Size rainwater downpipes and horizontal storm drains for a roof. Enter the roof area and design rainfall, and the calculator gives the runoff, the downpipe size from IPC capacities and the horizontal pipe size at your slope.
SI: Q (L/s) = C × i (mm/h) × A (m²) ÷ 3,600
US: Q (gpm) = C × i (in/h) × A (ft²) ÷ 96.25
For roofs C = 1.0, since practically all the rain runs off. Rainfall intensity is the key input: codes usually ask for the 5-minute storm with a 100-year return period for roofs, which ranges from about 75 mm/h (3 in/h) in dry regions to 200 mm/h (8 in/h) or more in tropical and storm-prone areas.
| Downpipe | Capacity | Capacity | Roof area at 100 mm/h | Roof area at 4 in/h |
|---|---|---|---|---|
| 2" (DN50) | 30 gpm | 1.9 L/s | 68 m² | 722 ft² |
| 2.5" (DN65) | 54 gpm | 3.4 L/s | 123 m² | 1,299 ft² |
| 3" (DN80) | 92 gpm | 5.8 L/s | 209 m² | 2,214 ft² |
| 4" (DN100) | 192 gpm | 12.1 L/s | 436 m² | 4,620 ft² |
| 5" (DN125) | 360 gpm | 22.7 L/s | 818 m² | 8,662 ft² |
| 6" (DN150) | 563 gpm | 35.5 L/s | 1,279 m² | 13,547 ft² |
| 8" (DN200) | 1,208 gpm | 76.2 L/s | 2,744 m² | 29,068 ft² |
Vertical conductor capacities as given in the IPC; check the edition adopted locally. Rectangular downpipes need the same cross-sectional area as the round size.
Horizontal rainwater pipes are sized to flow full at their slope: Q = (1 ÷ n) × A × R2/3 × S1/2, with R = D ÷ 4 for a full round pipe. With n = 0.015 the results match the code tables; a smooth PVC pipe (n = 0.011) carries about a third more. Doubling the slope increases capacity by about 40%.
800 m² roof, 100 mm/h, 4 downpipes, main drain at 1/4 in/ft:
Size the drainage from fixtures with the Fixture Unit Calculator and check pipe falls with the Drainage Slope Calculator. For drawings, see our storm water drainage shop drawing.
Find the runoff with Q = C × i × A (in SI, L/s = C × mm/h × m² ÷ 3,600), divide it between the downpipes, and choose the smallest downpipe whose capacity exceeds the flow. Size horizontal drains with Manning's equation at their slope.
A 4-inch (DN100) vertical leader carries about 192 gpm (12.1 L/s): roughly 435 m² at 100 mm/h, or 4,620 ft² at 4 in/h.
The design storm in your plumbing code, usually the 5-minute duration with a 100-year return period for roofs, taken from local rainfall maps or the meteorological authority.
Usually yes. The IPC requires secondary (emergency) roof drains or scuppers where parapets could trap water, sized for the same flow as the primary drains and discharging separately.
Found a number that looks wrong, or a case the tool does not cover? Say so here — corrections are welcome and get answered.
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