Flow & geometry

Pipe Velocity Calculator

Calculate pipe velocity from flow rate and actual internal diameter. Enter L/s, L/min, m³/h or U.S. gpm with mm or inches, and see mean velocity in m/s and ft/s.

119 / CALCULATE A SECTION AVERAGE

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INPUT GROUP 01

Full-pipe measurements

Choose units before entering your measurements. Changing a unit keeps the displayed number; enter the matching value for that unit.

Flow unitMeasured volumetric flowUse a volumetric rate at the same operating conditions as this pipe section.Diameter unitActual internal diameterUse the inside dimension rather than nominal pipe size or outside diameter.

Check the assumptions before acting on a result.

Read the method and example

YOUR CALCULATION

Mean velocity is volumetric flow divided by a full circular pipe area.

Mean flow velocity (m/s)

1.273

Mean flow velocity (ft/s)

4.177

Internal cross-sectional area (m²)

0.008

Same volumetric flow (m³/h)

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Illustrative inputs loaded. Replace them with your own assumptions.

Pipe velocity formula: flow rate divided by area

v = Q ÷ A = 4Q ÷ (πD²)

v is mean velocity in m/s, Q is volumetric flow in m³/s, A is the full internal area in m², and D is actual internal diameter in meters. The calculator converts the selected flow and diameter units before applying this relationship.

How to calculate velocity in a pipe

Choose the units, then enter measured volumetric flow and actual internal diameter. Changing a unit selector keeps the entered number, so check both together.
Calculate internal area with A = πD² ÷ 4.
Divide flow by area to find the section-average velocity. Read the result in m/s or ft/s.

How does diameter change velocity?

At the same flow, doubling the internal diameter makes the area four times larger and the mean velocity one-quarter as large.

Illustrative full-pipe comparison at 10 L/s
Actual internal diameter	Mean velocity
50 mm	5.093 m/s
100 mm	1.273 m/s
200 mm	0.318 m/s

These hypothetical dimensions illustrate the relationship. They do not select an acceptable speed or pipe size. Nominal pipe size is not generally the actual internal diameter, which also depends on the pipe dimensions and wall thickness.

To work through the next calculation, use Reynolds number with matching fluid properties or the pipe pressure-drop scenario with a supplied friction factor. The water flow unit converter converts flow units separately.

How the pipe flow velocity calculator works

Mean pipe velocity v = Q/A = 4Q/(πD²), using volumetric flow Q in m³/s and actual internal diameter D in meters. Area A = πD²/4 is the full circular cross-section. Flow rate measures volume per time; velocity measures distance per time. The reported velocity is a section average, not the speed at every point in the pipe.

The selected flow unit is converted to m³/s: divide L/s by 1,000, L/min by 60,000, or m³/h by 3,600. One U.S. liquid gallon is 0.003785411784 m³, so U.S. gpm is multiplied by that volume and divided by 60; imperial gallons are different. Millimeters divide by 1,000 and inches multiply by 0.0254 to give meters. Divide m/s by 0.3048 for ft/s. These unit conversions do not change the physical measurement.

The hourly volume rate is the same entered flow expressed in m³/h. The pipe must be full and circular. Actual internal diameter must be positive; zero volumetric flow gives zero mean velocity in both output units. At a fixed flow, doubling internal diameter multiplies area by four and divides velocity by four. π is evaluated at normal floating-point precision, and display rounding does not change the input dimensions.

This is a section-average kinematic calculation. It does not determine an acceptable speed, pipe diameter, pump size, pressure loss, transient pressure, flow regime or system capacity. Compressible or partially full flows need a matching physical model and operating basis.

Worked example

A 100 mm internal diameter has about 0.007854 m² area. A flow of 10 L/s, 600 L/min or 36 m³/h gives approximately 1.273 m/s, or 4.177 ft/s. At that same flow, a 200 mm internal diameter gives approximately 0.318 m/s, one quarter of the 100 mm result. Separately, 100 U.S. gpm through an actual 2-inch internal diameter gives approximately 3.113 m/s, or 10.212 ft/s; 2 inches here is a measured inside diameter, not a nominal pipe size.

Sources and assumptions

The source material explains the underlying calculation or pricing structure. Entered prices, costs, schedules and scenario choices are your assumptions; this tool does not obtain supplier quotes or predict actual results.

Calculation methods, testing and sources · Practical calculation guides