result above is displayed in three different forms: as a decimal (which could be rounded), in scientific notation (scientific form, standard index form or standard form in the United Kingdom) and as a fraction (exact result). Every display form has its own advantages and in different situations particular form is more convenient than another. For example usage of scientific notation when working with big numbers is recommended due to easier reading and comprehension. Usage of fractions is recommended when more precision is needed.
If we want to calculate how many Square Miles are 400 Acres we have to multiply 400 by 1 and divide the product by 640. So for 400 we have: (400 × 1) ÷ 640 = 400 ÷ 640 = 0.625 Square Miles
So finally 400 acres = 0.625 sq mi