Pipe Size by Flow Velocity Calculator

Back-solve the minimum pipe inside diameter from flow rate and target velocity, then estimate Darcy pressure drop, head loss and Reynolds number for that diameter.

Inputs

View results

Flow rate (Q)

Volumetric flow through the pipe.

L/min

Target velocity (v)

Velocity target used to back-solve pipe inside diameter.

m/s

Pipe length (L)

Straight equivalent pipe length used for pressure-drop screening.

Fluid density (rho)

Water is about 998 kg/m^3 near room temperature.

kg/m³

Dynamic viscosity (mu)

Water is about 1 cP near room temperature. Glycol mixes can be much higher.

Roughness (epsilon)

Equivalent internal roughness. Smooth plastic/copper is low; rough steel is higher.

Fitting loss coefficient (sum K)

Sum of minor-loss K values for elbows, tees, valves and fittings.

Results

Default result

Edit inputs

Required pipe ID(D)

26.6mm

Pass

Minimum actual ID for the entered velocity target.

Also computed

Flow area(A)555.6mm²

Pressure drop(dp)0.3244bar

f = 0.0221, turbulent flow

Head loss(h)3.315m

Pipe velocity(v)1.5m/s

Reynolds number(Re)39,810

turbulent

Darcy friction factor(f)0.02207

Method notes 3 notes

Required area = Q / v, and required ID = sqrt(4A/pi). Pressure loss uses the same Darcy-Weisbach model as the pipe pressure-drop calculator.
Select a nominal pipe whose actual inside diameter is at least this result, then re-check pressure drop with the actual pipe ID.
Final plumbing, hydronic and process designs still need code requirements, pump curves, pipe schedule, fittings, valves, water hammer, erosion/corrosion and balancing checks.

Pipe size by velocity starts from continuity: A = Q/v and D = sqrt(4A/pi). This calculator back-solves the minimum actual inside diameter for a target flow and velocity, then runs the same Darcy-Weisbach pressure-drop model at that solved diameter. Use the result to choose a nominal pipe, then re-check with the actual ID.

How to use this calculator

Enter flow rate. Use the design flow through the pipe.
Choose target velocity. Pick a velocity target suitable for the fluid, noise, erosion and pressure-loss limits.
Add loss inputs. Enter pipe length, fluid properties, roughness and fitting K for a pressure-drop screen.
Select actual pipe. Choose a nominal pipe whose actual ID meets or exceeds the result.
Re-check pressure drop. Use the actual selected ID in the pipe pressure-drop calculator.

How it works

Pipe sizing by velocity starts from continuity: A = Q / v D = sqrt(4A / pi) The result is the minimum actual inside diameter for the entered flow and velocity target. The page then estimates Darcy pressure drop at that diameter.

After choosing a real pipe size, use the pipe flow pressure drop calculator with the actual ID. For hydraulic oil hose, use the hydraulic hose pressure drop calculator.

Worked example

Verified against the live calculator

A 50 L/min water flow at a 1.5 m/s velocity target needs about 26.6 mm actual inside diameter. With the default 30 m length and fitting allowance, the calculator also reports the estimated head loss and pressure drop for that solved diameter.

Frequently asked questions

How do you size pipe from flow and velocity?

Convert flow to volume per second, divide by the target velocity to get flow area, then solve ID = sqrt(4A/pi).

Is this nominal pipe size?

No. The result is required actual inside diameter. Compare it with a pipe schedule or tubing table, then re-check pressure drop using the selected actual ID.

Does this include pressure drop?

Yes. After solving the required ID, the calculator runs a Darcy-Weisbach pressure-drop screen using length, density, viscosity, roughness and fitting K.

Can I use this for plumbing and hydronic systems?

Use it as an early screen for water-like fluids. Final plumbing or hydronic work still needs code, pump curves, pipe material, fittings, valves, water hammer and balancing checks.

Method & assumptions

The ID result is an actual inside diameter, not NPS, DN, copper tube size or PEX nominal size.
Pressure drop uses Darcy-Weisbach with laminar f = 64/Re and Swamee-Jain turbulent friction.
Use fluid viscosity at operating temperature; glycol and process fluids can change the result materially.
Final systems still need fittings, valves, pump curves, code, water hammer, erosion/corrosion and balancing checks.

Pipe Size by Flow Velocity Calculator

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