TOOL GUIDE
Water pipe sizing
Size/check internal diameters for pressurized water or Manning gravity drainage.
Purpose & scope
Size/check internal diameters for pressurized water or Manning gravity drainage.
Pure water; does not replace glycol, surge or branched-network checks. Manning assumes steady uniform partially full circular-pipe flow; maximum depth ratio 10–80%.
Inputs
Use drawings, design conditions and manufacturer data. Units and example values follow the calculator; example values are not universal design requirements.
| Parameter | Value / interpretation |
|---|---|
| System / design preset · mm | Select the material/system appropriate to the actual application; verify reference values. |
| Water flow rate | Volumetric flow at the calculation location; check the selected unit. |
| Internal diameter · mm | Actual internal diameter from pipe dimensions/wall thickness, not DN. |
| Material · mm | Select the material/system appropriate to the actual application; verify reference values. |
| Maximum velocity · m/s | Project maximum velocity, not a universal limit. |
| Maximum friction loss · Pa/m | Maximum friction loss per metre, in Pa/m. |
| Water temperature · °C | Water temperature used for fluid properties. |
| Absolute roughness ε · mm | Absolute roughness of the internal pipe/duct surface, in mm. |
| Pipe section length · m | Actual section length; include both supply and return where required. |
| Slope · % | Longitudinal gravity-pipe slope; the field uses %, so enter 1 for 1%. |
| Maximum depth y/D · % | Water depth/internal diameter ratio y/D, a depth limit rather than a flow rate. |
| Manning n | Manning roughness coefficient for material/condition; distinct from Darcy absolute roughness ε. |
| Minimum velocity · m/s | Minimum velocity required by the drainage design. |
| Maximum velocity · m/s | Select the material/system appropriate to the actual application; verify reference values. |
Steps
- Select chilled water, pressurized water supply or gravity drainage before entering data.
- For pressure pipes use internal diameter, temperature, roughness and velocity/Pa/m limits. For drainage use slope, n, depth ratio and velocity limits.
- Select a candidate or check an existing pipe, review warnings and save each section.
Worked example
This reproducible training example uses the values below. Select the appropriate mode and overwrite restored project values. Example rates, K values and equipment losses are assumptions, not mandatory design criteria.
Examples use a decimal point and no thousands separator.
All example inputs
| Parameter | Value / interpretation |
|---|---|
| System | Chiller / pressure |
| Flow | 10 |
| Unit | m³/h |
| Internal diameter | 100 mm |
| Temperature | 7 °C |
| Roughness | 0.045 mm |
| Length | 10 m |
| Maximum velocity | 2 m/s |
| Maximum friction rate | 300 Pa/m |
Use check mode for the main example, not automatic size selection. The water example first uses the pressurized chilled-water preset; its second example uses gravity drainage.
Expected results (rounded)
| Parameter | Value / interpretation |
|---|---|
| Velocity | 0.35 m/s |
| Friction loss rate | 16.01 Pa/m |
| Section loss | 0.16 kPa |
| Gravity example: internal D 150 mm, Q 10 m³/h, slope 1%, n 0.013, maximum depth ratio 70% → actual depth ratio | 28.92 % |
| Gravity velocity | 0.66 m/s |
Small differences in displayed rounding are normal. A result that passes the entered limits does not establish suitability for the complete system.
Method & limitations
Pure water; does not replace glycol, surge or branched-network checks. Manning assumes steady uniform partially full circular-pipe flow; maximum depth ratio 10–80%.
Pressurized pipes: V = Q/A; Re = ρVD/μ; R = fρV²/(2D). Darcy factor uses 64/Re for laminar flow and Colebrook for Re ≥ 4,000. The 2,300–4,000 transition region is interpolated and flagged. Density and viscosity are approximated for pure water at 1–80 °C.
Gravity drainage: Q = A·Rh^(2/3)·√S/n, with Rh = A/P. Normal flow depth in a circular pipe is solved from flow, slope and Manning n. Maximum depth is limited to 10–80% to use the monotonic branch. Only pipes with sufficient capacity and velocities within the minimum and maximum limits are retained.
Straight pipe sections only; excludes fittings, valves, equipment, elevation changes, required residual pressure, unsteady flow and branched networks. Material and design presets are editable reference values.
EPA EPANET — pressurized pipes · EPA SWMM — drainage hydraulicsCommon mistakes
Entering DN instead of internal diameter; misreading slope units; assuming hydraulic capacity verifies venting or the whole network.
Projects & reports
- Choose or create an engineering project and enter your actual design inputs.
- Project inputs/results are autosaved in this browser. Where a saved-list panel is available, name and add the selected section or alternative to compare multiple options.
- Review the report before exporting PDF. In the print dialog select Save as PDF. Export JSON for backup and use Open file to restore it; PDF cannot restore project data.
Saved results do not automatically recalculate when another tool changes. Review any dependency warning, recalculate and update the affected results.
References & related tools
The tool implements the methods and limits stated above; linking a standard does not establish complete compliance with that standard.