Darcy-Weisbach friction loss, Reynolds number, Moody friction factor, and minor losses via K-factor method. Real-time comparison across all standard pipe sizes.
| Size | ID (in) | Velocity (fps) | Re | Regime | f | Friction ΔP (psi) | Minor ΔP (psi) | Total ΔP (psi) | Velocity Check |
|---|---|---|---|---|---|---|---|---|---|
| ½" | 0.622 | 21.12 | 89.9k | Turbulent | 0.0188 | 108.917 | 10.513 | 119.430 | Too fast |
| ¾" | 0.824 | 12.03 | 67.9k | Turbulent | 0.0197 | 28.045 | 3.413 | 31.459 | Too fast |
| 1" | 1.049 | 7.42 | 53.3k | Turbulent | 0.0207 | 8.793 | 1.300 | 10.092 | Acceptable |
| 1¼" | 1.38 | 4.29 | 40.5k | Turbulent | 0.0219 | 2.365 | 0.434 | 2.798 | Acceptable |
| 1½" | 1.61 | 3.15 | 34.7k | Turbulent | 0.0227 | 1.132 | 0.234 | 1.366 | Good |
| 2" | 2.067 | 1.91 | 27.1k | Turbulent | 0.0240 | 0.344 | 0.086 | 0.430 | Acceptable |
| 2½" | 2.469 | 1.34 | 22.7k | Turbulent | 0.0251 | 0.148 | 0.042 | 0.190 | Acceptable |
| 3" | 3.068 | 0.87 | 18.2k | Turbulent | 0.0265 | 0.053 | 0.018 | 0.070 | Acceptable |
| 4" | 4.026 | 0.50 | 13.9k | Turbulent | 0.0284 | 0.014 | 0.006 | 0.020 | Acceptable |
| 6" | 6.065 | 0.22 | 9.2k | Turbulent | 0.0317 | 0.002 | 0.001 | 0.003 | Too slow |
This simulator calculates pipe friction loss, Reynolds number, and minor losses for water flowing through any standard pipe size and material, using the Darcy-Weisbach equation and the Colebrook-White friction factor. Engineers use it to size domestic water supply, hydronic, fire protection, and process piping by finding the pipe size that meets velocity and pressure drop targets.
The Darcy-Weisbach equation gives friction head loss as hf = f × (L/D) × V²/(2g), where f is the Moody friction factor, L is pipe length in feet, D is inside diameter in feet, V is velocity in ft/s, and g is gravitational acceleration (32.174 ft/s²). Reynolds number Re = ρVD/μ determines whether flow is laminar (Re < 2300), transitional (2300–4000), or turbulent (Re > 4000). In laminar flow, f = 64/Re exactly. In turbulent flow, f is determined from the Colebrook-White equation: 1/√f = −2 log(ε/D/3.7 + 2.51/(Re√f)), which is solved implicitly or using the Swamee-Jain explicit approximation used here.
Minor losses from fittings and valves are computed using the K-factor method: hm = K × V²/(2g). Each fitting type has a characteristic K value — a globe valve fully open has K ≈ 10, while a gate valve fully open has K ≈ 0.2. Total pressure drop in psi is obtained by summing friction and minor losses and converting from feet of head: ΔP (psi) = h (ft) × 0.4335.
IPC Section 604 and Table 604.3 specify minimum pipe sizes for domestic water supply based on fixture unit loads and Hunter's curve peak demand. NFPA 13 Section 16 uses Hazen-Williams (Q = 0.442 × C × d^2.63 × ΔP^0.54) for fire protection pipe sizing at velocities up to 10 fps. ASHRAE recommends 2–4 fps for hydronic heating and cooling systems. AWWA standards govern water main sizing in municipal distribution systems. ASME B31.1 and B31.3 govern power and process piping systems with pressure and temperature limits by material and schedule.
Pipe material roughness has a large effect on friction factor at high Reynolds numbers. Galvanized steel (ε = 0.006 inches) has about 3× the friction factor of copper (ε = 0.000059 inches) for the same pipe size and flow. For very turbulent flow (Re > 100,000), friction factor approaches a fully rough value that depends only on relative roughness ε/D. Below 0.5 fps sediment can accumulate in horizontal pipes; above 8 fps in copper, flow-accelerated corrosion and erosion at bends become significant. CPVC and PVC should not be used above their pressure-temperature ratings, particularly at elevated water temperatures where creep deformation can occur.
Select the pipe material and the specific pipe size for the flow diagram. Enter the design flow rate in GPM and pipe run length in feet. Add fittings using the increment/decrement buttons — the K values are automatically summed. The flow diagram updates to show velocity, Reynolds number, flow regime, and total pressure drop. The comparison table at right shows pressure drop for every standard pipe size at the same flow, making it easy to identify which sizes meet design targets. Use the highlighted results panel on the left for the selected pipe size's full hydraulic breakdown.
Darcy-Weisbach is the physically rigorous method that works for any fluid, any Reynolds number, and any pipe material. Hazen-Williams is an empirical approximation valid only for turbulent water flow in the velocity range 2–10 fps and is not applicable to viscous fluids or laminar flow. Use Darcy-Weisbach for design calculations involving glycol solutions, hot or cold water outside the Hazen-Williams validity range, or when you need to accurately model the laminar-to-turbulent transition. Hazen-Williams is still widely used for fire protection and water main sizing because of its simplicity.
Relative roughness is the ratio of pipe wall roughness height ε to inside diameter D (ε/D). On the Moody chart, friction factor curves converge to unique constant values at high Reynolds numbers for each relative roughness — this is the fully rough turbulent regime where f depends only on ε/D and not on Re. Smooth pipes (copper, plastic) maintain lower friction factors at all Reynolds numbers than rough pipes (galvanized, cast iron), especially in the transition zone between hydraulically smooth and fully rough flow.
ASHRAE and most design standards recommend a maximum of 4 fps for copper pipe in hydronic systems and 8 fps for domestic water supply. At higher velocities, flow-accelerated corrosion (FAC) occurs at fittings, elbows, and tees where turbulence disrupts the protective oxide film. Erosion-corrosion is accelerated by dissolved CO2, low pH, and soft water. The AWWA copper pipe design guide and CPPA recommendations confirm the 8 fps limit for cold water and 5 fps for hot water above 140°F.
1 foot of water head = 0.4335 psi at 60°F water density. Therefore ΔP (psi) = ΔP (ft of head) × 0.4335. Conversely, 1 psi = 2.31 feet of head. For fluids other than water, multiply by the specific gravity: ΔP (psi) = ΔP (ft of head) × SG × 0.4335. For a 50% propylene glycol solution at 40°F with SG ≈ 1.06, the conversion factor becomes 0.459 psi/ft.
Published K-factors (from Crane Technical Paper 410 and ASHRAE Fundamentals) are for fully open valves. A partially open gate or ball valve has an exponentially increasing K as it closes — a gate valve at 50% open has K ≈ 2, compared to K = 0.2 fully open. Globe valves throttle more linearly. For control valve sizing, do not use K-factors; instead use the Cv (flow coefficient) method: GPM = Cv × √(ΔP/SG), where Cv is published by the valve manufacturer for each stem position.
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