Pipe Pressure Drop Calculator

Calculate pipe friction pressure drop using the Darcy-Weisbach equation with Swamee-Jain friction factor. Handles laminar, transition, and turbulent flow regimes.

About this calculator

Despite living under the "insulation thickness" slug, this engine actually computes pipe friction pressure drop using the Darcy-Weisbach equation, ΔP = f·(L/D)·(ρv²/2). It converts the entered volumetric flow rate and pipe diameter into an average velocity, then computes the Reynolds number to classify the flow regime. Below Re = 2300 it uses the exact laminar solution f = 64/Re; above Re = 4000 it switches to the Swamee-Jain explicit approximation, a curve-fit that avoids solving the implicit Colebrook-White equation iteratively; in between, it linearly blends the two regimes as an approximation for the transition zone, where real flow behavior is genuinely unstable and hard to predict precisely. The pipe roughness input represents absolute surface roughness (steel ≈0.045 mm, copper/PVC ≈0.0015 mm) and directly reflects how much a rougher pipe interior increases turbulent friction.

Results are reported in kPa, bar, and psi, plus head loss expressed as an equivalent column height of the flowing fluid and the velocity head (v²/2g) used separately when estimating fitting and valve losses. A key limitation: this treats a single straight run and does not include minor losses from elbows, tees, valves, or fittings — those must be added via an equivalent length or K-factor method on top of this result. Because velocity, not just flow rate, drives most of the pressure loss (it enters squared), doubling flow rate roughly quadruples pressure drop for a fixed pipe size.

Inputs

Volumetric flow rate (m³/hr)

Pipe inner diameter (mm)

Pipe length (m)

Pipe roughness ε (mm)

Fluid density (kg/m³)

Dynamic viscosity (Pa·s)

Tab to move between fields

Results

Pressure drop (kPa)

1.49

Flow velocity (m/s)

0.35

Pressure drop (bar)0.01

Pressure drop (psi)0.22

Reynolds number35,297

Darcy friction factor0.02

Head loss (m)0.15

Velocity head (m)0.01

Flow RegimeTurbulent

How to Use This Calculator

Enter Volumetric flow rate (m³/hr), Pipe inner diameter (mm), and Pipe length (m).
Set Pipe roughness ε (mm), Fluid density (kg/m³), and Dynamic viscosity (Pa·s).
Review Pressure drop (kPa) and Flow velocity (m/s).
Use Pressure drop (bar) and Pressure drop (psi) to inform your decision.

How the result changes with Pipe inner diameter (mm)

Pipe inner diameter (mm)	Pressure drop (kPa)	Flow velocity (m/s)
50	45.62	1.41
75	6.1	0.63
150	0.21	0.16
250	0.02	0.06

What each input means

Volumetric flow rate (m³/hr): Fluid volumetric flow rate through the pipe.
Pipe inner diameter (mm): Internal diameter of the pipe.
Pipe length (m): Total equivalent length of straight pipe.
Pipe roughness ε (mm): Absolute roughness: steel ≈ 0.045 mm, copper ≈ 0.0015 mm, PVC ≈ 0.0015 mm.
Fluid density (kg/m³): Water ≈ 998, air ≈ 1.2, oil ≈ 850 kg/m³.
Dynamic viscosity (Pa·s): Water at 20°C ≈ 0.001 Pa·s, oil ≈ 0.1–1 Pa·s.

What each result means

Pressure drop (kPa): Total friction pressure loss along the pipe.
Pressure drop (bar): Pressure drop in bar.
Pressure drop (psi): Pressure drop in pounds per square inch.
Flow velocity (m/s): Average fluid velocity in the pipe.
Reynolds number: Dimensionless number indicating flow regime (<2300 laminar, >4000 turbulent).
Darcy friction factor: Dimensionless friction factor from Swamee-Jain (turbulent) or 64/Re (laminar).
Head loss (m): Pressure drop expressed as meters of fluid column.
Velocity head (m): v²/(2g) — useful for fitting loss calculations.

How this is calculated

Worked example, using the default values

Identify Input Parameters
4 parameters
Volumetric flow rate (m³/hr) = 10, Pipe inner diameter (mm) = 100, Pipe length (m) = 100, Pipe roughness ε (mm) = 0.045 = 6 input(s) provided
Calculate Pressure drop
Pressure drop = deltaPPa / 1000
1.49 = 1.49
Calculate Flow velocity
Flow velocity = Q / A
0.3537 = 0.3537
Calculate Pressure drop
Pressure drop = deltaPPa / 100000
0.0149 = 0.0149
Calculate Pressure drop
Pressure drop = deltaPPa * 0.000145038
0.22 = 0.22

Engine last updated September 1, 2026. Checked against 3 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.

Frequently Asked Questions

Why does the calculator switch formulas depending on the Reynolds number?

Below Re = 2300 the flow is laminar and the friction factor has an exact closed-form solution, f = 64/Re. Above Re = 4000 the flow is turbulent and the calculator uses the Swamee-Jain explicit approximation instead, since the exact turbulent equation (Colebrook-White) can only be solved iteratively. Between 2300 and 4000 is a genuinely unstable transition zone, so the tool linearly blends the laminar and turbulent results as an approximation rather than claiming precision it can't have.

Does this include losses from elbows, valves, and fittings?

No — this calculator only computes straight-pipe friction loss via the Darcy-Weisbach equation. Minor losses from elbows, tees, valves, and other fittings have to be added on top separately, typically using either an equivalent-length method (treating each fitting as an extra length of straight pipe) or a K-factor method applied to the velocity head this calculator already reports.

Why does pressure drop increase so much faster than flow rate?

Velocity appears squared in the ρv²/2 term of the Darcy-Weisbach equation, and velocity itself is proportional to flow rate for a fixed pipe diameter. That means doubling the flow rate through the same pipe roughly quadruples the pressure drop, which is why upsizing a pipe even slightly can meaningfully cut pumping costs.

What roughness value should I use for my pipe material?

Roughness represents the absolute surface roughness of the pipe's interior wall — commercial steel is about 0.045 mm, while smooth-bore materials like copper or PVC are closer to 0.0015 mm. A rougher pipe increases the turbulent friction factor (and therefore pressure drop) at the same Reynolds number, so using the wrong material's roughness can meaningfully skew the result in turbulent flow.

The questions that sit next to this one — chosen by subject, including calculators filed under a different category.

Chemical Engineering