The use of glass-fibre-reinforced polymer (GFRP) reinforcement as an alternative to steel for use in reinforced concrete (RC) structures has developed significantly in recent years. With excellent corrosion resistance, a high tensile strength to weight ratio and being non-magnetic and non-conductive, GFRP is an excellent solution for projects requiring improved corrosion resistance or reduced maintenance costs. However, despite a number of recent studies illustrating the effective use of GFRP rebars as longitudinal reinforcement for concrete compression members, the current international design codes do not recommend including GFRP reinforcement in the compression member capacity calculations. A test programme was thus carried involving the construction and testing of 17 rectangular concrete columns reinforced with steel or GFRP. This paper provides full derivations of the interaction diagrams for both steel- and GFRP-reinforced concrete columns. The interaction diagrams fitted the experimental data very well for both types of RC column. It was found that the GFRP-reinforced columns did not have a ‘balance point’ on the interaction diagram, and this was clearly shown for longitudinal reinforcement ratios above 3%. It was found that excluding the strength and stiffness of GFRP reinforcement from concrete compression calculations is conservative. Theoretical capacities better represent the experimental data when the strength and stiffness of GFRP reinforcement are included. The resulting factored interaction curves were exceeded by all experimental capacities.