How do I calculate flow rate in a pipe?

Use the formula Q = A × v, where A is the internal cross-section area of the pipe (m²) and v is the mean flow velocity (m/s). For a circular pipe: A = π × (D/2)², where D is the inside diameter in metres. Example: 100 mm ID pipe at 2 m/s → A = π × 0.05² = 0.00785 m², Q = 0.00785 × 2 = 0.01571 m³/s = 56.5 m³/h. If you know Q and pipe size, rearrange to find velocity: v = Q / A — use this to check whether velocity is within acceptable limits.

How do I convert flow rate between m³/h, L/min, and GPM?

1 m³/s = 3,600 m³/h = 1,000 L/s = 60,000 L/min = 15,850 US gpm = 13,198 UK gpm. Quick conversions: m³/h to L/min: multiply by 16.667. L/min to m³/h: multiply by 0.06. US gpm to L/min: multiply by 3.785. US gpm to m³/h: multiply by 0.2271. These conversions are commonly needed when comparing flowmeters, pump curves, and piping specs that use different unit systems.

What is the recommended velocity for water in industrial pipes?

For water in carbon steel pipes: pump discharge lines 1.5–3 m/s, pump suction lines 0.5–1.5 m/s. For pump suction, keeping velocity below 1 m/s reduces NPSHa loss and prevents cavitation. For cooling water: 1.5–2.5 m/s. For fire-fighting mains: 2–3 m/s. For slurries: minimum velocity must exceed the settling velocity (typically 1.5–3 m/s). Higher velocities increase erosion and pressure drop; lower velocities risk solids settling and biological growth (in cooling water).

How do I find pipe size (diameter) from flow rate and velocity?

Rearrange Q = A × v: A = Q / v, then D = 2 × √(A / π). Example: flow rate 100 m³/h = 0.02778 m³/s, target velocity 2 m/s: A = 0.02778 / 2 = 0.01389 m², D = 2 × √(0.01389/π) = 2 × √0.00442 = 2 × 0.0665 = 0.133 m = 133 mm. Select the nearest standard pipe bore: 150 mm NB (DN150). Recheck actual velocity with the standard bore: v = Q / A = 0.02778 / (π × 0.075²) = 0.02778 / 0.01767 = 1.57 m/s — acceptable.

How do I find flow rate if I know pressure drop but not velocity?

Use the Darcy-Weisbach equation rearranged: ΔP = f × (L/D) × (ρv²/2). Rearrange for v: v = √(2 × ΔP × D / (f × L × ρ)), then Q = A × v. For orifice plates and venturi meters, the flow rate from differential pressure is: Q = Cd × A_throat × √(2 × ΔP / ρ), where Cd is the discharge coefficient (0.6–0.98). Use the pressure drop calculator on this site to work backwards from ΔP to flow velocity.

Can I use this flow rate calculator for gas or compressed air?

Yes, for incompressible or low-velocity gas flows (Mach number < 0.3), Q = A × v is a valid approximation. However, gas volumetric flow changes with pressure and temperature. To convert actual volumetric flow to standard conditions: Q_std = Q_actual × (P_actual / P_std) × (T_std / T_actual), where P is absolute pressure and T is absolute temperature (Kelvin). For compressed air systems, flow is usually specified in Nm³/h (at 0°C, 1.01325 bar) or Sm³/h (at 15°C, 1 bar).

What is the difference between volumetric flow rate and mass flow rate?

Volumetric flow rate (Q, m³/s or m³/h) measures the volume of fluid passing a cross-section per unit time. It changes with temperature and pressure for gases. Mass flow rate (ṁ, kg/s or kg/h) = Q × ρ, where ρ is fluid density. Mass flow is conserved regardless of conditions — ideal for process control, energy balance, and combustion calculations. Use volumetric flow for pipe sizing; use mass flow for process design and heat/mass balances.

How accurate is Q = A × v for real industrial piping?

Q = A × v gives good accuracy (±2–5%) for fully developed turbulent pipe flow — the dominant condition in industrial systems. Accuracy is reduced near: pipe fittings, bends, valves (velocity profile distorted — flowmeters need 10–20 pipe diameters of straight run upstream), partial obstructions, pump discharge, or very low velocities (laminar flow). For precise flow measurement, use calibrated flowmeters (electromagnetic, Coriolis, vortex, or ultrasonic) rather than velocity × area estimates.

Flow Rate Calculator — Pipe Flow, Velocity & Volume (m³/h, L/min, GPM)

Calculate volumetric flow rate (Q = A × v) from pipe diameter and velocity, with unit conversion between m³/s, m³/h, L/min, and GPM.

Calculator

No signup required. Results are indicative—verify for your standards.

Inner diameter (mm)

Average velocity (m/s)

Flow rate: 0.0628 m³/s · 3770 L/min

Formula

Q = A × v, where A = π × (D/2)² for a circular pipe, v is average axial velocity (m/s), Q is in m³/s. Convert: m³/h = Q × 3600; L/min = Q × 60,000; US gpm = Q × 15,850.

Example calculation

200 mm ID pipe, flow velocity v = 2 m/s: A = π × 0.1² ≈ 0.0314 m², Q = 0.0314 × 2 = 0.0628 m³/s = 226 m³/h = 3,768 L/min = 995 gpm.

