Permeable System Infiltration Area Calculator

Calculator Inputs

• Runoff volume: V = A × P × C, where A is catchment area (m²), P is rainfall depth (m), and C is runoff coefficient (0–1).
• Effective infiltration rate: k_eff = (k / 1000) × R, where k is soil rate (mm/hr) and R is the reduction factor.
• Area from drawdown: A_inf = V / (k_eff × T), where T is drawdown time (hr).
• Area from storage: A_stor = V / (D × n), where D is storage depth (m) and n is void ratio.
• Recommended footprint: A_rec = max(A_inf, A_stor) × SF, where SF is the safety factor.

1. Select a runoff volume method. Use catchment sizing for early planning.
2. Enter rainfall, catchment area, and runoff coefficient, or enter volume.
3. Provide soil infiltration rate and choose a reduction factor for conservatism.
4. Set drawdown time based on local guidance and site constraints.
5. Enter storage depth and void ratio for your chosen stone or media.
6. Press Calculate to view footprint, capacities, and downloadable reports.

For final design, confirm rates with a field test and include pretreatment to reduce clogging.

Permeable systems work best when the infiltration area is sized to manage runoff volume, soil intake, and storage together. An undersized footprint can cause surface ponding and slow drawdown, while an oversized footprint raises costs. This calculator helps balance those tradeoffs with consistent units and conservative options.

For planning, runoff volume is estimated from catchment area, rainfall depth, and a runoff coefficient. For example, a 60 m² roof with 25 mm rainfall and coefficient 0.9 produces about 60 × 0.025 × 0.9 = 1.35 m³ of runoff. Use a lower coefficient for landscaped areas.

Field infiltration varies with soil structure, compaction, moisture, and clogging risk. Designers often apply a reduction factor (such as 0.3–0.8) to convert a test rate into a long‑term effective rate. Using a lower factor is prudent where fines, traffic, or sediment loading are expected.

Drawdown time links volume to area: Area = Volume ÷ (effective rate × time). Shorter drawdown targets require larger footprints. Many projects target complete drain‑down within a day or two, but local guidance and soil constraints should lead the final choice.

A permeable base often provides temporary storage before infiltration. Storage capacity depends on layer depth and void ratio (typical clean stone can range roughly 0.30–0.45). If storage governs, the footprint must be large enough that Volume ≤ Area × Depth × Void Ratio.

A safety factor accounts for construction variability, aging, and uncertainties in rainfall and soil data. Common sizing practice is to calculate both infiltration‑limited and storage‑limited areas, then adopt the larger value times the safety factor. The recommended footprint can be split into multiple cells for better constructability.

Site geometry matters as much as math. Steep slopes can drive lateral flow and reduce effective intake, while tight boundaries may limit footprint options. Maintain appropriate separation from foundations and utilities, and consider overflow routing for storms that exceed the design event.

Long‑term performance depends on keeping pores open and sediment out of the system. Routine sweeping or vacuuming, controlling upstream soil loss, and using pretreatment (like filter strips or sump boxes) help preserve infiltration. Re‑evaluate the effective rate if the site use changes or clogging indicators appear.

1) What runoff coefficient should I use?

Use values closer to 0.9–1.0 for roofs and sealed pavements, and lower values for landscaped areas. When uncertain, choose the higher coefficient for a conservative footprint.

2) How do I pick a reduction factor?

Start with 0.5 for typical soils and moderate sediment risk. Use 0.3–0.4 when clogging risk is higher, and 0.7–0.8 only when pretreatment and maintenance controls are strong.

3) What if my infiltration test is in inches per hour?

Convert to mm/hr by multiplying inches per hour by 25.4. Enter the converted rate, then apply a reduction factor to represent long‑term performance.

4) Which area should I build: infiltration or storage?

Build for the larger controlling area. If infiltration capacity is low, the drawdown method will dominate. If soils infiltrate quickly but storage is shallow, the storage method will dominate.

5) Can I split the recommended area into multiple zones?

Yes. Multiple smaller cells can improve grading, shorten flow paths, and simplify construction. Keep the total footprint at or above the recommended area and provide safe overflow between cells.

6) What drawdown time should I enter?

Use the target required by your project criteria or local guidance. If none is specified, 24–48 hours is commonly used for planning, then refined with site constraints and soil limitations.

7) Why does the calculator show a larger area after adding a safety factor?

Safety factors intentionally increase footprint to cover uncertainties in testing, construction, and aging. It reduces the risk of ponding and helps maintain acceptable drain‑down as the system accumulates fine sediment over time.