The interval package for real-valued interval arithmetic allows one to evaluate functions over subsets of their domain. All results are verified, because interval computations automatically keep track of any errors. These concepts can be used to handle uncertainties, estimate arithmetic errors and produce reliable results. Also it can be applied to computer-assisted proofs, constraint programming, and verified computing. The implementation is based on interval boundaries represented by binary64 numbers and is conforming to IEEE Std 1788-2015, IEEE standard for interval arithmetic. Related software: INTLAB, C-XSC, filib++.