If you have a tablet it might be worth downloading the free version of this app. The sharp-eyed may be able to use it on a phone, but the extra real estate of a tablet will be appreciated.
If you watch the LifeHacker demo below you will see some clever ways to capture dimensions and use them to build a floor plan model - no tape measure needed.
Give it a try and see if you find it useful. Note that it does not claim to have CAD level accuracy.