Lot Coverage Ratio Calculator Formula

Understand the math behind the lot coverage ratio calculator. Each variable explained with a worked example.

Use the Lot Coverage Ratio Calculator

Formulas Used

Building Coverage Ratio

building_coverage = building_coverage_pct

Total Impervious Coverage

impervious_coverage = impervious_coverage_pct

Remaining Building Capacity

remaining_building = max_building_sqft - building_footprint

Remaining Impervious Capacity

remaining_impervious = max_impervious_sqft - total_impervious

Open / Pervious Area

open_space_area = open_space

Open Space Percentage

open_space_pct = lot_area_sqft > 0 ? (open_space / lot_area_sqft) * 100 : 0

Variables

VariableDescriptionDefault
lot_area_sqftTotal Lot Area (sq ft)(sq ft)10000
building_footprintBuilding Footprint (sq ft)(sq ft)2500
driveway_patioDriveway & Patio Area (sq ft)(sq ft)800
other_imperviousOther Impervious Surfaces (sq ft)(sq ft)200
max_building_coverage_pctMax Building Coverage (zoning)(%)35
max_impervious_pctMax Impervious Coverage (zoning)(%)60
total_imperviousDerived value= building_footprint + driveway_patio + other_imperviouscalculated
building_coverage_pctDerived value= lot_area_sqft > 0 ? (building_footprint / lot_area_sqft) * 100 : 0calculated
impervious_coverage_pctDerived value= lot_area_sqft > 0 ? (total_impervious / lot_area_sqft) * 100 : 0calculated
open_spaceDerived value= lot_area_sqft - total_imperviouscalculated
max_building_sqftDerived value= lot_area_sqft * max_building_coverage_pct / 100calculated
max_impervious_sqftDerived value= lot_area_sqft * max_impervious_pct / 100calculated

How It Works

Lot Coverage Ratio

Lot coverage measures how much of a parcel is covered by buildings and impervious surfaces. Zoning codes use coverage limits to control density, manage stormwater, and preserve neighborhood character.

Building Coverage Ratio

Building Coverage = Building Footprint / Lot Area x 100

This measures only the building footprint (first floor area), not total floor area.

Impervious Coverage Ratio

Impervious Coverage = Total Impervious Surfaces / Lot Area x 100

Includes buildings, driveways, patios, walkways, and any surface that prevents water absorption.

Why Coverage Matters

  • Zoning compliance: Most zones cap building coverage at 25-50% and impervious at 50-75%
  • Stormwater management: Higher impervious coverage increases runoff and may require detention facilities
  • Neighborhood character: Coverage limits prevent oversized buildings on small lots
  • Permitting: Exceeding coverage limits requires a variance, which is not guaranteed

    • Worked Example

    10,000 sq ft lot, 2,500 sq ft building footprint, 800 sq ft driveway/patio, 200 sq ft other impervious, 35% max building coverage, 60% max impervious.

    lot_area_sqft = 10000building_footprint = 2500driveway_patio = 800other_impervious = 200max_building_coverage_pct = 35max_impervious_pct = 60
    1. 01Building coverage: 2,500 / 10,000 = 25.0%
    2. 02Total impervious: 2,500 + 800 + 200 = 3,500 sq ft
    3. 03Impervious coverage: 3,500 / 10,000 = 35.0%
    4. 04Remaining building capacity: 3,500 - 2,500 = 1,000 sq ft
    5. 05Remaining impervious: 6,000 - 3,500 = 2,500 sq ft
    6. 06Open space: 10,000 - 3,500 = 6,500 sq ft (65.0%)

    Ready to run the numbers?

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