Quickly size drainage basins for engineering decisions today. Switch units, add coordinates, and validate geometry. Download clear outputs that support audits and approvals easily.
| Scenario | Inputs | Expected area |
|---|---|---|
| Rectangular catchment | Length 120 m, Width 80 m | 9,600 m² (0.96 ha) |
| Equivalent circular basin | Radius 65 m | ≈ 13,273 m² (≈ 1.327 ha) |
| Irregular polygon | 0,0 · 120,0 · 120,80 · 0,80 | 9,600 m² (0.96 ha) |
A drainage area represents the land surface contributing runoff to a defined outlet. In practice, delineation begins with contours, flow direction grids, or surveyed breaklines. Small shifts in the outlet location can change contributing area and influence downstream sizing. For urban sites, include roof leaders, curb lines, and inlets that redirect flow. Record the datum, map scale, and boundary sources so assumptions remain traceable during review.
Rectangular, triangular, and trapezoidal planforms are useful for early estimates when boundaries are simplified. For near‑radial basins, an equivalent circle can represent average travel paths and compact shapes. When boundaries are irregular, coordinates provide higher fidelity and reduce bias at bends. Use trapezoids for tapering parcels and triangles for wedges between ridges. Choose the method that best preserves the true perimeter and avoids overstating area at corners.
Engineering teams often mix length units from different references. This tool converts inputs to meters, computes area in square meters, then converts to your selected output. Use hectares for hydrology worksheets, square kilometers for regional studies, and acres for legacy design records. For drainage design, keep consistent units across intensity, area, and discharge calculations. Always state the unit beside the final value and round appropriately.
For the polygon method, enter points around the boundary in order, either clockwise or counterclockwise. The shoelace approach sums cross products of adjacent vertices to calculate planar area. Avoid duplicate consecutive points and ensure at least three unique vertices. If you digitize from GIS, export the same projection used for measurement to prevent distortion. If the area returns near zero, check ordering, repeats, and coordinate magnitude.
Drainage area supports runoff estimation, detention sizing, and inlet layout decisions. With rainfall intensity and runoff coefficient, the tool estimates peak discharge using the Rational Method. Metric: Q ≈ 0.00278 × C × i(mm/hr) × A(ha). US: Q ≈ 1.008 × C × i(in/hr) × A(acres). Use results to compare options, validate GIS outputs, and attach exports for audit readiness.
Use the polygon option with coordinates traced from a survey or GIS layer. It preserves bends and indentations better than simplified shapes, improving downstream sizing confidence.
No. Enter each vertex once in order around the boundary. The calculation automatically closes the polygon by connecting the last point back to the first.
Select the unit used by your design workflow. Hectares and square kilometers suit metric hydrology summaries, while acres can match legacy drainage records and permit submittals.
This usually indicates repeated points, insufficient unique vertices, or inconsistent ordering. Remove duplicates, ensure at least three distinct points, and keep the boundary sequence continuous.
Treat it as a screening value. The Rational Method depends on appropriate intensity selection, time of concentration, and defensible runoff coefficients aligned with local standards.
After a successful calculation, the page stores your latest results for the current session. Use the CSV or PDF buttons to export the same values for documentation.
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.