Size a pipe run — a chain reaction

Start with flow and length

20 litres per second through fifty metres of 2-inch steel. The tool works out the velocity, the Reynolds number and the friction factor, then the pressure drop that follows from them.

Flow20 L/s

Pipe2 in steel

Run50 m

Rule of thumbKeep liquid velocity around 1–3 m/s. Below that you are paying for pipe; above it you are paying for pumping and noise.

Open Pipe Flow & Pressure Drop with these numbers →

Go one size up and watch the drop collapse

Pressure drop scales roughly with the fifth power of diameter for a given flow. That is why a single size increase does not shave the loss — it demolishes it.

2 inbaseline drop

3 ina small fraction of it

Scaling≈1/D⁵

Rule of thumbOne pipe size up fixes most pressure problems. It is nearly always cheaper than the pump that would otherwise be needed.

Open Pipe Flow & Pressure Drop with these numbers →

Change the fluid and everything moves

Viscosity drives the friction factor, and it is strongly temperature dependent. Oil at 20 °C and water at 20 °C are not remotely the same problem through the same pipe.

Water at 20 °Cbaseline

Thicker fluidmuch higher drop

Rule of thumbDensity and viscosity are independent. A dense fluid is not necessarily a viscous one — mercury is heavy and runny.

Open Fluid Properties →

Convert the answer into money

Pressure drop is pumping power, and pumping power is a standing cost. Convert the head loss into kilowatts and run it through the energy tool at your own tariff to see what the undersized pipe costs per year.

Drop→ pump power

Pump power→ kWh/yr

kWh/yr→ money

Rule of thumbPipe is a one-off cost; pumping energy is forever. Size for the life of the installation, not the purchase order.

Open Energy Cost Calculator with these numbers →

Start with flow and length

Go one size up and watch the drop collapse

Change the fluid and everything moves

Convert the answer into money

The fifth power is the whole lesson