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Area inside the site boundary that drains to the point being sized.
Trace it from the contours after grading, not before. Re-grading a pad routinely moves a divide by several metres, and the area that ends up draining to a low point is often not the one the survey suggested.
Fraction of rainfall on the site area that becomes surface runoff.
Take the value from the table in the governing manual for the surface as it will be built, and weight it where the area is mixed. Coefficients published for the same surface differ between jurisdictions, and a reviewer will check yours against their own list rather than a textbook.
Area beyond the boundary whose water crosses onto the site and has to be carried through it.
This is the area most often left out, because the survey stops at the fence and the divide does not. Walk the boundary uphill and find where the water actually comes from — a neighbouring roof, a road crown or a field is under no obligation to drain away from you.
Runoff fraction for the surface beyond the boundary, which is rarely the same as the site's.
Offsite ground is frequently softer than the developed site — grass, scrub or an unmade track — and gets a lower coefficient accordingly. Where the neighbouring land is itself likely to be developed later, some reviewers require the future condition instead of the present one.
Average intensity of the design storm over the catchment's time of concentration (25.4 mm/hr is 1 in/hr).
Intensity falls as duration rises, so the design storm is the one whose duration equals the time it takes water to travel from the furthest point of the catchment to the outlet. A short, intense burst on a paved catchment and a long, gentle one on a large grassed one can produce very different peaks from the same curve.
Peak discharge
1.49 ft³/s
The Rational Method returns a peak flow and nothing else — no hydrograph, no volume, no timing. It is accepted for small catchments and routinely rejected above a size limit written into the local ordinance, so check that limit before using this figure in a submission.
- Total contributing area
- 34,450 ft²
- Area-weighted runoff coefficient
- 0.63 (C)
- Contribution from beyond the boundary
- 0.31 ft³/s
- Share of the peak arriving from offsite
- 20.79 %
They open the calculator with your figures already in it
Rational Method Peak Runoff Calculator: 1.49 ft³/s — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- The Rational Method, Q = C x i x A, as adopted by local stormwater ordinances for small catchments; the runoff coefficients and the design-storm intensity belong to the reviewing jurisdiction and are entered here rather than supplied
- Intensity is read from the jurisdiction's intensity-duration-frequency curve at the design return period, for a storm duration equal to the catchment's time of concentration
- Worked in SI throughout — Q equals C times the intensity in metres per second times the area in square metres — then restated in the unit the visitor's market uses. No 1.008 or 1/360 shortcut factor is applied anywhere
Inputs used
- On-Site Contributing Area
- 21530 sq ft
- On-Site Runoff Coefficient
- 0.8
- Offsite Contributing Area
- 12920 sq ft
- Offsite Runoff Coefficient
- 0.35
- Design-Storm Rainfall Intensity (mm/hr)
- 75
Intermediate steps
- Total contributing area
- 34,450 ft²
- Area-weighted runoff coefficient
- 0.63 (C)
- Contribution from beyond the boundary
- 0.31 ft³/s
- Share of the peak arriving from offsite
- 20.79 %
Confidence note: The Rational Method returns a peak flow and nothing else — no hydrograph, no volume, no timing. It is accepted for small catchments and routinely rejected above a size limit written into the local ordinance, so check that limit before using this figure in a submission.
What this calculation does not cover
- Assumes uniform rainfall over the whole catchment for the full duration, and a single time of concentration for both areas.
- Gives no runoff volume, so it cannot size detention or infiltration storage — those need a hydrograph method.
Add the equipment this sizes
This result is a specification — 1.49 ft³/s — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-08-30 · in the site-wide review of 2026-09-06 · v1.0.0
Regulatory standards & verification citations3
- The Rational Method, Q = C x i x A, as adopted by local stormwater ordinances for small catchments; the runoff coefficients and the design-storm intensity belong to the reviewing jurisdiction and are entered here rather than supplied
- Intensity is read from the jurisdiction's intensity-duration-frequency curve at the design return period, for a storm duration equal to the catchment's time of concentration
- Worked in SI throughout — Q equals C times the intensity in metres per second times the area in square metres — then restated in the unit the visitor's market uses. No 1.008 or 1/360 shortcut factor is applied anywhere
Cite this page
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