Storm & Roof Drainage Calculator — Runoff in GPM
A 2,000 square foot roof in a 4-inch-per-hour storm sheds 83.1 gallons a minute — about thirty showers running at once, off an ordinary house. This gives you that design flow from first principles, and checks a proposed horizontal drain against it hydraulically.
Scope: the flow here is derived and exact. The pipe size is not a code answer — the IPC sizes storm drainage from Table 1106.2 (vertical leaders) and Table 1106.3 (horizontal storm drains), which this deliberately does not reproduce. Use the flow, then go to those tables.
Work out the runoff
Plan area, not slope length — plus a share of any wall draining onto it.
100-year, one-hour rate for your site. For orientation only: Pacific Northwest / Great Lakes ~2 · Northeast / Midwest ~3 · Mid-Atlantic / Central ~4 · Southeast / Gulf coast ~5.
Splits the load for the per-leader figure.
Used for the hydraulic capacity check.
Design runoff
83.1 gpm off 2,000 ft² at 4.0 in/hr. A 4" horizontal drain at 0.125 in/ft carries 115 gpm on the hydraulics — confirm against IPC Table 1106.3 before you build it.
Per leader
41.6 gpm
Hydraulic check suggests
4"
See the breakdown
The flow is derived and exact. The suggested size is a Manning hydraulic check only — IPC Tables 1106.2 and 1106.3 govern, and secondary (overflow) drainage is a separate requirement. A licensed plumber and your AHJ have final say.
The formula, explained in plain English
Rain falling on an area is a volume per unit time. The rest is unit conversion.
Horizontal projection, not surface area
Rain falls vertically, so a steep roof catches no more than its plan area. Use the footprint, and add a share of any wall that sheds onto it — walls are the part people forget.
Full-bore, not half-full
Sanitary drains are sized part-full so they scour solids. Storm drains carry clean water at a sustained rate and are designed to run full — which is why the check here doubles the half-full flow.
Overflow is not optional
A flat roof with a blocked primary drain and no secondary becomes a tank. Ponded water weighs 5.2 lb per square foot per inch of depth, so this stops being a plumbing problem and becomes a structural one quickly.
Storms exceed the design rate
The design figure is a 100-year hourly rate, not a ceiling. Short cloudbursts routinely beat it, which is exactly what the secondary system and any freeboard are there for.
Worked examples
A house, the same house on the Gulf coast, and a warehouse where the numbers get serious.
2,000 ft² house at 4 in/hr
across 2 leaders = 41.6 gpm each
4" horizontal at 1/8 in/ft carries 115 gpm ✓
Result: 83.1 gpm — more flow than the whole house's supply system will ever see, arriving through the gutters.
The same roof on the Gulf coast
Mid-Atlantic, 4 in/hr: 83.1 gpm
Gulf coast, 6 in/hr: 124.7 gpm
→ 3× the flow for the identical building
Result: location matters more than anything else on this page. The same drawing needs three times the drainage capacity in Louisiana as in Oregon — which is why a rainfall figure copied from a previous job is a real hazard.
A 20,000 ft² warehouse roof
= 1.85 cubic feet per second
across 6 leaders = 139 gpm each
6" horizontal at 1/8 in/ft carries 344 gpm
Result: over 831 gpm, which is municipal-scale flow — and on a flat roof it needs a full secondary system alongside. At this size the storm drainage is a bigger pipe exercise than the entire sanitary system.
