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I only know of one, the floor area is the numerator, the lot size is the denominator. So if you have a 2000 square foot house on a 6000 square foot lot, the FAR would be 2000/6000, or 1/3 or .33. For an office building, you might have 100,000 square feet of building on a 50,000 square foot lot; 100,000/50,000 or 2/1, FAR of 2.0
Different places then have different policy on what constitutes actual floor area, and what constitutes lot size.
Linden is right, but some jurisdictions, like the City of Ketchum, ID, incorporate incentives into the calculation by not requiring certain spaces to be included in the floor area. For instance if you were trying to encourage underground parking below buildings or affordable apartments on the upper floors of commercial buildings or quasi-public atriums or similar spaces in buildings you could exempt them from the floor area, or count them in at a discount.