Storm Drain Pipe Sizing Calculator
Find the right stormwater / drainage pipe size for your design flow. This tool uses Manning’s equation to compute the full-bore capacity of a circular storm drain from its diameter, slope and roughness — so you can pick the smallest standard pipe (8–36 in) that carries your peak runoff with a self-cleansing velocity. US customary units.
For educational purposes only. Not a substitute for professional engineering judgment.
Manning's equation is empirical and assumes uniform, steady flow conditions
Not valid for pressurized (surcharged) pipe flow - use pressure flow equations instead
Accuracy decreases for very smooth pipes (n < 0.010) or very rough pipes (n > 0.035)
Does not account for entrance/exit losses or other minor losses
Partial flow calculations assume free surface (atmospheric pressure at water surface)
Maximum Q/Qfull occurs at approximately y/D = 0.94, not at full flow
Air entrainment effects near the pipe crown are not considered
References formatted in APA 7th Edition style.
- Chow, V. (1959). Open-Channel Hydraulics. New York, NY: McGraw-Hill.textbook
- Federal Highway Administration (2009). Urban Drainage Design Manual (3rd ed.). Washington, DC: U.S. Department of Transportation. https://www.fhwa.dot.gov/engineering/hydraulics/pubs/10009/10009.pdfView Source manual
- United States Army Corps of Engineers (1994). Hydraulic Design of Flood Control Channels. Washington, DC: U.S. Army Corps of Engineers. https://www.publications.usace.army.mil/Portals/76/Publications/EngineerManuals/EM_1110-2-1601.pdfView Source manual
How to size a storm drain pipe
Sizing a storm drain is a two-step problem: first work out how much water the pipe must carry, then find the smallest standard pipe that carries it at an acceptable velocity and slope.
- Find the design flow (Q). For most sites the peak stormwater flow comes from the Rational Method, Q = CiA, where C is the runoff coefficient, i is the rainfall intensity (in/hr) for your design storm and time of concentration, and A is the drainage area in acres. The result is in cubic feet per second (cfs). Use the Rational Method calculator to get Q.
- Set the pipe slope. Match the pipe to the available ground slope, or set a minimum self-cleansing slope (see below). Storm drains run by gravity, so slope drives both capacity and velocity.
- Check standard sizes against the design flow. Enter a candidate diameter, your slope, and Manning’s n (0.013 for concrete or smooth-wall pipe) in the calculator above. It returns the full-bore capacity Qfull and the full-flow velocity. Pick the smallest standard size whose capacity exceeds your design Q with a sensible margin.
- Verify the velocity. Confirm the full-flow velocity is at least about 2–3 ft/s (self-cleansing) and not so high it risks erosion. Then confirm the size against your local storm-sewer standard — many agencies set a minimum public storm drain of 12 or 15 inches regardless of the calculated capacity.
Manning’s equation for a circular pipe is
Q = (1.486 / n) · A · R2/3 · S1/2,
where A is the flow area, R is the hydraulic radius (R = D/4 for a full pipe) and S is
the slope. The calculator on this page evaluates exactly this equation.
Standard storm drain pipe sizes & capacity
Full-bore capacity (Qfull) and velocity for common circular storm drain sizes, computed from Manning’s equation with n = 0.013 (concrete / smooth-wall RCP, HDPE, PVC) at two typical slopes. The last column is the slope at which the full pipe reaches a 2 ft/s self-cleansing velocity. Capacity scales with √slope, so doubling the slope multiplies capacity by about 1.41.
| Pipe dia. | Qfull @ 0.5% | V @ 0.5% | Qfull @ 1.0% | V @ 1.0% | Slope for 2 ft/s |
|---|---|---|---|---|---|
| 8 in | 0.9 cfs | 2.45 ft/s | 1.2 cfs | 3.46 ft/s | 1:300 |
| 10 in | 1.5 cfs | 2.84 ft/s | 2.2 cfs | 4.02 ft/s | 1:400 |
| 12 in | 2.5 cfs | 3.21 ft/s | 3.6 cfs | 4.54 ft/s | 1:515 |
| 15 in | 4.6 cfs | 3.72 ft/s | 6.5 cfs | 5.26 ft/s | 1:695 |
| 18 in | 7.4 cfs | 4.20 ft/s | 10.5 cfs | 5.94 ft/s | 1:885 |
| 21 in | 11.2 cfs | 4.66 ft/s | 15.8 cfs | 6.59 ft/s | 1:1085 |
| 24 in | 16.0 cfs | 5.09 ft/s | 22.6 cfs | 7.20 ft/s | 1:1300 |
| 30 in | 29.0 cfs | 5.91 ft/s | 41.0 cfs | 8.36 ft/s | 1:1745 |
| 36 in | 47.2 cfs | 6.67 ft/s | 66.7 cfs | 9.44 ft/s | 1:2225 |
Figures are full-flow (pipe flowing just full) and assume concrete/smooth-wall pipe at n = 0.013. Corrugated metal pipe (CMP, n ≈ 0.024) carries roughly 45% less at the same slope. Always confirm sizes and minimum diameters against your local storm-sewer design standard.
Where the design flow comes from: the Rational Method
A storm drain pipe is only as “right-sized” as the flow you size it for. For drainage areas up to roughly 200 acres, the Rational Method is the standard way to estimate the peak stormwater flow:
Q = C · i · A
- C — runoff coefficient (0.10 for lawns up to 0.95 for pavement)
- i — rainfall intensity in in/hr for your design storm (often the 10-year storm for storm drains) at the time of concentration
- A — contributing drainage area in acres
Because 1 acre-inch per hour happens to almost exactly equal 1 cfs, Q comes out directly in cfs — the same units this pipe calculator reports. Compute Q in the Rational Method calculator, then bring it here and size the pipe so Qfull comfortably exceeds it.
