Impervious Surface & Runoff Coefficient Calculator
Calculate a site's impervious percentage and weighted runoff coefficient from roofs, paving, gravel, lawn and woods, the extra runoff volume a storm produces compared with the undeveloped site, the detention or rain-garden volume to capture it, and a stormwater fee estimate.
How it works
Impervious % = (roofs + paving + ½ permeable) ÷ total; C_weighted = Σ(area × C) ÷ total; runoff volume = C × depth ÷ 12 × area (ft³) × 7.48 gal; rain garden area = extra volume ÷ ponding depth
Rain that falls on a forest mostly soaks in; rain that falls on a roof runs off almost entirely, so the share of a site that is impervious is the single best predictor of how much stormwater it sends downstream and how degraded the receiving stream will be — measurable impacts start around 10% impervious cover in a watershed. The area-weighted runoff coefficient combines each surface's tendency to shed water into one number used in the rational method, and comparing the developed and pre-development coefficients gives the extra runoff volume that detention ponds, rain gardens and cisterns are sized to hold. Because cities now bill stormwater by impervious area and require developments to hold post-development runoff to pre-development rates, every square foot of pavement has a running cost as well as a construction cost.
Worked example
A 12,000 ft² lot with 3,000 ft² of roof, 1,800 of paving, 400 of permeable pavers, 6,000 of lawn and 800 of woods on loam soil is 42% impervious with a weighted C of 0.48 — three times the 0.15 of a natural site. A 1.5″ storm sheds about 5,400 gallons, 3,700 more than before development; a 1,000 ft² rain garden ponded 6″ deep would capture the difference, and the impervious area would draw about $240 a year in stormwater fees at $4 per 1,000 ft².
Frequently asked questions
Why is permeable paving only counted at 50%?
Well-built permeable pavers over a stone reservoir infiltrate most storms, but codes credit them conservatively because they clog without maintenance and pass large storms; many jurisdictions credit 50–70%.
How does this relate to the rational method Q = CiA?
The weighted C here is the same coefficient; multiply it by rainfall intensity (in/h) and area (acres) to get peak flow in cfs for pipe and culvert sizing. This calculator gives volumes; the rational method calculator gives peak rates.
Related calculators
Planning arithmetic using the definitions most zoning codes share; the actual FAR, setbacks, height, coverage, parking ratios and density rules are set by the local ordinance and overlay districts, and many cities exclude or bonus certain areas. Confirm with the zoning code and a land-use professional before relying on a yield.