How to Calculate the Floor Space Index of a Particular Property

Obtain the square footage value for all the buildings built on the property. You can measure from the outside or inside, but if you measure rooms from the inside remember account for the thickness of the walls. City or county records offices will usually have a copy of the plot plan from the lot as it was originally built. If additional rooms or buildings were added later, these values must be added.

Determine the square footage of the lot. If the lot is a simple square or rectangle, you can multiply the length times the width for an initial estimate, but to ensure accuracy, contact the local tax office or check the property deed. If you have the property professionally surveyed, you may find that parts of the lot lie under roadways and over fence lines.

Divide the square footage value of the buildings by the square footage value of the lot. The resulting number is the FSI.