•Developed a mix design method for fly ash geopolymer concrete using MARS model.•Key variables are water/solid, activator/fly ash, Na2SiO3/NaOH and NaOH molarity.•Contour plots can be used to design fly ash geopolymer concrete mix proportions.•Targeted 28-day strength from 30 MPa to 55 MPa achieved in laboratory experiments. Many research studies have been conducted during recent years on the topic of geopolymer materials based on the engineering performance of the concrete. What has been missing is the combination of this research in a way that would provide a simple to use design tool for geopolymer concrete as a replacement to concrete based on Portland Cement. This research paper addresses this requirement for developing a standard mix design method for Class F, low calcium fly ash based geopolymer concrete using Multivariate Adaptive Regression Spline (MARS) model. Published geopolymer concrete research data was combined into a database and analysed to give the ratios of water/solid, alkaline activator/fly ash, Na2SiO3/NaOH, and NaOH molarity. Targeted compressive strengths ranging from 30 MPa to 55 MPa at 28 days were achieved with laboratory experiments, using the proposed MARS mix design methodology. Thus, this tool has the capability to provide a novel approach for the design of geopolymer concrete mixes to achieve the desired compressive strength appropriate for the construction requirement.