Storm Pipe Sizing Calculator

Calculator overview

Inputs and outputs

This summary comes from the calculator's published input and output contract.

Inputs

Selected Pipe Diameter: About this input

The trial pipe diameter you want to check, in inches. The tool reports its capacity and also recommends the smallest adequate size.

Unit in Default 24 Range At least 0
Slope: About this input

The pipe slope, as a fall in feet per foot of length. For example 0.005 is a half-percent grade.

Unit ft/ft Default 0.005 Range At least 0
Pipe Material: About this input

The pipe material, such as concrete, PVC or corrugated metal. It sets the Manning roughness coefficient. The listed roughness values are illustrative and must be confirmed against the governing drainage standard.

Default Concrete pipe, ordinary Allowed Concrete pipe, smooth, Concrete pipe, ordinary, PVC or HDPE, smooth interior, HDPE, corrugated interior, Corrugated metal, annular, Corrugated metal, helical, Vitrified clay, Cast iron
Design Flow: About this input

The peak flow the pipe must carry, in cubic feet per second, typically the runoff from the contributing area.

Unit cfs Default 12 Range At least 0
Length: About this input

The length of the pipe run, in feet, used to report the fall from inlet to outlet.

Unit ft Default 120 Range At least 0

Outputs

Selected Pipe Area: About this output

The full cross-sectional area of the selected pipe, in square feet.

Unit sf
Selected Fullflow Velocity: About this output

The velocity in the selected pipe when flowing full, in feet per second.

Unit ft/s
Selected Fullflow Capacity: About this output

The flow the selected pipe carries when flowing full, in cubic feet per second, from Manning's equation.

Unit cfs
Selected Size Is Adequate: About this output

Whether the selected pipe carries the design flow within its full-flow capacity. Reads no when the design flow exceeds capacity.

Unit yes/no
Velocity Within Limits: About this output

Whether the recommended size would run within the usual velocity limits for the material, 2 to 12 ft/s, at FULL flow. It is a capacity-side check on the recommendation, not the operating condition; the design-flow condition is the separate self-cleansing check.

Unit yes/no
Velocity At The Design Flow: About this output

The design flow divided by the full cross-section of the selected pipe, in feet per second: a conservative lower bound on the true velocity at partial depth. This is the number the self-cleansing check reads; the true partial-depth velocity is somewhat higher at low fills.

Unit ft/s
Selfcleansing At Design Flow: About this output

Whether the design-flow velocity estimate in the selected pipe meets the self-cleansing minimum of 2 ft/s. The estimate is the design flow over the full cross-section, a strict lower bound on the true partial-depth velocity: a Yes is therefore reliable, and a No is conservative -- check the partial-depth hydraulics before concluding the pipe will silt.

No unit declared
Recommended Pipe Diameter: About this output

The smallest standard pipe diameter whose full-flow capacity carries the design flow at the given slope, in inches. This is a sizing aid, not a design or a check of one, and storm drainage design must be performed and sealed by a licensed engineer to the codes adopted where the work is built.

Unit in
Its Capacity: About this output

The full-flow capacity of the recommended diameter, in cubic feet per second.

Unit cfs
Fall Over The Length: About this output

The drop in pipe invert from inlet to outlet, in feet: slope times length.

Unit ft
Capacity Utilisation: About this output

The design flow as a percent of the selected pipe's full-flow capacity. Above 100 percent the pipe surcharges.

Unit %
Its Fullflow Velocity: About this output

The full-flow velocity of the recommended diameter, in feet per second.

Unit ft/s
Model Status: About this output

The overall check on your entries, shown above the results. It reads OK when the inputs are usable, NOT VALID with a reason when an entry makes the model meaningless, or CHECK with a reason when a result is valid but worth a second look. Read it before you trust the numbers below.

No unit declared
Manning N In Use: About this output

The Manning roughness coefficient for the chosen material used in the flow calculation. Smoother pipe has a lower value and carries more flow.

Unit n
Its Utilisation At The Design Flow: About this output

The design flow as a percent of the recommended pipe's full-flow capacity, showing the headroom the recommended size leaves.

Unit %

What it is

The Storm Pipe Sizing Calculator checks a gravity storm drain against a design flow and recommends a size. You give it a trial diameter, the pipe material, the slope and the flow the pipe must carry, and it reports the full-flow capacity and velocity of that trial pipe by Manning's equation, whether it is adequate, and the smallest standard diameter that would carry the flow.

