Pipe Pressure Drop Calculator

Calculate pressure drop, head loss, and Reynolds number for pipe flow using the Darcy-Weisbach equation.

About this calculator

This calculator applies the Darcy-Weisbach equation, the standard method for computing frictional pressure loss in pipe flow across chemical and process engineering. Total Pressure Drop combines two additive terms: straight-pipe friction loss, which scales with pipe length divided by diameter times the friction factor and the velocity squared, and fittings loss, which scales with a summed K-factor covering every valve, elbow, and tee in the line. Because both terms scale with velocity squared, pressure drop is far more sensitive to flow velocity than to almost any other single input -- doubling velocity roughly quadruples pressure drop.

Head Loss simply converts pressure drop into the equivalent height of fluid column (dividing by fluid density and gravity), the figure pump sizing calculations actually use. Reynolds Number and Flow Regime classify whether the flow is laminar (smooth, layered, Reynolds number under about 2,300) or turbulent (chaotic, mixing, above that threshold) -- this matters because the Darcy friction factor itself should be looked up or calculated differently depending on which regime you're in (the Moody chart or the Colebrook equation for turbulent flow, a simple 64/Re relationship for laminar flow). Reynolds Number depends on Fluid Dynamic Viscosity as well as density, velocity, and diameter -- water, air, and oil have very different viscosities, so entering the correct fluid's viscosity (not just its density) is essential for an accurate flow-regime classification.

Inputs

Pipe Length

Pipe Inner Diameter

Flow Velocity

m/s

Fluid Density

kg/m³

Fluid Dynamic Viscosity

mPa·s

Darcy Friction Factor

Total Fittings K-Factor

Tab to move between fields

Results

Total Pressure Drop

90,000 Pa

Head Loss

9.17 m

≈ 5 adult heights

Reynolds Number100,000

Flow RegimeTurbulent

How to Use This Calculator

Enter the Pipe Length in meters and the Pipe Inner Diameter in mm.
Enter the Flow Velocity in m/s — typical process piping velocities are 1–3 m/s for liquids and 10–30 m/s for gases.
Enter the Fluid Density in kg/m³: water ≈ 1000, air at ambient ≈ 1.2, typical process oils ≈ 850.
Enter the Fluid Dynamic Viscosity in mPa·s (water ≈ 1, air ≈ 0.018, oils ≈ 20-1000+) — this drives the Reynolds number and flow regime, not the pressure drop itself.
Enter the Darcy Friction Factor — for fully turbulent flow use the Moody chart or Colebrook equation; turbulent flow values typically range from 0.01–0.05.
Enter the Total Fittings K-Factor — sum the K values for all valves, elbows, tees, and reducers from the K-method tables.
Read the Total Pressure Drop in Pa and Head Loss in m, and confirm the Reynolds Number to verify the assumed flow regime.

How the result changes with Flow Velocity

Flow Velocity	Total Pressure Drop	Head Loss
1	22,500 Pa	2.29 m
1.5	50,625 Pa	5.16 m
3	202,500 Pa	20.64 m
5	562,500 Pa	57.34 m

What each input means

Pipe Length: Total straight length of the pipe in meters.
Pipe Inner Diameter: Internal diameter of the pipe in millimeters.
Flow Velocity: Average fluid velocity in the pipe.
Fluid Density: Density of the fluid. Water is approximately 1000 kg/m³.
Fluid Dynamic Viscosity: Dynamic viscosity of the fluid (1 mPa·s = 1 cP). Water ≈ 1 mPa·s, air ≈ 0.018 mPa·s, light oils ≈ 20-100 mPa·s, heavy oils ≈ 200-1000+ mPa·s. Drives the Reynolds number, not the pressure drop itself.
Darcy Friction Factor: Darcy-Weisbach friction factor. Typical values: 0.01-0.05 for turbulent flow.
Total Fittings K-Factor: Sum of resistance coefficients for all fittings, valves, and bends.

How this is calculated

Worked example, using the default values

Identify Input Parameters
7 parameters
Pipe Length = 100, Pipe Inner Diameter = 50, Flow Velocity = 2, Fluid Density = 1000, Fluid Dynamic Viscosity = 1, Darcy Friction Factor = 0.02, Total Fittings K-Factor = 5 = 7 input(s) provided
Calculate Total Pressure Drop
Total Pressure Drop
90000 = 90000
Calculate Head Loss
Head Loss
9.174 = 9.174
Calculate Reynolds Number
Reynolds Number
100000 = 100000
Calculate Flow Regime
Flow Regime
2 = 2

Engine last updated August 23, 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 is Total Pressure Drop so sensitive to Flow Velocity?

Both the straight-pipe friction term and the fittings term in the Darcy-Weisbach equation scale with velocity SQUARED, not velocity itself, so doubling Flow Velocity roughly quadruples Total Pressure Drop while other inputs like Pipe Length or Fluid Density only scale it linearly. This squared relationship is why oversizing a pipe (which lowers velocity for a given flow rate) is one of the most effective ways to cut pumping costs.

Why does Fluid Dynamic Viscosity matter if it doesn't appear in the pressure drop formula?

Viscosity drives Reynolds Number, which classifies Flow Regime as laminar or turbulent -- and that classification is what tells you which method to use for looking up the Darcy Friction Factor input in the first place (a simple formula for laminar flow, the Moody chart or Colebrook equation for turbulent flow). Water, air, and oil have very different viscosities, so using water's viscosity for an air or oil calculation would misclassify the flow regime even though it wouldn't change Total Pressure Drop directly.

What's the difference between the straight-pipe loss and the fittings loss?

Straight-pipe loss accounts for friction along the pipe's own length, scaling with Pipe Length divided by Pipe Inner Diameter, the Darcy Friction Factor, and velocity squared. Fittings loss accounts for the extra turbulence every valve, elbow, tee, and reducer introduces, scaling with the summed Total Fittings K-Factor and velocity squared but not with pipe length at all -- a short run with many fittings can lose more pressure than a long, straight run with few.

Does a larger Pipe Inner Diameter always reduce Total Pressure Drop?

Yes, holding flow velocity fixed at the value you enter: Total Pressure Drop's straight-pipe term is inversely proportional to diameter, so a larger diameter directly reduces that term. In real system design, though, increasing diameter at a FIXED VOLUMETRIC flow rate also lowers velocity (which reduces pressure drop even further, since it enters squared) -- this calculator takes Flow Velocity as a direct input, so remember to reduce it yourself if you're modeling a diameter change at constant flow rate.

Is the Reynolds Number here exact for any fluid I enter?

It uses the standard Reynolds number formula (density times velocity times diameter, divided by dynamic viscosity), so it is accurate for whatever density and viscosity you enter -- just make sure both values describe the same real fluid at your actual operating temperature, since viscosity in particular changes significantly with temperature for most liquids.

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