This paper presents two program packages, FormCalc and LoopTools, for evaluating one-loop amplitudes. These programs can work in either dimensional regularization or constrained differential renormalization (CDR), with the latter being equivalent to dimensional reduction at the one-loop level. The authors implemented three methods—dimensional regularization, dimensional reduction, and CDR—in these packages. FormCalc produces symbolic expressions that can be further evaluated numerically, while LoopTools provides numerical evaluation of these expressions. The programs are designed to handle large numbers of diagrams and are used for processes such as $ ZZ \rightarrow ZZ $ in the Standard Model. FormCalc is based on Mathematica and simplifies Feynman diagrams, while LoopTools is a numerical package that supports Fortran, C++, and Mathematica interfaces. The paper discusses the equivalence between CDR and dimensional reduction, and how they can be used interchangeably for one-loop calculations. It also describes the implementation of the packages, their use in numerical evaluations, and their availability on various platforms. The programs are useful for automating one-loop calculations in quantum field theories, particularly in gauge and supersymmetric theories.This paper presents two program packages, FormCalc and LoopTools, for evaluating one-loop amplitudes. These programs can work in either dimensional regularization or constrained differential renormalization (CDR), with the latter being equivalent to dimensional reduction at the one-loop level. The authors implemented three methods—dimensional regularization, dimensional reduction, and CDR—in these packages. FormCalc produces symbolic expressions that can be further evaluated numerically, while LoopTools provides numerical evaluation of these expressions. The programs are designed to handle large numbers of diagrams and are used for processes such as $ ZZ \rightarrow ZZ $ in the Standard Model. FormCalc is based on Mathematica and simplifies Feynman diagrams, while LoopTools is a numerical package that supports Fortran, C++, and Mathematica interfaces. The paper discusses the equivalence between CDR and dimensional reduction, and how they can be used interchangeably for one-loop calculations. It also describes the implementation of the packages, their use in numerical evaluations, and their availability on various platforms. The programs are useful for automating one-loop calculations in quantum field theories, particularly in gauge and supersymmetric theories.