Pressure Drop Calculator
Frictional pressure loss along a pipe by Darcy–Weisbach, with fitting K-values, equipment losses, and static elevation head.
When to use this calculator
Use for hydraulic checks, pump duty selection, and proving a line pressure budget closes. It handles the whole loss stack rather than pipe friction alone: major loss along the run, minor loss from a list of fittings and valves, fixed drops for equipment such as filters or exchangers, and the static ρgΔZ term for a change in elevation. Roughness presets cover commercial steel, stainless, PVC, cast iron, galvanised, and cement-lined pipe.
Required inputs
- Flow rate and inside diameter (NPS presets available)
- Straight length and elevation change ΔZ
- Absolute roughness — presets by pipe material
- Fluid density and dynamic viscosity (water presets at 20/60/90 °C)
- Fittings and valves with K-value and count; fixed equipment drops
Expected outputs
- Velocity and Reynolds number with the flow regime
- Darcy friction factor
- Pressure drop split into major, minor, fixed, and static components
- Total pressure drop and total head loss
Formula overview
SI internally: ΔP in Pa (displayed in bar/kPa), L and D in m, ρ in kg/m³, μ in Pa·s, V in m/s. Roughness is entered in mm.
ΔP = f·(L/D)·(ρV²/2) + ΣK·(ρV²/2) + ΣΔP_fixed + ρ·g·ΔZ
V = Q / A
Re = ρ·V·D / μ
f = 64 / Re for Re < 2300 (laminar)
f = Haaland (1983), explicit for Re > 4000 (turbulent)
interpolated between the two across 2300 ≤ Re ≤ 4000Worked example
Water at 20 °C in DN 50 commercial steel: D = 50 mm, L = 100 m,
ε = 0.045 mm, ρ = 998.2 kg/m³, μ = 0.001002 Pa·s, V = 2 m/s,
plus fittings totalling ΣK = 2.0, ΔZ = 0
Re = 998.2 × 2 × 0.05 / 0.001002 = 99 600 → turbulent
f = 0.0216 (Haaland, ε/D = 0.0009)
ρV²/2 = 998.2 × 2² / 2 = 1996 Pa
ΔP_major = 0.0216 × (100/0.05) × 1996 = 86 300 Pa
ΔP_minor = 2.0 × 1996 = 4 000 Pa
ΔP_total ≈ 90 300 Pa ≈ 0.90 bar (9.2 m head)Common mistakes
- Using nominal diameter instead of the actual inside diameter. ΔP scales roughly with D⁻⁵, so a DN 50 line taken as 50 mm instead of its true 52.5 mm ID over-predicts the drop by about 12%.
- Sizing a pump on friction alone. Add the static ρgΔZ term, and remember it is a lift or a credit depending on the direction of the elevation change.
- Trusting a friction factor in the transitional band. Between Re 2300 and 4000 the value is interpolated and genuinely uncertain — if a line sits there, treat the answer as indicative and design with margin.
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