# How to Calculate the Floor Space Index of a Particular Property

Floor Space Index (FSI), also referred to as Floor Area Ratio (FAR), is the ratio of developed floor space on a piece of land to the amount of raw space available on that land. Municipal zoning ordinances may establish the FSI within a particular range to limit the amount of construction and the size of buildings in a particular area. Because the FSI is a measure of the combination of the height and footprint of a building, using it to regulate size allows more flexibility in building design than using either of the measurements by themselves.

1. #### 1.

Measure the dimensions of the lot. If it is rectangular, this may simply involve measuring its length and width with a 100-foot tape measure. If the lot has an irregular shape, however, it may be more expedient to obtain a copy of the lot description from the county recorder.

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Calculate the area of the lot, either by multiplying its length by its width, or by performing an alternate calculation based on its shape. If it's triangular, measure the longest side, or "b," and the distance to the apex, or "h," and use the formula 1/2 x b x h. If circular, measure the diameter, "d," and use the formula 1/2 x 3.14 x (d/2). If your lot is irregular, divide it into a combination of these basic shapes, calculate the area of each shape and add them together.

3. #### 3.

Measure the ground dimensions of all the buildings on the property -- not their heights -- with a 100-foot tape measure and calculate the ground area occupied by each building. If any buildings have more than one story, multiply the ground area by the number of stories. Include lofts and attics that have headroom exceeding 7 feet, but exclude basements.

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Add the areas of all the buildings on the lot and divide this number by the area of the lot itself to obtain the FSI.
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