[en] EFFI calculates the electromagnetic field and vector potential in coil systems of arbitrary geometry. The coils are made from circular arc and/or straight segments of rectangular cross-section conductor. EFFI can also calculate magnetic flux lines, magnetic force, and inductance. The methods used for the calculations are based on a combination of analytical and numerical integration of the Biot--Savart law for a volume distribution of current. These methods yield accurate field values inside and outside the conductor. All input to EFFI is format-free and is checked for validity before any calculations are done. Any errors detected during the check produce a diagnostic that lists the error, the code's objection to it, and the number of the offending data card. EFFI produces output in both printed and graphical form. Each page of output is labeled with the title of the problem, the time, the computer and date of the run, and the version number and compilation date for EFFI. In addition, each column of numbers on each page is appropriately labeled. Examples from the coil design for the Mirror Fusion Test Facility (MFTF) and a divertor design for a Tokamak reactor are used for illustration