Self-cleansing velocity & minimum slope
Minimum velocity (self-cleansing)
A storm drain should run fast enough to keep grit and sediment moving. The widely used minimum is a full-flow velocity of about 2 ft/s, with many agencies requiring 2.5–3 ft/s. Below ~2 ft/s, sediment settles and the pipe slowly clogs. The calculator flags velocities under 2 ft/s.
Maximum velocity (erosion)
Very high velocities scour pipe walls and damage joints. Keep full-flow velocity below roughly 10 ft/s for normal materials (up to ~15 ft/s with abrasion- resistant pipe and energy dissipation at the outlet). The calculator warns above these thresholds.
Minimum slope follows from the minimum velocity: set the slope so the full pipe reaches at least 2–3 ft/s. The “Slope for 2 ft/s” column in the table above gives that value for each size — for example a 12-inch pipe needs about 1:515 (0.19%) and a 24-inch about 1:1300 (0.08%). Larger pipes self-cleanse at flatter slopes. Many jurisdictions also impose an absolute floor (commonly 0.5%) on small storm drains.
Worked example
Size a storm drain for a 0.75-acre parking lot. Runoff coefficient C = 0.90 (asphalt), design rainfall intensity i = 4.0 in/hr, drainage area A = 0.75 acres.
- Design flow (Rational Method): Q = C · i · A = 0.90 × 4.0 × 0.75 = 2.7 cfs.
- Available slope: the run can be laid at S = 0.5% (0.005 ft/ft) to match the site grade.
- Check a 12-inch pipe (concrete, n = 0.013) at 0.5% in the calculator: full-bore capacity Qfull ≈ 2.5 cfs at V ≈ 3.2 ft/s. That is just short of the 2.7 cfs design flow — undersized.
- Step up to a 15-inch pipe: Qfull ≈ 4.6 cfs at V ≈ 3.7 ft/s. Capacity now exceeds 2.7 cfs with margin, and the velocity is well above the 2 ft/s self-cleansing minimum.
- Result: use a 15-inch storm drain at 0.5%. (A 12-inch pipe would have worked if the slope were steepened to about 0.6%, lifting its capacity past 2.7 cfs — try it in the calculator.)
These capacities are computed by the calculator above from Q = (1.486/0.013) · A · R2/3 · S1/2.
Frequently asked questions
How do I size a storm drain pipe?
First estimate the peak design flow (Q) reaching the pipe, usually with the Rational Method, Q = CiA. Then use Manning’s equation to find the full-bore capacity of candidate pipe sizes at your available slope and pick the smallest standard size whose capacity comfortably exceeds Q. Finally check that the velocity stays roughly between 2 and 10 ft/s, and confirm the size against your local storm-sewer standards (many agencies set a minimum public storm drain of 12 or 15 inches).
What size storm drain pipe do I need?
It depends on the peak flow and the pipe slope, not just the area drained. As a rough guide for concrete or smooth-wall pipe at a 0.5% slope (n = 0.013), full-bore capacity is about 2.5 cfs for a 12-inch pipe, 7.4 cfs for an 18-inch, 16 cfs for a 24-inch, and 47 cfs for a 36-inch pipe. Compute your design flow first, then choose the size whose capacity exceeds it with margin. Use the calculator on this page to check any size at your exact slope.
What is the minimum slope for a storm drain pipe?
The minimum slope is whatever keeps the flow moving fast enough to be self-cleansing — typically a full-flow velocity of about 2 to 3 ft/s. For n = 0.013 pipe, a velocity of 2 ft/s at full bore needs roughly 1:300 (0.33%) for an 8-inch pipe, about 1:515 (0.19%) for a 12-inch, and about 1:1300 (0.08%) for a 24-inch. Larger pipes can run flatter and still self-cleanse. Many agencies also impose an absolute minimum such as 0.5% on small storm drains regardless of the velocity calculation.
What is the minimum velocity in a storm drain?
A full-flow velocity of about 2 ft/s (0.6 m/s) is the widely used minimum for self-cleansing in storm drains carrying grit and sediment; some agencies require 2.5 to 3 ft/s. Below roughly 2 ft/s, sediment settles and the pipe gradually clogs. This calculator flags velocities under 2 ft/s as a low-velocity warning.
What Manning’s n should I use for storm drain pipe?
Use n = 0.013 for concrete (RCP) and smooth-wall HDPE or PVC storm drain pipe — this is the standard design value and what the table on this page assumes. Corrugated metal pipe (CMP) is much rougher, around n = 0.024, so it carries less flow at the same slope. Always match Manning’s n to your actual pipe material.
Can this calculator size both storm sewers and culverts?
It sizes pipes by full-bore (gravity) capacity using Manning’s equation, which is the correct approach for storm sewers and for culverts flowing in inlet- or barrel-control by open-channel flow. It does not model pressurized (surcharged) flow or culvert headwater/tailwater effects — for those, use a dedicated culvert calculator. For most storm-drain sizing the full-bore Manning’s capacity is the governing check.
Why is my storm drain pipe flowing more than 80% full?
Storm drains are usually designed to flow full or just under full at the peak design storm, but flowing consistently above about 80% leaves little margin and risks surcharging (pressurized flow) if the storm exceeds the design event. If the calculator shows a high depth ratio, step up to the next standard pipe size or increase the slope to add capacity headroom.