fireLib is a C language function library for predicting the spread rate, intensity, flame length, and scorch height of free-burning surface fires. It is derived directly from the BEHAVE fire behavior algorithms for predicting fire spread in two dimensions, but is optimized for highly iterative applications such as cell- or wave- based fire growth simulation. fireLib was developed to give fire growth modelers a simple, common, and optimized application programming interface to use in their simulations. While fireLib contains 13 functions, as few as 4 function calls are required to create a simple yet efficient and functional fire growth simulator. fireLib is written entirely in ANSI standard C and compiles under a wide range of C and C++ compilers on a variety of DOS, Windows, and Unix computers.