Pipe Flow Rate Calculator
Find flow rate from pipe diameter and velocity.
Results update as you type.
How it works
The volumetric flow rate through a pipe tells you how much fluid passes per unit time, and this calculator finds it from the pipe's inside diameter and the fluid's velocity. You enter the internal diameter and the flow velocity, and it returns the flow rate in liters per minute and cubic meters per second. The relationship is the continuity equation: flow rate equals the cross-sectional area of the pipe multiplied by the average velocity of the fluid. The area comes from the inside radius, half the internal diameter, so a wider pipe carries much more flow at the same velocity because area grows with the square of the diameter. This calculation is fundamental in plumbing, HVAC, irrigation, process engineering, and fluid mechanics for sizing pipes, pumps, and systems. Engineers often target a velocity range, commonly around one to three meters per second for water, to balance adequate flow against excessive friction, noise, and erosion at high speeds. Note that this uses the inside diameter, which is smaller than the nominal pipe size, and assumes the pipe flows completely full with a uniform average velocity. For a required flow rate, you can rearrange to find the velocity or the diameter needed. Enter the pipe diameter and velocity to compute the flow rate.
Q = A · v = π·(d/2)² · v
Volumetric flow rate equals the pipe's cross-sectional area times the fluid velocity. The area is pi times the inside radius squared, using the internal diameter. The result in cubic meters per second is converted to liters per minute by multiplying by 60,000. It assumes the pipe runs full and the velocity is the average across the cross-section.
Source:Fluid mechanics
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter pipe inside diameter and fluid velocity.
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Flow rate updates instantly as you type — no button to press.
Frequently asked questions
How do I calculate pipe flow rate?
Multiply the pipe's cross-sectional area by the fluid velocity. The area is pi times the inside radius squared. For a 100-millimeter pipe at 2 meters per second, the flow is about 942 liters per minute. Enter the inside diameter and velocity and the calculator returns the flow rate in liters per minute and cubic meters per second.
Why does diameter matter so much?
Flow rate depends on the cross-sectional area, which grows with the square of the diameter. So doubling the pipe diameter quadruples the flow at the same velocity. This strong relationship is why even small increases in pipe size can carry substantially more fluid, and why choosing the right diameter is central to system design.
Should I use inside or outside diameter?
Always use the inside, or internal, diameter, since that is the space the fluid actually flows through. The nominal or outside diameter includes the wall thickness and would overstate the area and flow. Pipe schedules affect wall thickness, so look up the true internal diameter for your specific pipe for an accurate result.
What is a good fluid velocity?
For water in typical systems, velocities around one to three meters per second balance adequate flow against friction losses, noise, and pipe erosion. Too low wastes pipe capacity, while too high causes excessive pressure loss and wear. The ideal range depends on the fluid and application, so check design guidelines for your specific system.
Does this assume the pipe is full?
Yes. The calculation assumes the pipe flows completely full and uses the average velocity across the cross-section. For partially full pipes, like gravity drains, the effective area is smaller and the calculation differs. It also assumes steady, uniform flow, which is a good approximation for most pressurized pipe systems.