90° elbow Elbow 90° standard (threaded) K 0.75 · L/D 30
2
Pressure Drop Calculator
The flagship calculator. Add segments, tap fittings, get pressure drop and annual energy cost in one place — the way pump-vendor tools should have worked.
System inputs
Pipe segments
Fittings on this segment 3
Fittings & valves
Tap to add. Equivalent length and Σ K both update live and feed the friction calc.
3 added Σ K 1.67 Σ L/D 68Elbows
2000Tees
00Valves
10000000000Transitions
0000Entry / Exit
000
Schematic
Live preview of the system you've configured.
Flow regime — what's actually happening inside the pipe
Live cross-section of the peak-velocity segment. Profile shape, particle speed, and wall eddies update with your inputs.
Per-segment breakdown
Velocity, Reynolds, friction factor, and head loss for each pipe segment.
| # | Material | D × L | v (ft/s) | Re | f | hmajor (ft) | hminor (ft) | Σ K |
|---|---|---|---|---|---|---|---|---|
| 1 | PVC | 2in × 100 ft | 5.11 | 7.87e+4 | 0.0191 | 4.63 | 0.68 | 1.67 |
| System total | 4.63 | 0.68 | ||||||
Moody diagram — operating point
Live (Re, f) for the peak-velocity segment. Curves show ε/D = 1e-6 (smoothest) to 2e-2 (very rough). Laminar reference (dashed) and the transitional band (yellow) are shown.
Annual energy cost of friction
The retrofit conversation. Pumping water through friction loss costs real money — most engineers never put a $ figure on it.
Hydraulic power
0W
Shaft power (incl. η)
0kW
Energy / year
0kWh
Annual cost of friction
$0 /yr
How this works
Major loss (Darcy-Weisbach): hf = f · (L / D) · v² / (2g)
Minor loss (fittings): hm = Σ Ki · v² / (2g)
Friction factor (Colebrook–White, iterative): 1 / √f = −2 · log₁₀(ε / 3.7D + 2.51 / (Re · √f))
Pressure drop: ΔP = ρ · g · (hf + hm + hstatic)
The flagship combines two well-validated equations: Darcy-Weisbach for the straight-pipe friction term and the K-factor sum for fittings. Friction factor is the awkward bit — analytical only in the laminar regime (Re ≤ 2,300, f = 64/Re); above that it requires solving Colebrook implicitly. We do the iteration honestly and seed it from Swamee-Jain to keep convergence under ~6 steps.
Fluid properties are temperature-dependent. Water viscosity drops by a factor of six between 0 °C and 100 °C — a friction-factor calculator that hard-codes 20 °C is fine for a back-of-envelope but wrong for hot-water or chiller-loop sizing. The fluid library here is fitted to NIST IAPWS-IF97 saturated-liquid data; for non-water fluids, the same Darcy formula applies once you supply the correct ρ and μ.
The "energy cost of friction" panel converts head loss into a kWh-per-year and dollar figure. This is the single most underused output in pipe-flow tools — most engineers never put a price on the friction they're designing in. A 20-ft increment of head loss running 2,000 hours on a 50-GPM pump at $0.14/kWh is in the ballpark of $200/year, every year, forever.
| Hazen-Williams | Darcy-Weisbach | |
|---|---|---|
| Form | Empirical | Physically derived |
| Fluid | Water only | Any Newtonian fluid |
| Temperature range | ~10–25 °C | Any (handles ν and ρ explicitly) |
| Friction factor | Bundled into C-factor | Solved (Colebrook / Haaland / Swamee-Jain) |
| Computational cost | One closed-form expression | Iterative on f |
| Use when | Sizing potable / irrigation pipes near room temp | Anything chilled, hot, oily, or non-water |
Common questions
Why does pressure drop change so much when I add fittings?
Real systems get most of their pressure drop from fittings, not straight pipe. A 90° standard elbow alone has K=0.75; a globe valve has K≈6. The calculator applies Σ K · v²/2g and adds it to the Darcy-Weisbach friction term.
How is the friction factor computed?
Below Re = 2,300 we use the exact laminar form f = 64/Re. From Re = 4,000 upward we iterate Colebrook–White to a 1e-10 tolerance, seeded from Swamee–Jain. The transitional band 2,300 < Re < 4,000 is linearly interpolated between the two endpoints — the regime is genuinely undefined and any free-running calculator is making the same compromise.
Why is my velocity flagged as a water-hammer risk?
Most plumbing and irrigation guides cap continuous water velocity around 2.4 m/s (~7.9 ft/s). Above 3.0 m/s the kinetic energy at sudden valve closure is high enough to cause noise, vibration, and pipe-failure modes. Increase the diameter or split into parallel runs.
Does the pipe material library use new or aged C-factors?
New, by default. Each material exposes both new and aged values where they differ — toggle by selecting an "aged" variant (e.g. cast iron, aged) for end-of-life sizing. The Hazen-Williams calculator exposes the toggle directly.
Can I share my configured calculation in a forum post?
Yes — copy the share link from the panel below the result; all inputs round-trip through the URL. Useful for engineering forum threads and code-review style discussions.
When should I switch to Darcy-Weisbach instead?
When the fluid is not clean cold water, when temperature is far from 20 °C, when you need physically grounded results (Hazen-Williams is empirical and water-only). Our /darcy-weisbach calculator solves the same system using a fluid-property table.