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IMPERVIOUS AREA CALCULATOR CALIFORNIA
Impervious & Pervious Area Calculator
Calculate roof, pavement, removal and remaining pervious area from measurements you supply. Local zoning and stormwater rules determine which surfaces count.
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BEFORE YOU START
Lot coverage versus impervious area →Get a more useful result
Enter measured roofs, paving and any removed areas. The remaining pervious area is mathematical only; local rules determine whether each surface is counted.
- Confirm legal parcel area and the surfaces your jurisdiction counts.
- Enter existing roofs, paving, new impervious surfaces and removals without overlap.
- Add a maximum only when verified with your zoning or stormwater reviewer.
Example: Example: 6,000-square-foot lot with 1,500 square feet roof, 800 paving, 100 other; 240 new roof and 200 paving; remove 100.
INTERACTIVE ANALYSIS
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HOW IT WORKS
Useful Answers Without Pretending Every Property Is the Same
Run the tool logic
Project-specific rules and calculations evaluate the inputs.
Check local context
Official building and planning sources appear when available.
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WHY THIS TOOL MATTERS
Impervious Coverage Is Not Building Coverage
Impervious coverage considers surfaces that the local ordinance treats as impervious, which may include roofs, conventional paving, concrete or other materials. Building coverage is a separate zoning calculation commonly focused on structures and their footprints. The two figures are not interchangeable. This tool totals only the surfaces you classify as counted impervious area.
Existing, Added and Removed Areas
Enter existing counted roof, paving and other surfaces separately. Proposed roof and paving increases are added, while areas being removed are subtracted exactly once. Removed impervious area must not exceed the current counted total. Label material types and the actual measured areas in the drawing, taking care not to count an area twice.
How Pervious Area Is Estimated
Remaining pervious area equals lot area minus total counted impervious area. This arithmetic does not establish infiltration capacity or guarantee landscape compliance. Gravel, grasscrete or permeable paving may be classified differently by a specific California city or county. Enter those surfaces under the treatment accepted by the reviewing agency.
SOURCE & CONFIDENCE LAYER
Strong Tools Explain What Is Known, What Is Estimated, and What Still Needs Verification.
Verified sourceOfficial local or state resource when available
Preliminary resultCalculated or rules-based planning guidance
User suppliedFacts you entered about the property or project
Needs verificationProperty-specific issue the tool should not guess
DETAILED GUIDANCE
Use the Result Correctly Before You Commit Time or Money
Compare Against a Verified Local Maximum
If a city or county provides an impervious-surface limit for your property, you can enter that percentage for a conditional comparison. The report shows remaining area allowance or the amount over that entered limit. There is no statewide default used in this tool. Verify the appropriate zoning or drainage ordinance before drawing a permitting conclusion.
How to Show Impervious Areas on the Site Plan
A permit site plan can include a schedule showing lot area, existing roof and paving areas, proposed new areas, removed paving, final counted impervious area and remaining landscaped/pervious area. A reviewer should be able to trace each area to an outlined and labeled surface on the drawing.
Drainage Design Is a Separate Question
Changing site paving may alter runoff or low points. Surface coverage alone does not establish that drainage is controlled. A grading plan, swale section, drainage computation or legal discharge review may be necessary depending on site conditions. This calculator does not size drainage pipes, determine retention capacity or engineer a proposed stormwater system.
Before Relying on the Numbers
Use the correct parcel area, measured improvement dimensions and the local definition of imperviousness. Parcel GIS can help initial research but may not provide surveyed precision. This tool does not verify property lines, easements, lot restrictions, or agency approval. Ask for a site-plan review if the dimensions or classifications are uncertain.
FREQUENTLY ASKED QUESTIONS
Impervious & Pervious Area Calculator FAQs
These answers explain how to interpret the tool and when local or professional verification still matters.
What is the impervious area formula?
Existing counted roof plus existing counted paving plus other existing counted surfaces, plus new roof and paving, minus removed counted surfaces. Divide the result by lot area and multiply by 100 for percent.
Do permeable pavers count as pervious?
Not automatically. Classification depends on local ordinance and approved installation system.
Is this the same as lot coverage?
No. Building lot coverage and impervious coverage can include different surfaces.
Can I add a local maximum percentage?
Yes. The comparison uses only the verified limit you enter. No statewide maximum is assumed.
What if my entered impervious area exceeds the lot?
The calculator rejects inconsistent dimensions or removal totals and asks you to correct the inputs.
Can these calculations appear on a permit drawing?
Yes, when the figures and surface definitions are verified, a matching area table can be incorporated into the site plan.
Does the tool calculate drainage capacity?
No. Hydrology, discharge and engineering design are separate.