Runoff by area and rainfall rate
Design flow in gpm, computed from the same code the calculator runs.
| Roof area | 2 in/hr | 3 in/hr | 4 in/hr | 5 in/hr | 6 in/hr |
|---|---|---|---|---|---|
| 1,000 ft² | 20.8 | 31.2 | 41.6 | 51.9 | 62.3 |
| 2,000 ft² | 41.6 | 62.3 | 83.1 | 103.9 | 124.7 |
| 3,000 ft² | 62.3 | 93.5 | 124.7 | 155.8 | 187.0 |
| 5,000 ft² | 103.9 | 155.8 | 207.8 | 259.7 | 311.7 |
Horizontal drain capacity — hydraulic check
Manning's equation for Schedule 40 flowing full. This is a sanity check, not the code answer — IPC Table 1106.3 governs the actual size.
| Size | Slope | Capacity flowing full | Velocity |
|---|---|---|---|
| 3" | 0.125 in/ft | 56 gpm | 2.42 ft/s |
| 3" | 0.25 in/ft | 79 gpm | 3.43 ft/s |
| 4" | 0.125 in/ft | 115 gpm | 2.91 ft/s |
| 4" | 0.25 in/ft | 163 gpm | 4.11 ft/s |
| 6" | 0.125 in/ft | 344 gpm | 3.82 ft/s |
| 6" | 0.25 in/ft | 486 gpm | 5.40 ft/s |
Sources & standards: the runoff conversion is derived from 7.48052 gallons per cubic foot, not quoted from a table. Design rainfall comes from the 100-year hourly rate — IPC Figure 1106.1 and NOAA precipitation frequency data. Pipe sizing is governed by IPC Tables 1106.2 and 1106.3, which are deliberately not reproduced here; the capacity column is Manning's equation as a hydraulic sanity check only. Secondary (overflow) drainage under IPC 1107 is a separate requirement. Roughly fifteen states use the UPC or a derivative. Local amendments override the model code.
Frequently asked questions
Common questions about roof runoff, rainfall rates, and storm drain sizing.
How much water comes off a roof in a storm?
More than people expect. A 2,000 ft² roof at a 4 in/hr design rainfall sheds 83.1 gpm — roughly what thirty showers use at once, off an ordinary house. The arithmetic is area × rainfall ÷ 96.25, and that divisor is derived here rather than quoted.
Where does the 96.25 divisor come from?
From unit conversion, not a table. One inch per hour falling on one square foot is 1/12 of a cubic foot per hour; a cubic foot is 7.48052 gallons; divide by 60 minutes and you get 0.0103896 gpm per ft² per in/hr. The reciprocal is 96.250. Published references usually round it to 96.23 — this computes it so you can see where it comes from.
What rainfall rate should I use?
The 100-year, one-hour rainfall for your location, which the IPC provides as a map (Figure 1106.1) and which NOAA also publishes. It ranges from roughly 2 in/hr in the Pacific Northwest to 5 in/hr or more on the Gulf coast. The examples in the dropdown are for orientation only — get the real figure for your site before designing anything.
Does this size the pipe to code?
No, and it says so deliberately. The code sizes storm drainage from IPC Table 1106.2 (vertical leaders) and Table 1106.3 (horizontal storm drains), which this calculator does not reproduce. What it gives you is the design flow — the number that is genuinely hard to work out — plus a Manning hydraulic check so a proposed size can be sanity-tested. Take the flow to the code tables for the actual size.
Why is a storm drain sized on full-bore flow when a sanitary drain is not?
Because the loads behave differently. A sanitary drain carries intermittent slugs with solids in them and is sized to run part-full so it scours. A storm drain carries clean water at a sustained rate during a storm, so it is designed to run full. That is why the capacity check here doubles the half-full figure rather than using it directly.
What about a flat roof or a parapet?
Flat roofs need secondary (overflow) drainage as well as primary — a separate system sized for the same flow, discharging somewhere visible so a blockage announces itself. A parapet turns a roof into a container: if the primary drain blocks and there is no overflow, water ponds, and ponded water is very heavy. This is a structural problem as much as a plumbing one.
Do I include walls that drain onto the roof?
Yes. Any vertical surface that sheds onto the roof adds to its effective area, and the code has a method for adding a share of adjacent wall area. Sloped roofs also project a smaller horizontal area than their actual surface — it is the horizontal projection that catches rain, so use the plan area, not the slope length.
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