Estimate runoff, peak flow, and drain capacity. Build better garden drainage plans with practical stormwater sizing guidance today.
| Surface Type | Typical Runoff Coefficient | Rainfall Intensity | Drainage Note |
|---|---|---|---|
| Lawn | 0.15 to 0.35 | 25 to 50 mm/hr | Higher infiltration and lower peak runoff |
| Compacted Soil | 0.40 to 0.60 | 35 to 65 mm/hr | Common in traffic zones and paths |
| Gravel Path | 0.30 to 0.50 | 30 to 60 mm/hr | Moderate runoff with some storage |
| Paved Patio | 0.70 to 0.95 | 40 to 90 mm/hr | Fast runoff and stronger drain demand |
The calculator uses the Rational Method for peak runoff.
Q = 0.00278 × C × i × A
Here, Q is peak flow in cubic meters per second. C is runoff coefficient. i is effective rainfall intensity in millimeters per hour. A is catchment area in hectares.
Effective intensity is estimated as rainfall intensity minus infiltration rate. Runoff depth is calculated from effective intensity, storm duration, and runoff coefficient. Runoff volume equals runoff depth converted into meters, multiplied by catchment area.
A safety factor raises the final design flow. Surface slope is used as a simple adjustment to suggest practical drain capacity for garden drainage planning.
Garden runoff affects plant health, soil stability, and surface safety. Too much water can drown roots, erode beds, and damage nearby paving. A runoff calculator helps estimate stormwater before it becomes a problem. This supports better drainage choices for lawns, patios, borders, and low spots.
Catchment area is the first driver of runoff. A larger surface sends more water toward drains or swales. Rainfall intensity matters because intense storms create faster flow. Runoff coefficient also matters. Hard paving sheds water quickly. Soil and turf absorb more water and reduce discharge.
Infiltration lowers effective rainfall. Sandy or amended soil may absorb more water. Compacted ground absorbs less. When infiltration is ignored, drain sizing may become too conservative. When it is overstated, flooding risk may rise. Realistic values produce better drainage design for practical garden work.
Peak flow helps size channels, pipes, gravel trenches, and collection points. The result shows how much water may move during the design storm. The adjusted flow includes a safety factor. This gives extra capacity for uncertain weather, blocked inlets, or future surface changes.
Use the calculator early in the planning stage. Compare lawn, gravel, and paving scenarios before construction. Direct runoff away from foundations and soft planting zones. Combine pipe sizing with grading, swales, rain gardens, and soil improvement. Good drainage is usually a system, not one product.
This tool supports smarter stormwater design in home gardens. It helps reduce standing water, muddy paths, and maintenance issues. By checking runoff volume and peak discharge, you can choose drainage components with more confidence and protect your landscape during heavy rain.
A runoff coefficient shows how much rainfall becomes surface runoff. Hard surfaces have higher values. Grass and loose soil usually have lower values because they absorb more water.
Yes. For mixed surfaces, use a weighted average runoff coefficient. Estimate each surface share, multiply by its coefficient, then add the values for one combined input.
Infiltration represents water soaking into the soil. When soil absorbs part of the storm, less water remains on the surface, so runoff depth and peak flow both decrease.
This tool is best for preliminary planning in gardens and small landscapes. Large or regulated projects should be checked by a qualified drainage engineer using local standards.
Use a local design storm value from weather records, municipal guidance, or site engineering data. Pick a storm intensity that matches your project risk level.
The safety factor adds reserve capacity. It helps account for blocked grates, uncertain soil behavior, aging surfaces, and storms that exceed normal expectations.
Slope mainly affects how quickly water moves, not the total water volume. Steeper areas can create faster concentration and stronger drain demand during storms.
Yes. After entering values, use the CSV or PDF buttons. The file includes the major runoff metrics, drain capacity estimate, and suggested pipe diameter.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.