Flow Rate Calculator
What the Flow Rate Calculator does
Irrigation lines, hot water systems, ventilation ducts and hydraulic circuits all come down to one question: how much fluid passes per second? The Flow Rate Calculator answers it, with a Unit system switch between Imperial and Metric and two solving routes. Pick whichever route matches the numbers you actually measured on the job.
Plumbers size pipes with it, HVAC technicians check duct velocity, and pool owners estimate turnover. Aquarium keepers rely on the same turnover math, and students like it because the velocity method is the cleanest introduction to fluid flow.
Velocity times area, or pressure driven
The velocity method is Q = v x A. For a Round pipe of 0.1 m diameter, A = pi x 0.05^2 = 0.00785 m2, so water at 2 m/s gives Q = 2 x 0.00785 = 0.0157 m3/s, about 15.7 liters per second.
The pressure method is for times when you know pressures rather than speed: enter Pressure at start and the downstream pressure, and the calculator derives velocity through the pipe's resistance, then multiplies by the flow area. Rectangular ducts take width and height instead of a diameter.
How to use the calculator
Choose the Unit system first, Imperial or Metric, so every label and unit follows your preference. Next set Solve via to Flow velocity or Pressure difference, and declare The pipe is Round or Rectangular. Round pipes need one diameter; rectangular ducts need width and height.
Enter the pipe dimensions, then the velocity, or the two pressures. Press Calculate and read the volume flow, easily converted to liters per second or gallons per minute. Tip: use the inner diameter, because pipe walls reduce the true flow area more than people expect.
Frequently Asked Questions
What is the basic flow rate formula?
Q = v x A, velocity times cross-section area. Water moving 2 m/s in a 100 mm pipe flows at about 15.7 liters per second.
How do I convert cubic meters to liters?
Multiply by 1000, since one cubic meter holds 1000 liters. A modest 0.005 m3/s is therefore 5 L/s, about 79 gallons per minute.
Does pipe size affect flow rate?
Yes, because area scales with diameter squared. Doubling pipe diameter quadruples the area, so the same velocity moves four times more fluid.
When should I use pressure difference?
When you know upstream and downstream pressures instead of velocity, the calculator derives the flow from the pressure drop across the pipe.
Why does the pipe shape matter?
Round and Rectangular pipes use different area formulas. A round pipe's area is pi times radius squared, while ducts use width times height.