Estimate runoff depth, volume, and peak flow quickly. Useful for stormwater screening, detention sizing, and watershed planning tasks today.
This tool supports fast screening. For final design, verify qᵤ with official TR-55 tables, watershed conditions, rainfall distribution, and local criteria.
| Drainage Area (ac) | CN | P (in) | Tc (hr) | Fp | qᵤ Method |
|---|---|---|---|---|---|
| 25 | 82 | 4.2 | 1.3 | 1.00 | Estimate |
| 40 | 88 | 5.1 | 0.9 | 0.95 | Manual qᵤ = 420 |
| 12 | 74 | 3.0 | 1.8 | 1.00 | Estimate |
Potential maximum retention: S = (1000 / CN) - 10
Initial abstraction: Ia = λ × S, where λ is usually 0.20 or 0.05
Runoff depth: Q = (P - Ia)² / (P - Ia + S), when P > Ia
Runoff volume: Volume = A × Q / 12, reported in acre-feet
Peak discharge: qp = qu × Ami² × Q × Fp × Safety Factor
Time to peak for estimate mode: Tp = D/2 + 0.6 × Tc
Estimated qu: a practical screening estimate based on Tc, Ia/P, storm depth, and Tp. Manual qu entry is best when official values are known.
This calculator helps engineers estimate runoff depth, runoff volume, and peak discharge from a small watershed. It uses a TR-55 style workflow. The form is practical and quick. It works well for detention planning, culvert checks, grading studies, and concept design.
The runoff depth uses the NRCS curve number method. That method links rainfall depth and land response. Higher curve numbers usually create more runoff. Lower curve numbers usually hold more rainfall in storage. This gives a direct way to compare site conditions during early design.
Peak discharge depends on how much runoff forms and how quickly it arrives. Time of concentration is very important. Shorter travel times often raise the peak. The calculator also includes a pond and swamp factor. This lets you reduce peak flow when storage and routing effects are present.
Some users know the unit peak discharge from charts or approved references. Others only need a screening estimate. This tool supports both approaches. You can enter qᵤ manually for stronger control. You can also estimate it automatically for fast comparisons between alternatives.
The runoff volume output helps with storage sizing. The peak flow output helps with inlet, pipe, swale, and outlet review. A safety factor is included for conservative testing. The example table also makes it easier to validate data entry before using the tool on a live project.
Use this calculator for planning, concept design, and educational review. Final design should still match local standards, published rainfall data, watershed segmentation, routing assumptions, and official TR-55 source material.
TR-55 helps estimate runoff depth, runoff volume, and peak discharge for small watersheds. It is often used in stormwater planning and preliminary drainage design.
Curve number represents runoff potential. It reflects land cover, soil group, treatment, and hydrologic condition. Higher values usually mean greater runoff and lower infiltration.
Time of concentration affects when runoff reaches the outlet. Shorter times usually raise peak flow. Longer times usually spread runoff and reduce the peak rate.
Enter qᵤ manually when you already have approved values from official TR-55 charts, local design criteria, or a reviewer-required method. That improves consistency in final checks.
It reduces peak flow to reflect watershed storage effects. Use values below 1.00 when storage is meaningful. Use 1.00 when no reduction is justified.
No. This is a planning and screening calculator. Detailed design may require routing, subarea breakdowns, local rainfall standards, and approved drainage software.
Area is entered in acres. Rainfall and runoff depth use inches. Time of concentration uses hours. Peak discharge is reported in cubic feet per second.
If rainfall does not exceed initial abstraction, direct runoff does not form in the equation. In that case, runoff depth, volume, and peak discharge are reported as zero.