Calculator
Example Data Table
| Scenario | Flow | Length | Limits | Material C | Suggested Size |
|---|---|---|---|---|---|
| Domestic riser | 2.5 L/s | 60 m | V≤2.0 m/s, h≤6 m | 150 | 50 mm |
| Equipment feed | 1.2 L/s | 35 m | V≤1.8 m/s, h≤4 m | 140 | 40 mm |
| Fire branch | 500 GPM | 180 ft | V≤3.0 m/s, h≤8 m | 120 | 150 mm |
Formula Used
- Velocity:
V = 4Q / (πD²) - Velocity-based diameter:
D = √(4Q / (πV_allow)) - Hazen–Williams headloss (metric):
h_f = 10.67 · L · Q^1.852 / (C^1.852 · D^4.871) - Pressure drop from headloss:
ΔP = ρ g h_f
How to Use This Calculator
- Enter the expected flow rate and choose its unit.
- Enter pipe length and select meters or feet.
- Select a sizing method: velocity, headloss, or both.
- Set allowable velocity and allowable headloss or pressure drop.
- Choose a Hazen–Williams C value for your pipe material.
- Add a minor loss allowance to represent fittings.
- Click Calculate to see the recommended pipe size.
- Use Download buttons to save CSV or PDF results.
Design intent and common constraints
Pipe sizing balances deliverable flow, acceptable friction losses, and stable operation. In buildings, designers often limit velocity to control noise, erosion, and water hammer risk, while also keeping headloss within available pump head or supply pressure. This calculator checks both limits and recommends the smallest standard diameter that satisfies your selected criteria.
How flow and safety factors affect diameter
Flow is converted to a consistent unit and then increased by a safety factor to cover peak demand, future expansion, and uncertainty. Diameter is strongly influenced by flow: if design flow increases, velocity rises for the same pipe, and headloss increases even faster. Applying a realistic safety factor early helps avoid undersized mains and costly rework during commissioning.
Hazen–Williams inputs and material impact
The Hazen–Williams roughness coefficient (C) represents internal smoothness and aging. Higher C values reduce calculated headloss for a given diameter and flow. Use project specifications where possible, and consider lower C for older or tuberculated lines. Minor losses are approximated by adding an equivalent-length percentage to the straight run.
Interpreting the results for construction decisions
Review the recommended size along with the resulting velocity, headloss, and pressure drop. If the size meets criteria, it typically supports stable service levels at the stated length and fittings allowance. If no size meets criteria, you can increase diameter, reduce length or fittings, select a smoother material, or adjust limits to match realistic site conditions and authority requirements.
Example data for a quick check
Example input set: flow 2.5 L/s, length 60 m, safety factor 10%, minor loss allowance 15%, allowable velocity 2.0 m/s, allowable headloss 6 m, and C value 150. The candidate table shows checked diameters and flags which sizes satisfy the chosen limits. Export the CSV or PDF output to attach calculations to RFIs, submittals, and QA records.
FAQs
1) What diameter does the calculator recommend?
It recommends the smallest standard nominal size that meets your selected velocity and/or headloss limits using the design flow and equivalent length you provided.
2) Why does headloss change so much with diameter?
Friction loss is highly sensitive to diameter. A small increase in diameter can significantly reduce headloss, especially at higher flows, improving pressure at fixtures.
3) Should I use velocity, headloss, or both?
Use both when you want balanced performance. Velocity helps control noise and surge effects, while headloss ensures adequate pressure remains after friction losses.
4) What does the minor loss allowance represent?
It approximates fittings, valves, bends, and transitions by increasing the straight length to an equivalent length, capturing extra losses without detailed component modeling.
5) How do I choose the Hazen–Williams C value?
Use published or specified values for the pipe material and condition. New smooth pipes have higher C, while older or rough pipes should use a lower C.
6) Can I size for pressure drop instead of headloss?
Yes. Enter allowable pressure drop in kPa and the calculator converts it to headloss using water density and gravity, then checks sizes against that limit.
7) Are nominal sizes the same as internal diameter?
Not always. Nominal sizes are labels. Actual internal diameter depends on pipe schedule, wall thickness, and standards. Confirm internal diameters for final hydraulic verification.