I agree with Linda as far as listing the square footage. If you estimate it, make sure you quote your source. If you are planning on listing your home with these measurements, you may want to hire a company to take the measurements and draw up a floor plan.
I use Measure Comp (see link). For about $99, they will provide you with very detailed floor plans, including square footage. The floor plans are something buyers love to see and you can quote the professional square foot measurements to help reduce any liability.... more