It works in United States customary units: inches for diameter, feet per foot for slope, feet for the run, and cubic feet per second for flow.

Use it to size or check a gravity storm pipe under uniform flow. It is a full-flow calculation with a conservative estimate of the velocity at the design flow, and it models nothing about the wider drainage network.

Methodology

Purpose and model boundary

This model calculates full-flow gravity capacity for a circular storm pipe with the U.S. customary Manning equation, checks a selected diameter, and identifies the smallest adequate diameter in a fixed standard-size table. It supports preliminary sizing under steady uniform flow. It does not solve a partially full pipe, hydraulic grade line, inlet system, or storm-drain network.

The spreadsheet remains the calculation authority. The page sends the named inputs to the workbook and presents the returned values, size comparison, and status without recreating the equations in browser code.

Inputs and units

Input	Meaning and unit
Pipe material	One of eight workbook choices that selects Manning roughness `n`.
Design flow, `Q_d`	Cubic feet per second.
Slope, `S`	Pipe fall divided by horizontal length, in feet per foot.
Selected diameter, `D`	Inches for the user's candidate pipe.
Length, `L`	Feet used only to report total fall and a travel-time estimate.

Governing relationships

The selected diameter is converted to feet. For a full circular pipe:

A = πD² / 4

R = D / 4

The workbook applies the U.S. customary Manning equation:

Q_full = (1.486 / n) × A × R^(2/3) × sqrt(S)

and:

full-flow velocity: V_full = Q_full / A;
capacity utilization: Q_d / Q_full;
nominal design-flow velocity: V_nominal = Q_d / A;
fall over the entered run: S × L;
full-flow travel time: L / V_full / 60 minutes.

The recommended diameter is the first size in the workbook's 12-through-72-inch standard table whose full-flow capacity is at least the design flow. Its capacity, full-flow velocity, and utilization are calculated with the same relationships.

Calculation sequence

Material selection looks up Manning n.
Diameter, slope, and n produce the selected pipe's full-flow area, hydraulic radius, capacity, and velocity.
The selected capacity is compared with design flow.
The workbook evaluates every standard table diameter and selects the first adequate size.
It calculates the recommended size's full-flow velocity and utilization and plots capacity by size against the design-flow reference.
It evaluates the conservative nominal self-cleansing indicator and then Model_Status in the precedence shown below.

Outputs and interpretation

Selected_Fullflow_Capacity and Selected_Size_Is_Adequate describe the user-entered diameter. Recommended_Pipe_Diameter is the smallest adequate size in the fixed table; it is blank when none qualifies. Velocity_Within_Limits refers to the recommended pipe's full-flow velocity against the workbook limits. Velocity_At_The_Design_Flow is Q_d divided by the full circular area, not a solved partial-depth velocity. Selfcleansing_At_Design_Flow uses that value as a conservative lower-bound screen: a No does not prove actual partial-depth velocity is below the minimum.

Validation and status logic

The workbook evaluates status in this order:

Condition	Returned status
Design flow is less than or equal to zero	`NOT VALID: design flow must be greater than zero`
Slope is less than or equal to zero	`NOT VALID: slope must be greater than zero`
Selected diameter is less than or equal to zero	`NOT VALID: the selected diameter must be greater than zero`
No diameter in the workbook's size table carries the design flow	`NOT VALID: no size in the table carries the design flow`
The selected diameter's capacity is below design flow	`CHECK: the selected size is too small; see the recommended size`
The recommended size's full-flow velocity is outside the workbook limits	`CHECK: the recommended size gives a velocity outside the limits`
`Q_d / A` for the recommended size is below the self-cleansing minimum	`CHECK: at the design flow the water moves below the self-cleansing minimum; the pipe may silt`
None of the preceding conditions applies	`OK`

The ordering matters: inadequate selected size is reported before either velocity message.

Assumptions and limitations

Flow is steady, uniform, gravity-driven, and evaluated at full circular depth with one constant Manning roughness.
The model does not solve partial-depth geometry. Actual operating velocity at a flow below full capacity can differ materially from Q_d / A.
The workbook omits inlet control, outlet control, headwater, tailwater, backwater, surcharge, pressure flow, entrance and junction losses, bends, debris, sediment, and hydraulic-grade-line interaction.
Roughness is a representative material value; age, joints, deposition, condition, and local specification can change it.
The size recommendation is limited to the stored 12-through-72-inch table and does not consider availability, cover, structural class, loading, minimum diameter, or local criteria.
Velocity checks are screening references, not erosion, abrasion, or sediment-transport design.

