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Pipe Pressure Drop Calculator

Find how much head and pressure water loses to friction in a pipe — from the flow, the actual inside diameter, the length and the water temperature — with the working shown.

Method reviewed October 26, 2026. Version 1.0.0. How it works

Your inputs

Unit system

The actual inside diameter from the pipe or tube manufacturer — it differs from the nominal size.

From the fitting or valve manufacturer’s data. Leave at 0 to calculate straight pipe only.

Water only. Glycol mixtures are more viscous and lose more pressure.

Result

Pressure drop
3.44psi
Head loss
7.95ft

3.44 psi (7.95 ft of head) over 100 ft at 3.93 ft/s.

Head loss per 100 ft of pipe
7.95ft
Velocity
3.93ft/s
Reynolds number
27,628
Darcy friction factor
0.028
Show the working
  1. Water at 60 °F (IAPWS, checked against NIST)
    ρ = 62.364 lb/ft³, μ = 1.1211 cP
  2. Velocity = flow ÷ area
    3.926 ft/s
  3. Reynolds number = ρ V D ÷ μ
    27,628
  4. Swamee–Jain: f = 0.25 ÷ [log₁₀(ε ÷ 3.7D + 5.74 ÷ Re⁰·⁹)]²
    f = 0.02821
  5. Darcy–Weisbach: h = f × (L ÷ D) × V² ÷ 2g
    0.02821 × (100 ÷ 0.085) × 3.926² ÷ (2 × 32.174) = 7.951 ft
  6. Pressure = head × density ÷ 144
    7.951 × 62.364 ÷ 144 = 3.444 psi

How it is calculated

Friction loss in a full pipe follows the Darcy–Weisbach equation: head loss = f × (L ÷ D) × V² ÷ 2g, where f is the friction factor, L the length, D the inside diameter, V the velocity and g the acceleration of gravity.

The friction factor depends on the Reynolds number (how turbulent the flow is) and on how rough the pipe wall is relative to its diameter. The calculator works it out the way the EPA’s EPANET software does: 64 ÷ Re for smooth, slow laminar flow; the Swamee–Jain approximation of the Colebrook equation for turbulent flow; and a smooth interpolation between them.

Why the water temperature matters

Cold water is more viscous than hot water: at 40 °F its viscosity is more than four times what it is at 180 °F. Higher viscosity means a lower Reynolds number and a higher friction factor, so the same flow loses more pressure in a chilled-water pipe than in a heating loop. The calculator uses density and viscosity for the temperature you enter.

Fittings, valves and equipment

Elbows, tees, valves, strainers and coils add loss on top of straight pipe. Enter their total equivalent length from the manufacturers’ data, or add their pressure drops separately. This site does not publish fitting tables. For the pump, the total head is the sum of every loss around the circuit.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Flow rate
10 gpm
Pipe inside diameter
1.025 in
Straight pipe length
100 ft
Equivalent length of fittings and valves
0 ft
Pipe material (new pipe roughness)
Steel, new — 0.00015 ft
Water temperature
60 °F

Result

Pressure drop
3.36 psi
Head loss
7.76 ft
Head loss per 100 ft of pipe
7.76 ft
Velocity
3.89 ft/s
Reynolds number
27,494
Darcy friction factor
0.028

3.36 psi (7.76 ft of head) over 100 ft at 3.89 ft/s.

Assumptions

  • Darcy–Weisbach head loss with the friction factor computed as EPA’s EPANET does: f = 64 ÷ Re below Re 2,000, the Swamee–Jain approximation of the Colebrook equation above 4,000, and EPANET’s cubic interpolation between.
  • Water density and viscosity for saturated liquid at the entered temperature, generated with CoolProp’s IAPWS formulation and checked against the NIST WebBook to better than 0.01 %.
  • New-pipe roughness from EPANET: plastic 0.000005 ft, steel 0.00015 ft, galvanized iron 0.0005 ft.

Limitations

  • Fittings, valves, coils and equipment add losses: enter their equivalent length from the manufacturer’s data. This site does not publish fitting tables.
  • Water only. For glycol mixtures use the fluid supplier’s viscosity and density data.

Frequently asked questions

What inside diameter should I use?

The actual inside diameter from the pipe or tube manufacturer. Nominal sizes are not inside diameters, and wall thickness differs between pipe types and schedules.

Does this work for glycol?

No. Glycol mixtures are denser and much more viscous than water, especially when cold, so they lose more pressure. Use the fluid supplier’s property data for those.

How do I convert head loss to psi?

Multiply the head in feet by the fluid density in lb/ft³ and divide by 144. For water near room temperature that is about 0.433 psi per foot of head.

Sources

  1. EPANET 2.2 User Manual (Rossman et al.) — U.S. Environmental Protection Agency. Table 3.2 Darcy–Weisbach roughness for new pipe (ft × 10⁻³): plastic 0.005, steel 0.15, galvanized iron 0.5, cast iron 0.85; §13: f = 64/Re below Re 2,000, Swamee–Jain above 4,000, cubic interpolation between (the printed Swamee–Jain uses the base-10 logarithm with its 0.25 constant, consistent with the manual’s Y3 = −0.86859 ln(…)).
  2. NIST Chemistry WebBook, SRD 69 — Thermophysical Properties of Fluid Systems (water) — National Institute of Standards and Technology. Saturated-liquid water density and viscosity, e.g. 62.364 lb/ft³ and 1.1211 cP at 60 °F; 61.377 lb/ft³ and 0.46602 cP at 140 °F.
  3. CoolProp — open-source thermophysical property library (version 8.0.0) — CoolProp developers (MIT licence). Equations of state for pure fluids and mixtures, incompressible fluids and brines, and IAPWS-IF97 water/steam; free for commercial use under its licence. Used here to generate the refrigerant saturation tables.

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