Calculator
Example data table
| Scenario | Area | C | Intensity | Slope | y/D | Material | Selected size | Design flow |
|---|---|---|---|---|---|---|---|---|
| Parking + roof catchment | 3.5 ha | 0.65 | 75 mm/hr | 0.70% | 0.80 | Concrete | 300 mm | 0.466 m³/s |
| Residential block | 6.0 acres | 0.45 | 2.4 in/hr | 1.00% | 0.80 | HDPE | 18 in | 13.08 cfs |
| Industrial yard | 8.0 ha | 0.80 | IDF (tc=12) | 0.50% | 0.90 | PVC | 600 mm | 1.603 m³/s |
Formula used
- Q = 0.00278 · C · i · A for metric, where i is mm/hr and A is hectares.
- Q = 1.008 · C · i · A for US customary, where i is in/hr and A is acres.
- Design flow applies a safety factor: Q_design = Q_peak · SF.
- Q = (1/n) · A · R^(2/3) · S^(1/2)
- For a circular pipe at a fixed depth ratio y/D, the wetted area and hydraulic radius are computed from circle geometry.
- θ = acos(1 − 2·(y/D)), central angle is 2θ (radians).
- A = D² · (2θ − sin(2θ)) / 8
- P = D · θ, R = A / P
How to use this calculator
- Choose your unit system and enter drainage area and runoff coefficient.
- Set rainfall intensity directly, or compute it using IDF options.
- Enter pipe slope, pick a material, and choose a target depth ratio.
- Adjust the safety factor and velocity check limits if needed.
- Click Size Pipe. Review warnings and export results.
Design flow selection and risk
Storm sewer sizing starts with a clear design storm, because pipe diameter follows peak discharge. This calculator applies the Rational Method and then multiplies the peak flow by a safety factor. For example, increasing the safety factor from 1.10 to 1.25 raises the design flow by 13.6%, which commonly shifts the next standard pipe size.
In many municipal standards, a 10-year storm governs minor systems, while critical corridors may require 25-year or 50-year checks. Pair the chosen return period with consistent inlet spacing, upstream capture assumptions, and tailwater boundary conditions for reliable sizing and clearer permitting documentation during reviews.
Runoff coefficient impact
Runoff coefficient C represents land cover response. Impervious pavement may be near 0.80–0.95, while landscaped areas can be 0.15–0.35. Holding intensity and area constant, moving C from 0.45 to 0.65 increases peak flow by 44.4%. Use local guidance and verify composite C for mixed catchments.
Intensity options and time of concentration
Rainfall intensity can be entered directly or computed from an IDF form i = a/(tc+b)^c. Shorter tc produces larger i, which increases Q. The optional Kirpich estimate links tc to flow length and slope, so longer flow paths or flatter grades increase tc and reduce intensity. Always confirm the IDF parameters against your jurisdiction’s curve.
Manning capacity and partial flow criterion
Hydraulic capacity is computed using Manning’s equation with circular geometry at your selected depth ratio y/D. Many agencies size storm sewers for 0.80–0.90 depth to preserve freeboard and allow surcharge storage. At a fixed slope, higher y/D increases wetted area and capacity, but the design criterion should match inlet spacing and HGL requirements.
Velocity checks and constructability
The calculator reports velocity at design flow and flags values outside your min and max checks. Low velocity can promote sediment deposition, while high velocity may require lining or drop structures. If velocity is high, consider a larger diameter or reduced slope where feasible. Document assumptions, then export CSV and PDF outputs for review.
FAQs
1) When should I use the Rational Method?
Use it for small urban catchments where runoff responds quickly and rainfall intensity can be tied to time of concentration. For large basins, storage and routing dominate, so a hydrograph method is usually more appropriate.
2) What depth ratio y/D should I select?
Common targets are 0.80 to 0.90 for storm sewers to limit surcharge and preserve capacity for debris. Use 1.00 for full-flow checks or where design standards explicitly require full pipe sizing.
3) How do I choose Manning’s n?
Select n based on material, joint condition, and aging. Smooth plastics often have lower n than concrete, while corrugated metal is higher. If local specifications publish design n values, prefer those over generic ranges.
4) Why does the calculator pick the next standard size?
Pipe is typically procured in nominal diameters. The tool computes a required diameter from capacity scaling, then selects the smallest standard size that meets or exceeds it, improving constructability and reducing undersizing risk.
5) What does the safety factor represent?
It provides conservatism for uncertainty in C, intensity, future development, and minor losses not modeled explicitly. Apply a value consistent with your design manual, and avoid stacking multiple conservatisms without documenting the rationale.
6) Does this include inlet, manhole, or bend losses?
No. The capacity check is based on uniform flow in a straight reach. For detailed design, evaluate minor losses, hydraulic grade line, tailwater, and junction energy balances using your standard method or a network model.