Step 1
Take accurate measurements of your townhouse or duplex exterior. For walls where you can't gain measurement from the exterior, measure the inside and add the thickness of the walls to the measurement, to obtain the exterior measurement.
Step 2
Measure each section of your home separately. For a townhouse or duplex shaped like a rectangle, you just need the length and width. For dwellings with other shapes, such as an "L" shape, you need to divide each room into sections of rectangles and take those measurements separately.
Step 3
Go to the second floor and any other floors above and to the basement to take measurements of total floor space on each level. Do not include open spaces, such as vaulted ceiling space, in your measures.
Step 4
Round all measurements taken to the closest 0.5 linear foot. For each section, multiply the length by the width to get the measurement for that section. Add the square footage of each section of your home to get a total measurement of your square footage for the townhouse or duplex. Make sure you round out your number to the nearest square foot.