Permeable Paver Drainage & Stormwater Calculator
Free permeable paver drainage calculator for rectangle, L-shaped and circular patios, with an optional fire-pit or garden cutout. Compute stormwater runoff in gallons, required gravel reservoir depth, and catch-basin count for a permeable paver patio.
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30 secLive Estimate
Permeable paving lets a storm soak through instead of sheet-flowing. Runoff volume is rainfall depth spread over your patio footprint; the gravel reservoir stores it and releases it slowly into the soil below.
How Our Formulas Work
The calculator uses standard estimating assumptions for material volume, waste, coverage and ordering allowances. Actual quantities vary by material, supplier, site conditions and installation method. We use real material bulk densities and compaction coefficients, so your orders match actual site needs. ⌈x⌉ means rounded up to the next whole unit.
Patio Footprint
Worked example: Example: 20 ft × 15 ft = 300 sq ft of patio
Pro rule of thumb: For an L, add the two rectangles. For a circle, use half the diameter as the radius r.
Stormwater Runoff
Worked example: Example: 300 sq ft × (2.5 in ÷ 12) × 7.48052 = 467.5 gallons
Pro rule of thumb: 7.48 gallons fill one cubic foot. The 2.5" storm is this calculator's default example / design assumption — the real 5-year, 24-hour rainfall varies by location, so use your local design rainfall for accurate sizing.
Reservoir Gravel
Worked example: Example: (300 sq ft × 4 in ÷ 12) ÷ 27 = 3.70 cu yd → × 1.4 = 5.19 tons
Pro rule of thumb: Use open-graded drain rock — no compaction buffer here, the voids are what store the water. Default loose bulk density: 1.40 tons/cu yd — actual density varies by aggregate type, moisture, and supplier.
Catch Basins
Worked example: Example: max(1, ⌈300 sq ft ÷ 250⌉) = 2 basins
Pro rule of thumb: The 1 basin per ~250 sq ft formula is a planning estimate / rule of thumb, not a universal engineering requirement. Actual requirements depend on grading, drainage layout, inlet capacity, soil conditions, and local code requirements.
Planning Estimate
Results are estimates based on the information and assumptions provided. Actual material quantities may vary with site conditions, material specifications, installation method, waste, and supplier measurements. For construction-critical decisions, verify quantities against your supplier's specifications and any local requirements before purchasing or building.
Cut buffer: 8% running bond → 20% curved / borders.
Crushed stone: +15% compaction allowance, ≈ 1.40 tons/yd³ loose bulk density.
Bedding sand: 50 lb bag ≈ 0.5 ft³, default 1″ screed bed.
Joint sand: one 50 lb bag ≈ 75 sq ft of 1/8″ joints.
Edge & fabric: 6 ft strips, spikes ≈ every 1.4 ft; fabric +10% for seams.
Prices: 2026 local-market averages (USD · CAD · GBP · EUR · INR · AUD · JPY) — override in "local prices".
Keep the Momentum Going
Your permeable drainage is just one layer of the build. Try a related tool or jump back to the full material list.