Drainage Pipe Sizing Calculator

Formula used

This calculator sizes a circular drainage pipe assuming full flow using the Manning equation:

• Q = (1/n) · A · R^(2/3) · S^(1/2)

Where Q is flow rate, n is roughness, A is area, R is hydraulic radius, and S is slope (m/m). For a full circular pipe: A = πD²/4 and R = D/4.

The calculator solves for the minimum diameter D needed to convey the design flow at the provided slope and roughness, then recommends the next standard size.

How to use this calculator

1. Enter the design flow and select its unit.
2. Enter the pipe slope and choose percent or m/m format.
3. Select a material preset, then confirm the roughness value.
4. Optionally set minimum and maximum velocity checks.
5. Click Calculate to view required and standard pipe sizes.
6. Use the download buttons to export results for reports.

Professional notes

1) Purpose of drainage pipe sizing

Drainage lines must pass peak flow without surcharge, while maintaining practical slopes and constructible sizes. This calculator estimates a minimum circular diameter for full-flow conveyance using Manning hydraulics, then recommends the next standard size for procurement and detailing. It is useful for storm drains, site collection lines, and gravity outfalls where open-channel behavior is expected.

2) Key inputs and typical data

Design flow should represent the governing event or fixture demand. You may enter L/s, m³/s, m³/h, gpm, or cfs. Slope is entered as percent or m/m; for example, 1.0% equals 0.010 m/m. Roughness (Manning n) depends on material and condition: smooth concrete is often around 0.013, PVC about 0.009, and corrugated metal can exceed 0.020.

3) Manning method and assumptions

Manning’s equation relates flow to area, hydraulic radius, and slope. This tool applies the full-flow geometry for a circular pipe (A = πD²/4 and R = D/4). Full-flow sizing is conservative for many gravity drains that run partially full, but it provides a clear baseline for selecting a safe nominal diameter.

4) Velocity checks for performance

After a standard size is selected, the calculator reports full-flow velocity and allows optional limits. A minimum velocity supports self-cleansing and reduces sedimentation, while a maximum velocity helps control abrasion, lining damage, and noise. Set targets that match your specifications, soil conditions, and permissible energy.

5) Using results in design documentation

Record the chosen diameter, slope, and roughness basis in calculation sheets. For example, a site line carrying 120 L/s at 1.0% slope with n = 0.013 typically selects around 300 mm, then confirms velocity is within limits. Export CSV for quick logs and PDF for submittals, then verify with profiles, cover, and local standards.

FAQs

1) Does this size for full flow or partial flow?

It sizes a circular pipe assuming full-flow geometry. Many gravity drains run partially full, but full-flow sizing provides a conservative diameter baseline for early design and checks.

2) What slope should I enter?

Enter the longitudinal grade of the pipe invert. Use percent (%) if you have grade values, or m/m if your design notes use ratios. Keep the value positive.

3) How do I choose Manning n?

Select a preset close to your material and condition, then adjust if your specification differs. Rougher interiors require higher n, which increases the required diameter.

4) Why does it recommend a “standard” diameter?

Construction typically uses stocked nominal sizes. The tool rounds up to the next common diameter so the selected pipe meets or exceeds the computed minimum capacity.

5) What do the velocity limits mean?

Minimum velocity helps reduce sediment deposition. Maximum velocity helps reduce abrasion and potential damage. Use limits from your agency, client criteria, or pipe manufacturer guidance.

6) Can I use this for pressurized systems?

No. This is intended for gravity drainage using Manning open-channel assumptions. For pressurized flow, use Hazen–Williams or Darcy–Weisbach with appropriate loss modeling.

7) Why might my field result differ from the calculator?

Real systems include bends, manholes, entrance losses, partial-flow behavior, sediment, and aging. Treat this as a sizing baseline, then confirm with detailed hydraulic and profile checks.