Calculation sequence
Step-by-step method
- Convert I-P diameter from inches to feet when required.
- Calculate full circular area and hydraulic radius.
- Apply the Manning equation and divide discharge by area for velocity.
PlumbingAdvancedSanitary and Storm Drainage
Manning Gravity-Drain Capacity Calculator
Estimate full-flow circular-pipe discharge and velocity using the Manning equation.
Why it matters: A transparent hydraulic screen is useful only when full-flow, roughness, slope, and boundary assumptions are explicit.
Calculated result
Engineering evaluation
Full-flow discharge (Q)0.561 cfs
- Full-flow discharge (Qˢ)
- 251.842 gpm
- Full-flow area (A)
- 0.196 ft²
- Hydraulic radius (Rʰ)
- 0.125 ft
- Mean velocity (v)
- 2.858 ft/s
Engineering interpretation
The entered full-flow Manning basis yields 0.5611066 cfs at 2.857692 ft/s.
Important limitations
- Full uniform gravity flow only. Do not use this screen as a code pipe-size table. Partial depth, inlet/outlet control, backwater, surcharge, blockage, local losses, venting, roughness selection, and adopted-code criteria require review.
Engineering knowledge center
Understand the calculation—not just the answer
Governing relationships
A=πD²/4
Rʰ=D/4
Q=(K/n)A(Rʰ)^(2/3)S^(1/2), K=1.486 I-P or 1 SI
Qˢ=448.83117Q (I-P) or 1000Q (SI)
v=Q/A
Current substitution
D=6 in; A=0.1963495; Rʰ=0.125; Q=0.5611066 cfs; Qˢ=251.8421 gpm; v=2.857692 ft/s; n=0.013; S=0.01.
Equation legend
Inputs and calculated quantities
- D
- Pipe internal diameter — in or m
- S
- Hydraulic slope — ft/ft or m/m
- n
- Manning roughness coefficient — dimensionless
- Q
- Full-flow discharge — cfs
- Qˢ
- Full-flow discharge — gpm
- A
- Full-flow area — ft²
- Rʰ
- Hydraulic radius — ft
- v
- Mean velocity — ft/s
Applications
How this calculation is used
Building design
Screen a full-flow circular-pipe capacity.
Construction
Check diameter, slope, and roughness inputs against approved documents.
Commissioning
Document a theoretical comparison without implying code sizing.
Quality controls
Assumptions and limitations
Assumptions
- Entered values describe one documented system and use a consistent unit, energy, pressure, elevation, and measurement basis.
- Measurements are representative and synchronized where the calculation combines field readings.
Limitations
- Uniform steady full gravity flow is assumed; inlet, outlet, backwater, surcharge, partial depth, local losses, and blockage are omitted.
- Sanitary drains normally operate partially full and adopted code tables or another approved method may govern.
Field use
Verification procedure
- Confirm the calculation boundary, source documents, formula symbols, and units.
- Record representative field values or approved project inputs and their sources.
- Review the result against the applicable code, criteria, manufacturer data, and measurement uncertainty.
Common engineering mistakes
- Combining values from different system boundaries, operating periods, temperature bases, or unit conventions.
- Treating a calculated screening value as a hidden pass/fail threshold or final design decision.
Technical basis
References and source standards
- International Plumbing Code, 2024The legally adopted edition and local amendments govern water, sanitary, storm, sump, and pumping requirements; protected code tables are not reproduced.
- American Society of Plumbing EngineersPlumbing design and engineering-method context; verify the current licensed reference and project criteria during professional review.
- NIST SI and Unit Conversion GuidanceUnit symbols, pressure, flow, energy, temperature, and dimensional conversions used by the application.
Engineering calculation disclaimer
These engineering tools are provided for educational, preliminary evaluation, field verification, and commissioning support. Results depend on the accuracy of user-entered information and the assumptions stated for each calculation. Every colored status and comparison is a screening indicator only; project criteria, contract requirements, applicable codes, manufacturer instructions, measurement uncertainty, and authorized engineering judgment govern. The tools do not replace project-specific engineering analysis or the judgment of a licensed professional engineer.
Read the full use and limitation statement