A computable formula for the class number of the imaginary quadratic field $Q(\sqrt{-p}), \ p=4n-1$.

A formula for the class number $h$ of the imaginary quadratic field $Q(\sqrt{-p}$ is obtained by counting on a specific way the quadratic residues of a prime number of the form $p=4n-1.$ Formulas for the sum of the quadratic residues are also found.