[en] EFFI calculates magnetic flux lines, fields, forces, and inductance for an arbitrary system of coils made from circular arc and/or straight segments of rectangular cross section conductor. The preparation of input to the code and the output options available are described. Several examples, including the magnet design for MFTF and a divertor design for a Tokamak are used for illustration. All input to EFFI is format free, and is checked for validity before any calculations are done. To aid the user in correcting mistakes, each error detected produces a diagnostic giving the objection to the data and the number of the offending data card. If no errors are detected, the program automatically expands memory to the size required for the problem. EFFI produces both printed and graphical output. Each page of output is labeled with the problem title; the time, machine and data of the run; and the version number and compile date for EFFI. In addition, each column of numbers on each page is appropriately labeled. Plots of field contours, field profiles, flux lines, field magnitude along a flux line, and forces can all be obtained at the option of the user. On field contour and flux line plots, lines of intersection between the coils and the plotting plane are also shown. 5 figures