DrainageCalculators

Storm Drain & Stormwater Pipe Sizing Calculator

Size storm drain and stormwater pipes from design flow, slope, and material. Compare RCP, HDPE, and CMP capacity using Manning’s equation.

Try a Common Scenario

Click to pre-fill the calculator with realistic values.

Parking-lot storm drain

Use this stormwater drainage calculator to find the right stormwater pipe size for your design flow. It applies Manning’s equation to the full-bore capacity of a circular storm drain from its diameter, slope and roughness — so you can screen standard pipe sizes (8–36 in) against your peak runoff. US customary units.

How this stormwater pipe sizing screen works

Enter design flow Q, pipe slope S, and material roughness. The calculator selects the smallest screened standard RCP, HDPE, or CMP diameter whose full-bore Manning capacity meets Q. Then verify velocity, the governing minimum diameter, cover, and the connected system’s hydraulic grade line. This is gravity-flow screening, not a surcharged-network model.

Sizing Mode

cfs

Peak flow the storm drain must convey; calculate it first with an approved hydrologic method.

Input Parameters

Pipe Properties

The calculator will screen standard 8–36 inch diameters.
ft/ft

Pipe slope (rise/run)

Roughness Coefficient

Select a material to auto-fill Manning's n, or enter a custom value below

Typical values: 0.009-0.015 for smooth pipes, 0.022-0.030 for corrugated metal

Don't know this value? Look it up

Flow Conditions

Enable to specify a flow depth less than the full pipe diameter

For educational purposes only. Not a substitute for professional engineering judgment.

Interactive model

Interactive partial-flow storm drain hydraulics

Change pipe diameter, slope, roughness, and relative depth to see how wetted geometry controls Q/Qfull and V/Vfull.

Open full model (opens in a new tab)

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.

  1. 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.
  2. 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.
  3. 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.
  4. 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.

Storm drain pipe size chart (capacity by diameter)

Full-bore capacity (Qfull) and velocity for common circular storm drain sizes, computed from Manning’s equation with n = 0.013 (concrete / RCP) at three typical slopes. Approximate metric equivalents are included for orientation; confirm the actual internal diameter. 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. Approx. metric Qfull @ 0.5% V @ 0.5% Qfull @ 1.0% V @ 1.0% Qfull @ 2.0% Slope for 2 ft/s
8 in 200 mm 0.9 cfs 2.45 ft/s 1.2 cfs 3.46 ft/s 1.7 cfs 1:300
10 in 250 mm 1.5 cfs 2.84 ft/s 2.2 cfs 4.02 ft/s 3.1 cfs 1:400
12 in 300 mm 2.5 cfs 3.21 ft/s 3.6 cfs 4.54 ft/s 5.0 cfs 1:515
15 in 375 mm 4.6 cfs 3.72 ft/s 6.5 cfs 5.26 ft/s 9.1 cfs 1:695
18 in 450 mm 7.4 cfs 4.20 ft/s 10.5 cfs 5.94 ft/s 14.8 cfs 1:885
21 in 525 mm 11.2 cfs 4.66 ft/s 15.8 cfs 6.59 ft/s 22.4 cfs 1:1085
24 in 600 mm 16.0 cfs 5.09 ft/s 22.6 cfs 7.20 ft/s 32.0 cfs 1:1300
30 in 750 mm 29.0 cfs 5.91 ft/s 41.0 cfs 8.36 ft/s 58.0 cfs 1:1745
36 in 900 mm 47.2 cfs 6.67 ft/s 66.7 cfs 9.44 ft/s 94.3 cfs 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.

Capacity by material and slope: RCP, HDPE, and CMP

The same nominal diameter can carry very different flow when its interior roughness changes. Values below are full-bore screening capacities in cfs; use the calculator for another n value or exact internal diameter.

