Measure the length and width of the room, in feet, using a tape measure. Write these measurements down. Multiply the width by length to arrive at the square footage of the room. Write this number down.
Launch the default web browser by selecting it from the “Start” > “All Applications” pop-up menu. Enter the URL web address in the top field for a website that provides a free air conditioning calculator, for example, the BTU Calculator on the Hearth website (see Resources).
Enter the “length” measurement you wrote down in the “Length” field on the website’s air conditioning calculator. Enter the “width” measurement you wrote down in the “Width” field.
Select the type of house you have from the drop-down menu or from a list on the web page (the selection process may vary, depending on the website).
Enter any other information that is requested on the web page, for example, how many windows there are in the room and whether it has a skylight or not.
Press the “Compute” or “Calculate” button on the web page (the actual name may vary, depending on the website).
Read the number of Btu’s that are needed in the “Btu” text field on the web page. Use this number when buying the portable AC unit for the room. Match the Btu number with that listed on the unit’s specifications -- typically found on a side or back panel. Purchase a unit that matches or slightly exceeds the Btu number that is needed.