Implementation math: FAR, density, parking, and more

Learning objectives

• Convert between square feet, acres, and square miles without hesitation.
• Calculate floor area ratio, building height from FAR and coverage, and lot coverage.
• Calculate gross and net density, and units allowed under a minimum-lot-area rule or density bonus.
• Calculate required parking and the land area it consumes, and peak-hour trip generation.
• Calculate housing cost burden, affordable monthly housing cost, and income as a percentage of area median income.

Key concepts

The AICP exam includes a handful of calculation questions. None needs advanced math, but they reward planners who set up the problem carefully. Each section below gives the formula, then a worked example. Try each example yourself before reading the answer.

Units you must know

• 1 acre = 43,560 square feet
• 1 square mile = 640 acres
• A percentage is a fraction of 100: 30% = 0.30.

9 more sections follow in the full lesson.

Key terms

• Acre: 43,560 square feet.
• Floor area ratio (FAR): Total floor area divided by lot area.
• Lot coverage: Footprint (or impervious area) divided by lot area.
• Gross density: Units per total site area.
• Net density: Units per site area after subtracting streets, open space, and other non-residential land.
• Minimum lot area per unit: A density rule expressed as land required for each dwelling unit.
• Parking ratio: Required parking spaces per unit of floor area or per dwelling unit.
• Trip generation rate: Average trips produced per unit of a land use in a given period.
• Cost-burdened household: A household paying more than 30% of gross income for housing.
• Area median income (AMI): The median household income for an area, used to set affordability limits.