Designs of electromagnetic (EM) coil have attracted a lot of attention in the research community due to its applications in several areas of human endeavours. However, the optimal selection of coil wire size and current in the design of Square Air-Core Multi-turn Multilayer Electromagnetic Coil (SAMMEC) with significant wire diameter for both safe and cost-effective products has not been given enough research attention. Therefore, the equation for the flux density produced by a rectangular loop of wire was adopted in the modelling of SAMMEC with significant wire diameter. A coil design chart was constructed based on the developed model and design specifications. Both the feasible and non-feasible design regions and the line of optimum magnetic flux density were identified on the constructed chart. The appropriate wire size and current for the coil were both determined from the design-chart. The diameter, length, resistance, copper loss, and weight of the selected wire for the generation of 0.06 T flux density were found to be 0.00326 m, 267.01 m, 0.5502 Ω, 263.87 W, and 19.86 kg respectively. The selected wire can produce an optimum flux density of 0.066 T with current of 24 Amp and associated copper loss of 316.92 W. Keywords—Air core, Electromagnetic coil, Magnetic flux density, Multilayer, Multi-turn