Compute flow rate, velocity, required pipe diameter, and Reynolds number for water, oil, and air in circular pipes.
Water ≈ 1e-6, oil ≈ 4.6e-5, air ≈ 1.5e-5
Common pipe sizes
Results
Flow rate
Velocity
Diameter & area
Reynolds number——
Velocity check: enter values to see recommendation.
Recommended velocity ranges
Application
m/s
ft/s
Domestic cold water
0.5 – 2.0
1.6 – 6.5
Domestic hot water
0.5 – 1.5
1.6 – 4.9
Pumped mains & risers
1.0 – 3.0
3.3 – 9.8
Pump suction line
0.6 – 1.2
2.0 – 3.9
Gravity drainage
0.6 – 2.4
2.0 – 7.9
Compressed air (main)
6 – 10
20 – 33
HVAC supply duct
4 – 8
13 – 26
Low-pressure steam
15 – 20
49 – 66
FAQ
What is the Reynolds number and why does it matter?
Reynolds number (Re) is a dimensionless ratio of inertial to viscous forces in a flowing fluid: Re = V · D / ν, where V is velocity, D is the pipe inside diameter, and ν is the kinematic viscosity. Flow is laminar when Re < 2300, transitional from 2300 to 4000, and turbulent above 4000. Laminar flow has smooth parallel streamlines and low friction loss. Turbulent flow mixes aggressively, has higher friction, and is the regime you will see in almost any practical water or air pipe — a 25 mm pipe at 1.5 m/s gives Re ≈ 37 000, well into turbulence.
How do I choose a pipe velocity for water?
For cold water service inside buildings, keep velocity below about 2 m/s (6.5 ft/s) to limit noise, erosion of copper, and water hammer. For long pumped mains a range of 1–3 m/s is a good compromise between pump energy (favoring low velocity) and pipe cost (favoring high velocity). On a pump suction line stay lower, 0.6–1.2 m/s, so you do not starve the pump. Hot water is usually held below 1.5 m/s to reduce copper pitting. If you exceed these ranges you can still make it work, but expect more noise, more pressure drop, and shorter pipe life.
How does flow rate relate to pipe diameter?
From continuity, Q = V · A, where A = π · (D / 2)². Because area scales with the square of the diameter, doubling the diameter quadruples the flow at the same velocity. That is why a small increase in pipe size often fixes pressure problems: going from 20 mm to 25 mm increases cross-section by 56 percent, and going from 25 mm to 32 mm adds another 64 percent. The flip side: at fixed flow, velocity drops by the same factor, which usually also reduces friction loss more than proportionally.
Why does friction loss rise so fast with velocity?
In turbulent flow, friction head loss is roughly proportional to velocity squared: h_f ∝ V². Doubling the velocity multiplies the pressure loss per meter by about four. That is the main reason oversizing slightly (one pipe step up) often pays back — pump power scales even faster because it is pressure times flow. A full friction calculation uses the Darcy-Weisbach equation with a friction factor that depends on Re and pipe roughness; this calculator gives you the velocity and Re so you can look up the factor.
Do these formulas work for gases like air?
The continuity equation Q = V · A is valid for any fluid, and Reynolds number uses the actual kinematic viscosity of the gas (air ≈ 1.5 × 10⁻⁵ m²/s at 20 °C), so the Reynolds result is correct. What breaks down for gases is the assumption of constant density: if pressure changes along the pipe, volumetric flow changes too. For low-pressure HVAC ducts and shop-air mains with modest pressure drop the incompressible approach is fine. For high-pressure gas, compressed-air distribution with large drops, or steam, you need a compressible flow calculation.
US gallons or Imperial gallons?
All GPM values here are US gallons per minute (1 US gal = 3.785 411 784 L). Imperial gallons (UK and some Commonwealth plumbing codes) are 4.54609 L, about 20 percent larger. To convert Imperial GPM to US GPM multiply by 1.2009; to go the other way multiply US GPM by 0.8327. The metric outputs (L/min, L/s, m³/h) are identical regardless of gallon system.
Results assume steady, incompressible flow in a full circular pipe. Friction loss depends additionally on pipe roughness and length; use Darcy-Weisbach or Hazen-Williams for detailed sizing.
This pipe flow calculator covers the three sizing questions engineers and plumbers meet every day. Mode 1 finds volumetric flow rate from an inside diameter and velocity using Q = V times area. Mode 2 reverses that and returns velocity from a known flow rate. Mode 3 solves for the required inside diameter given a target velocity, useful when choosing a pipe step from nominal sizes. Every result is shown in both metric and imperial units: L/min, L/s, m3/h, GPM, CFM for flow, and m/s, ft/s, km/h, mph for velocity. The Reynolds number is computed from pipe diameter, velocity, and the kinematic viscosity of the selected fluid (water, hot water, light oil, air, or a custom fluid), and the flow regime badge reports laminar, transitional, or turbulent. For a 25 mm pipe at 1.5 m/s with water, flow equals about 44 L/min and Reynolds around 37 000, well into turbulent flow.