BiGO interpreter and BiGO script together (the tool) calculate the upper bounds for primitive recursive functions. BiGO script is a notation language that is used to model the program flow execution in the terms of the time steps. BiGO interpreter is not acutally an interpreter, although it has some common characteristics with programming language interpreters; after creating the parse tree from a given BiGO script, the evaluator calculates the Big O from the parse tree without executing the given BiGO script. BigO script is complete to context sensitive languages. The documentation and syntax with examples can be found from the tool. The tool is available for MacOS, Linux and for Windows (x64).