  We show that the set $SCL^{rp}$ of stable commutator lengths on recursively
presented groups equals the set of non-negative right-computable numbers. Hence
all non-negative algebraic or computable numbers are in $SCL^{rp}$ and
$SCL^{rp}$ is not closed under subtraction. We also show that every
non-negative real number is the stable commutator length of an element in some
infinitely presented small cancellation group.
