This free pump flow rate calculator converts between flow rate, velocity and pipe size using the continuity equation. Enter a pipe’s inner diameter and the velocity to get the flow rate, or switch modes to find the velocity needed for a target flow. Results come in m³/h, L/min, L/s and US GPM, with a recommended-velocity check — handy as a pipe flow rate calculator and a flow velocity calculator for pump and piping sizing.
How to calculate flow rate
Flow rate is the volume of fluid passing a point per unit time, and it follows the continuity equation:
- Flow rate (Q) =
area × velocity - Pipe area (A) =
π/4 × inner diameter² - Velocity (v) =
flow rate ÷ area
Worked example: a pipe with a 100 mm inner diameter has an area of π/4 × (0.1 m)² = 0.00785 m². At a velocity of 2 m/s, the flow rate is 0.00785 × 2 = 0.0157 m³/s — which is about 56.5 m³/h, 942 L/min, or 249 US GPM. Always use the inner (bore) diameter, since the fluid only flows through the inside of the pipe.
What velocity should you design for?
Pipe and pump sizing is a balance: too fast wastes energy to friction and risks noise, water hammer and erosion; too slow oversizes the pipe and wastes capital. Typical water design velocities:
| Line | Typical velocity | Why |
|---|---|---|
| Pump suction | 0.6 – 2.0 m/s | Lower to protect NPSH and avoid cavitation. |
| Pump discharge | 1.5 – 3.0 m/s | Balances pipe cost against friction loss. |
| Long transfer lines | 1.0 – 2.0 m/s | Friction loss dominates running cost. |
This calculator flags when your velocity falls outside the usual 0.5–3 m/s band, so you can step the pipe size up or down before finalising the pump.
From flow rate to a sized pump
Flow rate is only half the pump-sizing story — you also need the total head (static lift plus friction losses) to pick the duty point and the motor. Once you have flow and head, the pump power calculator gives you hydraulic, shaft and motor power. For the tank side, the tank volume calculator sizes storage.
From spreadsheets to a real pump ERP
A flow calculator answers one sizing question. Running a pump or valve factory means turning that into BOMs, costed quotations, work orders and GST invoices — by model and material of construction. OEMup ERP is built for Indian pump & valve manufacturers: model-wise multi-level BOMs, weight-based casting/machining costing, foundry and machining job-work with ITC-04, serial/batch traceability with test certificates, and GST e-invoicing. See the pumps & valves solution or start free.
Pump Flow Rate Calculator — frequently asked questions
How do I calculate flow rate from diameter and velocity?
Flow rate = area × velocity, with area = π/4 × inner diameter². A 100 mm pipe (area 0.00785 m²) at 2 m/s gives 0.0157 m³/s ≈ 56.5 m³/h ≈ 942 L/min. The calculator converts all units instantly.
What is a good water velocity in a pipe?
About 1–2 m/s on suction and 1.5–3 m/s on discharge. Above ~3 m/s friction, noise and water-hammer risk rise sharply; too low oversizes the pipe.
How do I find velocity for a required flow rate?
Velocity = flow rate ÷ area. Use "Find velocity" mode, enter flow and inner diameter, and the tool returns m/s plus whether the pipe size is suitable.
Inner or outer diameter?
Inner (bore) diameter — the fluid flows only through the inside. For thick-walled pipe, subtract twice the wall thickness from the outer diameter.
Need more shop-floor tools? Browse all free calculators, or try the Pump Power Calculator and the Tank Volume Calculator.
Built for pump & valve manufacturers
OEMup runs model-wise BOMs, casting/machining costing, job-work ITC-04 and GST for Indian pump makers — in one cloud ERP.
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