Pressure Drop Calculator
A Hazen-Williams head loss tool for water systems: enter flow, pipe diameter, length and a C-factor, and get pressure drop in psi, kPa and metres of head. The C-factor table is on the page — around 150 for new PVC down to roughly 100 for aged cast iron — so pick honestly.
Friction head loss
| Loss per metre | |
| Loss per 100 m | |
| Effective length (incl. fittings) | |
| Flow velocity | |
| C factor used |
Hazen-Williams applies to water at ordinary temperatures in full, pressurised pipes. For other fluids, wide temperature ranges or extreme velocities, use Darcy-Weisbach instead.
Why the exponents matter more than the inputs
Hazen-Williams packs the whole behaviour of a water pipe into two exponents, and reading them tells you where to spend design effort. Loss climbs with flow to the power 1.852 — steeply, but forgivingly. It falls with diameter to the power 4.87, which is savage. Increase demand by a third and losses roughly double; drop one nominal pipe size at the same flow and they can triple. Every stubborn low-pressure complaint at the end of a long run traces back to that second exponent.
The roughness coefficient C sits quietly beside them and deserves scrutiny of a different kind. Manufacturers publish 150 for smooth new plastic, yet many design guides and codes deliberately specify lower values — 140 for plastic, 120 for new steel, 100 for aged cast iron — because pipes foul, scale and age, and a system designed on day-one smoothness has no margin left in year fifteen. Choosing C is less a physics question than a decision about which stage of the pipe's life you are designing for.
How to use the pressure drop calculator
- Enter the flow rate. In m³/h, L/s or US gpm. Use the actual design flow through this section of pipe, not the pump's total output where the line branches.
- Enter the true internal diameter. Head loss scales with diameter to the power of 4.87 — the single most sensitive input in the whole formula. A 5% error in bore becomes a 27% error in loss.
- Give the pipe length. Straight developed length along the route, following the actual pipe rather than the plan-view distance.
- Select the material and C factor. Smooth plastic carries a high C value, aged cast iron a low one. Where a specification names a C factor, use it in the custom field rather than the preset.
- Add a fittings allowance. Bends, valves and tees each behave like extra pipe. A 10–30% addition to length is a common shortcut when the fitting count is not yet known in detail.
- Read loss in metres and kPa. Head loss in metres adds directly into a pump head calculation; the kPa and psi figures suit pressure-availability checks at outlets.
Key formulas
- Hazen-Williams (SI): hf per metre = 10.67 × Q1.852 ÷ (C1.852 × d4.87), Q in m³/s, d in m
- Total loss: hf = per-metre loss × effective length
- Effective length: L × (1 + fittings allowance)
- Pressure: kPa = hf × 9.80665; psi = kPa ÷ 6.8948
- Velocity: v = Q ÷ (π d² ÷ 4)
Worked example
20 m³/h through 50 m of 80 mm bore new steel pipe (C = 120): Q = 0.005556 m³/s; per-metre loss = 10.67 × 0.0055561.852 ÷ (1201.852 × 0.084.87) = 0.02201 m/m. Over 50 m that is 1.10 m of head = 10.79 kPa (1.57 psi), at a velocity of 1.11 m/s — comfortable on all counts.
Hazen-Williams C factors
| Material / condition | C | Note |
|---|---|---|
| Plastic (PVC, CPVC, PE), new | 150 | Manufacturer value; guides often use 140 |
| Copper / brass | 140 | Stable over life |
| Cement-lined ductile iron | 140 | Lining resists tuberculation |
| Steel, new | 120 | Common design default |
| Steel / cast iron, aged | 100 | Scaled and tuberculated |
Conventional design values. Where a code or project specification names a C factor — fire protection standards in particular — that value takes precedence over any table.
Things to keep in mind
- Design for the aged pipe, not the new one. Systems are lived with for decades; a C value chosen for day one leaves nothing in reserve.
- Friction is only part of pump head. Add static lift and any required residual pressure at the outlet before sizing a pump.
- Check the least-favoured outlet. Average performance means little — the furthest, highest fixture is what determines whether the design works.
- Stay within the method's domain. Hazen-Williams is a water correlation; applying it to air, steam or oil produces confident nonsense.
- Count fittings for final design. The percentage allowance is a planning tool, not a substitute for equivalent-length tables.
Frequently asked questions
What is the Hazen-Williams formula used for?
It estimates friction head loss for water flowing full in pressurised pipes, which is why it dominates water distribution, irrigation and fire protection design. Its appeal is a single roughness coefficient C that needs no iteration, unlike Darcy-Weisbach with its friction-factor solution. Its limits are equally clear: it is empirical, calibrated for water near ambient temperature at ordinary velocities, and it should not be used for oil, air, steam or hot fluids.
What C factor should I use for PVC pipe?
Manufacturers publish 150 for new smooth plastic, and this tool offers that value. Many design guides and fire-protection standards deliberately specify a lower figure — often around 140 for plastic and 120 for new steel — to allow for ageing, fouling and site workmanship over the system's life. If a project specification or code names a C value, that number governs, and the custom field exists for exactly that case.
Why is pipe diameter so much more important than length?
Look at the exponents: loss rises with flow to the power 1.852 but falls with diameter to the power 4.87. Doubling the length doubles the loss, whereas dropping one nominal pipe size can multiply it several-fold. That asymmetry is why re-routing pipe to shorten a run rarely rescues a marginal system, while going up one diameter almost always does.
How do I account for valves and fittings?
Two ways. The rigorous method assigns each fitting an equivalent length from tables — a full-bore gate valve adds little, a globe valve adds a great deal — and totals them into the pipe length. The practical method, offered here as a percentage, adds 10–30% to the straight length depending on how congested the route is. Early in design the percentage is honest; for final calculations, count the fittings.
Should I use Hazen-Williams or Darcy-Weisbach?
Hazen-Williams for water at ordinary temperatures in the velocity ranges of building services and distribution work — it is simpler, well established and accurate enough there. Darcy-Weisbach for anything else: other fluids, wide temperature ranges, very high or very low velocities, or where you need physical rather than empirical grounding. Darcy-Weisbach is the more general method; Hazen-Williams is the convenient special case that happens to cover most water systems.
How much pressure drop is acceptable?
Building services commonly design around 100–300 Pa per metre — roughly 1–3 m of head per 100 m — with the total loss checked against what the pump or supply pressure can deliver while leaving adequate residual pressure at the least-favoured outlet. That last check is the one that matters: a system can meet a per-metre guideline everywhere and still starve its furthest fixture once static lift and fitting losses accumulate.
Last updated: 24 July 2026