Engineering notes

Velocity profiles are not flat (turbulent: Darcy, laminar: parabolic); Q = A × v uses mean velocity. For compressible gas flow at high ΔP, correct for density at operating conditions using ideal gas law. Recommended velocities: water discharge lines 1.5–3 m/s, suction lines 0.5–1.5 m/s; steam mains 25–40 m/s; compressed air mains 6–10 m/s; gas lines 5–15 m/s. Slurry lines: minimum 1.5–2.5 m/s to prevent settling.

When to use this calculator

Pipe sizing — verify that a proposed pipe bore gives acceptable flow velocity for the design flow rate
Flow meter selection — confirm the flow range matches the meter's maximum and minimum rangeability
Pump flow rate check — verify that a pump delivers the required flow at the design velocity
Velocity check — ensure liquid lines stay within 1.5–3 m/s and suction lines under 1 m/s to prevent cavitation
HVAC and ventilation — calculate air volume flow in circular ducts from duct diameter and measured velocity

Frequently asked questions

How do I calculate flow rate in a pipe?: Use the formula Q = A × v, where A is the internal cross-section area of the pipe (m²) and v is the mean flow velocity (m/s). For a circular pipe: A = π × (D/2)², where D is the inside diameter in metres. Example: 100 mm ID pipe at 2 m/s → A = π × 0.05² = 0.00785 m², Q = 0.00785 × 2 = 0.01571 m³/s = 56.5 m³/h. If you know Q and pipe size, rearrange to find velocity: v = Q / A — use this to check whether velocity is within acceptable limits.
How do I convert flow rate between m³/h, L/min, and GPM?: 1 m³/s = 3,600 m³/h = 1,000 L/s = 60,000 L/min = 15,850 US gpm = 13,198 UK gpm. Quick conversions: m³/h to L/min: multiply by 16.667. L/min to m³/h: multiply by 0.06. US gpm to L/min: multiply by 3.785. US gpm to m³/h: multiply by 0.2271. These conversions are commonly needed when comparing flowmeters, pump curves, and piping specs that use different unit systems.
What is the recommended velocity for water in industrial pipes?: For water in carbon steel pipes: pump discharge lines 1.5–3 m/s, pump suction lines 0.5–1.5 m/s. For pump suction, keeping velocity below 1 m/s reduces NPSHa loss and prevents cavitation. For cooling water: 1.5–2.5 m/s. For fire-fighting mains: 2–3 m/s. For slurries: minimum velocity must exceed the settling velocity (typically 1.5–3 m/s). Higher velocities increase erosion and pressure drop; lower velocities risk solids settling and biological growth (in cooling water).
How do I find pipe size (diameter) from flow rate and velocity?: Rearrange Q = A × v: A = Q / v, then D = 2 × √(A / π). Example: flow rate 100 m³/h = 0.02778 m³/s, target velocity 2 m/s: A = 0.02778 / 2 = 0.01389 m², D = 2 × √(0.01389/π) = 2 × √0.00442 = 2 × 0.0665 = 0.133 m = 133 mm. Select the nearest standard pipe bore: 150 mm NB (DN150). Recheck actual velocity with the standard bore: v = Q / A = 0.02778 / (π × 0.075²) = 0.02778 / 0.01767 = 1.57 m/s — acceptable.
How do I find flow rate if I know pressure drop but not velocity?: Use the Darcy-Weisbach equation rearranged: ΔP = f × (L/D) × (ρv²/2). Rearrange for v: v = √(2 × ΔP × D / (f × L × ρ)), then Q = A × v. For orifice plates and venturi meters, the flow rate from differential pressure is: Q = Cd × A_throat × √(2 × ΔP / ρ), where Cd is the discharge coefficient (0.6–0.98). Use the pressure drop calculator on this site to work backwards from ΔP to flow velocity.
Can I use this flow rate calculator for gas or compressed air?: Yes, for incompressible or low-velocity gas flows (Mach number < 0.3), Q = A × v is a valid approximation. However, gas volumetric flow changes with pressure and temperature. To convert actual volumetric flow to standard conditions: Q_std = Q_actual × (P_actual / P_std) × (T_std / T_actual), where P is absolute pressure and T is absolute temperature (Kelvin). For compressed air systems, flow is usually specified in Nm³/h (at 0°C, 1.01325 bar) or Sm³/h (at 15°C, 1 bar).
What is the difference between volumetric flow rate and mass flow rate?: Volumetric flow rate (Q, m³/s or m³/h) measures the volume of fluid passing a cross-section per unit time. It changes with temperature and pressure for gases. Mass flow rate (ṁ, kg/s or kg/h) = Q × ρ, where ρ is fluid density. Mass flow is conserved regardless of conditions — ideal for process control, energy balance, and combustion calculations. Use volumetric flow for pipe sizing; use mass flow for process design and heat/mass balances.
How accurate is Q = A × v for real industrial piping?: Q = A × v gives good accuracy (±2–5%) for fully developed turbulent pipe flow — the dominant condition in industrial systems. Accuracy is reduced near: pipe fittings, bends, valves (velocity profile distorted — flowmeters need 10–20 pipe diameters of straight run upstream), partial obstructions, pump discharge, or very low velocities (laminar flow). For precise flow measurement, use calibrated flowmeters (electromagnetic, Coriolis, vortex, or ultrasonic) rather than velocity × area estimates.

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