In the present paper we propose a variant of constraint interval arithmetic that operates with a single parameter (level) in each interval operand of an expression. This results in a computationally simple interval arithmetic that has several desirable properties not shared in general by interval operations previously defined in the literature. The actualization and usefulness of this approach to fuzzy (interval) arithmetic is accomplished via alpha-levels. The evaluation of arithmetic expressions using the proposed single level constrained arithmetic is discussed in detail and we extend this theory to the fuzzy context.