Material Manning’s n Diameter 0.5% 1.0% 2.0%
RCP / concrete 0.013 8 in / ~200 mm 0.9 cfs 1.2 cfs 1.7 cfs
RCP / concrete 0.013 10 in / ~250 mm 1.5 cfs 2.2 cfs 3.1 cfs
RCP / concrete 0.013 12 in / ~300 mm 2.5 cfs 3.6 cfs 5.0 cfs
RCP / concrete 0.013 15 in / ~375 mm 4.6 cfs 6.5 cfs 9.1 cfs
RCP / concrete 0.013 18 in / ~450 mm 7.4 cfs 10.5 cfs 14.9 cfs
RCP / concrete 0.013 21 in / ~525 mm 11.2 cfs 15.8 cfs 22.4 cfs
RCP / concrete 0.013 24 in / ~600 mm 16.0 cfs 22.6 cfs 32.0 cfs
RCP / concrete 0.013 30 in / ~750 mm 29.0 cfs 41.0 cfs 58.0 cfs
RCP / concrete 0.013 36 in / ~900 mm 47.2 cfs 66.7 cfs 94.3 cfs
Smooth-interior HDPE 0.012 8 in / ~200 mm 0.9 cfs 1.3 cfs 1.9 cfs
Smooth-interior HDPE 0.012 10 in / ~250 mm 1.7 cfs 2.4 cfs 3.4 cfs
Smooth-interior HDPE 0.012 12 in / ~300 mm 2.7 cfs 3.9 cfs 5.5 cfs
Smooth-interior HDPE 0.012 15 in / ~375 mm 4.9 cfs 7.0 cfs 9.9 cfs
Smooth-interior HDPE 0.012 18 in / ~450 mm 8.0 cfs 11.4 cfs 16.1 cfs
Smooth-interior HDPE 0.012 21 in / ~525 mm 12.1 cfs 17.2 cfs 24.3 cfs
Smooth-interior HDPE 0.012 24 in / ~600 mm 17.3 cfs 24.5 cfs 34.7 cfs
Smooth-interior HDPE 0.012 30 in / ~750 mm 31.4 cfs 44.4 cfs 62.8 cfs
Smooth-interior HDPE 0.012 36 in / ~900 mm 51.1 cfs 72.3 cfs 102.2 cfs
Annular CMP 0.024 8 in / ~200 mm 0.5 cfs 0.7 cfs 0.9 cfs
Annular CMP 0.024 10 in / ~250 mm 0.8 cfs 1.2 cfs 1.7 cfs
Annular CMP 0.024 12 in / ~300 mm 1.4 cfs 1.9 cfs 2.7 cfs
Annular CMP 0.024 15 in / ~375 mm 2.5 cfs 3.5 cfs 4.9 cfs
Annular CMP 0.024 18 in / ~450 mm 4.0 cfs 5.7 cfs 8.0 cfs
Annular CMP 0.024 21 in / ~525 mm 6.1 cfs 8.6 cfs 12.1 cfs
Annular CMP 0.024 24 in / ~600 mm 8.7 cfs 12.3 cfs 17.3 cfs
Annular CMP 0.024 30 in / ~750 mm 15.7 cfs 22.2 cfs 31.4 cfs
Annular CMP 0.024 36 in / ~900 mm 25.5 cfs 36.1 cfs 51.1 cfs

RCP uses n = 0.013, smooth-interior HDPE n = 0.012, and annular CMP n = 0.024. Source basis: FHWA HEC-22. Material condition, joints, corrugation, and local design criteria can require another value.

Circular pipe partial-flow ratios: y/D, Q/Qfull, and V/Vfull

For a circular pipe at the same slope and Manning’s n, relative capacity depends only on relative depth. Discharge reaches slightly more than full-flow capacity near y/D = 0.94 because hydraulic radius peaks before the water surface reaches the crown.

y/D A/Afull V/Vfull Q/Qfull
0.05 0.019 0.257 0.005
0.10 0.052 0.401 0.021
0.15 0.094 0.517 0.049
0.20 0.142 0.615 0.088
0.25 0.196 0.701 0.137
0.30 0.252 0.776 0.196
0.35 0.312 0.843 0.263
0.40 0.374 0.902 0.337
0.45 0.436 0.954 0.417
0.50 0.500 1.000 0.500
0.55 0.564 1.039 0.586
0.60 0.626 1.072 0.672
0.65 0.688 1.099 0.756
0.70 0.748 1.120 0.837
0.75 0.804 1.133 0.912
0.80 0.858 1.140 0.977
0.85 0.906 1.137 1.030
0.90 0.948 1.124 1.066
0.95 0.981 1.095 1.075
1.00 1.000 1.000 1.000

Geometry uses θ = 2 cos−1(1 − 2y/D), A = (D2/8)(θ − sinθ), P = Dθ/2, and R = A/P; Manning ratios then follow at fixed n and S. See Chow (1959) and FHWA HEC-22.

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.

  1. Design flow (Rational Method): Q = C · i · A = 0.90 × 4.0 × 0.75 = 2.7 cfs.
  2. Available slope: the run can be laid at S = 0.5% (0.005 ft/ft) to match the site grade.
  3. 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.
  4. 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.
  5. 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 storm-sewer pipes by full-bore gravity capacity using Manning’s equation. For culverts, that calculation is only a screening check of barrel capacity and the friction side of outlet control; inlet control and headwater require a separate FHWA HDS-5 evaluation using the inlet geometry and headwater conditions. It also does not model pressurized (surcharged) storm-sewer flow. Use the dedicated culvert tools for culvert-specific checks.

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.

Related calculators

Rational Method Calculator

Estimate the peak stormwater flow (Q = CiA) that your storm drain must carry.

Manning’s Pipe Flow Calculator

The full Manning’s pipe tool, including partial-flow (less-than-full) depth analysis.

Culvert Capacity Calculator

Screen circular culvert barrel sizes by Manning full-flow capacity, with culvert-specific limits and guidance.

Professional Review: For projects requiring regulatory approval or where public safety is involved, have your calculations reviewed and stamped by a licensed Professional Engineer (PE).

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Last verified: February 2026