The Rheem XE50M06ST45U1 Performance 50 Gal. Electric Water Heater has the following dimensions: Height: 53 inches Diameter: 24.38 inches12 Given your current water heater space measurements of 56 inches in height and 18 inches in width, the Rheem model will fit in terms of height but may be too wide for your space. The diameter of 24.38 inches exceeds your available width of 18 inches. You might need to consider a different model with a smaller diameter or reconfigure the space to accommodate the wider unit.