To determine the appropriate size of an air conditioning unit for a 1800 square foot space, there are several factors to consider. The most important factor is the square footage of the space, but other factors such as the number of occupants, the amount of sunlight the space receives, and the presence of heat-generating appliances should also be taken into account.

According to This Old House, for a central air conditioner, you can calculate the square footage of the space by multiplying the length of each room by its width, then adding them all together. Next, multiply the total square footage by 20 to see how many BTUs are needed to cool your space. Based on the air conditioner sizing charts provided by This Old House, a 1800 square foot space would require an air conditioner with a cooling capacity of approximately 27,000 BTUs or 2.25 tons.

It is important to note that these calculations provide a rough estimate, and other factors such as the climate zone and insulation of the space should also be taken into account. For a more accurate calculation, a professional can perform a cooling load calculation, also known as a "Manual J".