Restrictions and non-computing states

This calculator restricts material to eight declared choices and numeric inputs to nonnegative values. The workbook requires positive flow, slope, and diameter. A request whose required capacity exceeds the largest stored pipe has no recommendation and is non-computing for selection. Length may be zero, in which case fall and travel time are zero without changing hydraulic capacity. Protected formulas may display zero in invalid states; those values are not a valid size decision.

Errors and warnings

A rejected entry means the request did not satisfy the published input rules and no workbook sizing decision was made. A workbook NOT VALID status identifies a missing hydraulic basis or a flow beyond the table. CHECK means the calculation completed but selected-size adequacy or a velocity screen needs review. A connection or calculation-service failure is an availability problem, not a drainage conclusion.

References

The workbook derives its relations rather than reproducing any table, chart or figure from a specification, standard or agency publication. The capacity calculation is Manning's equation in its U.S. customary form, applied to a full circular section, and the standard-size list is an editable internal table.

Federal Highway Administration. Design Charts for Open-Channel Flow, Hydraulic Design Series No. 3 (HDS-3), which gives the Manning velocity relation and Q = A V, and explicitly treats a partly full pipe as open-channel flow. https://www.fhwa.dot.gov/engineering/hydraulics/library_listing.cfm
Federal Highway Administration. Urban Drainage Design Manual, Hydraulic Engineering Circular No. 22 (HEC-22), for storm drain design practice including velocity criteria.
Wikipedia. Manning formula. https://en.wikipedia.org/wiki/Manning_formula

Manning roughness coefficients shipped with the workbook are illustrative typical values and carry no authority. Values for design must come from the governing drainage manual, the pipe manufacturer, or local design data.

Additional source notes migrated from Methodology

The delivered audit reconciles the full-pipe Manning relationship with FHWA HDS-3, Design Charts for Open-Channel Flow and records the Engineering ToolBox Manning gravity-flow calculator as a comparator. Applicable drainage manuals, local criteria, and a qualified engineer govern real storm-drain design.

Frequently asked questions

Why are there two velocities, and which one should I check?

They answer different questions. The full-flow velocity is what the pipe does at capacity and is the figure conventionally compared against the 2 to 12 feet per second limits; the velocity-limit output refers to that condition. The velocity at the design flow is the design flow divided by the full cross-section, which is a lower bound rather than a true velocity. Check the full-flow velocity against the limits, and read the design-flow figure as a conservative self-cleansing indicator.

Why is the design-flow velocity only a lower bound?

Because a pipe carrying less than its capacity does not run full. It runs partly full, as an open channel, over a smaller flow area, so the actual velocity is higher than the design flow divided by the full area. Solving the real value needs partial-depth open-channel geometry, which this tool does not carry. Reporting the bound honestly is preferred to reporting a number that would overclaim.

My pipe says yes for self-cleansing. Can I rely on it?

Yes, and that is deliberate. Because the estimate can only understate the true velocity, a yes means the real velocity is at least the self-cleansing minimum and probably higher. A no is the softer verdict: it means the conservative estimate fell short, so a proper partial-depth check is warranted, not that the pipe will certainly silt.

The pipe is well within capacity, so why is it warned for silting?

Because those are opposite concerns. A generously oversized pipe has plenty of capacity but spreads a modest design flow thinly, so the water moves slowly and sediment settles. Utilisation and self-cleansing pull in different directions, which is why both are reported. Dropping to the recommended diameter usually fixes a silting warning.

What does the recommended diameter actually consider?

Capacity alone. It is the smallest standard size in the 12 to 72 inch table whose full-flow capacity carries your design flow at the slope you entered. It does not consider cover, bedding, structural loading, constructability, minimum-slope rules, or what is already installed upstream and downstream. It is a starting point for a size, not a selection.

Can I use this to design a storm drain network?

No. This is one pipe under steady uniform flow. It models nothing about the hydraulic or energy grade line, inlet and outlet control, junction and manhole losses, bends, surcharge, backwater from a downstream constraint, or tailwater. Any of those can govern in a real network, and a pipe that passes this check can still fail to perform once it is connected.

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This page is provided by LogicCommons for informational purposes only. Results are analysis outputs computed from the inputs you supply and are not engineering advice, a design, or a substitute for review by a licensed professional under the codes adopted where the work is built. Verify all inputs and results independently.

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