Lot Coverage Ratio Calculator Formula
Understand the math behind the lot coverage ratio calculator. Each variable explained with a worked example.
Formulas Used
Building Coverage Ratio
building_coverage = building_coverage_pctTotal Impervious Coverage
impervious_coverage = impervious_coverage_pctRemaining Building Capacity
remaining_building = max_building_sqft - building_footprintRemaining Impervious Capacity
remaining_impervious = max_impervious_sqft - total_imperviousOpen / Pervious Area
open_space_area = open_spaceOpen Space Percentage
open_space_pct = lot_area_sqft > 0 ? (open_space / lot_area_sqft) * 100 : 0Variables
| Variable | Description | Default |
|---|---|---|
lot_area_sqft | Total Lot Area (sq ft)(sq ft) | 10000 |
building_footprint | Building Footprint (sq ft)(sq ft) | 2500 |
driveway_patio | Driveway & Patio Area (sq ft)(sq ft) | 800 |
other_impervious | Other Impervious Surfaces (sq ft)(sq ft) | 200 |
max_building_coverage_pct | Max Building Coverage (zoning)(%) | 35 |
max_impervious_pct | Max Impervious Coverage (zoning)(%) | 60 |
total_impervious | Derived value= building_footprint + driveway_patio + other_impervious | calculated |
building_coverage_pct | Derived value= lot_area_sqft > 0 ? (building_footprint / lot_area_sqft) * 100 : 0 | calculated |
impervious_coverage_pct | Derived value= lot_area_sqft > 0 ? (total_impervious / lot_area_sqft) * 100 : 0 | calculated |
open_space | Derived value= lot_area_sqft - total_impervious | calculated |
max_building_sqft | Derived value= lot_area_sqft * max_building_coverage_pct / 100 | calculated |
max_impervious_sqft | Derived value= lot_area_sqft * max_impervious_pct / 100 | calculated |
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
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.
- 01Building coverage: 2,500 / 10,000 = 25.0%
- 02Total impervious: 2,500 + 800 + 200 = 3,500 sq ft
- 03Impervious coverage: 3,500 / 10,000 = 35.0%
- 04Remaining building capacity: 3,500 - 2,500 = 1,000 sq ft
- 05Remaining impervious: 6,000 - 3,500 = 2,500 sq ft
- 06Open space: 10,000 - 3,500 = 6,500 sq ft (65.0%)
Frequently Asked Questions
What is the difference between lot coverage and FAR?
Lot coverage measures the building footprint as a percentage of lot area (horizontal coverage). Floor Area Ratio (FAR) measures total building floor area relative to lot area (can exceed 100% with multi-story buildings). A 2-story building on 25% lot coverage has a 50% FAR.
Do decks count as lot coverage?
It depends on local zoning codes. Some jurisdictions count uncovered decks as impervious surface while others do not. Covered porches and patios typically count toward building coverage. Check your local zoning ordinance for specific definitions.
What if I exceed the maximum coverage?
You would need a zoning variance, which requires a public hearing and approval by the zoning board. Variances typically require proving a hardship related to the property itself (unusual shape, topography, etc.). Exceeding coverage without approval can result in stop-work orders and fines.
Ready to run the numbers?
Open Lot Coverage Ratio Calculator