Water Flow Rates and Friction Losses with Different Size Pipes
(Information supplied by Reefe Pumps and the Australian Water Pump Warehouse)
Friction loss is the resistance that a pipe imparts on water as it moves through it. It’s a bit like putting your hand out the window of your car — when you’re stopped there’s no resistance, but the faster you go, the more resistance there is. It’s also often compared to voltage drop in an electrical cable.
In essence, the faster water moves through a pipe, the more resistance builds up. This resistance is sometimes called pressure loss, and it’s directly tied to pipe diameter. For example, if we are moving 100L per minute through both a 2″ pipe and a 3/4″ pipe, the water will be moving faster in the smaller pipe — and hence there will be more resistance there.
Additionally, it makes sense that water flows quicker through a 1m long hose than a 100m long hose. This effect works on a per-metre basis. So, the smaller the pipe and the longer the run, the more resistance (and thus less flow) will result. The tables typically used for this calculation give figures based on 100 metres of pipe.
Pipework Size Is Critical
Pipework selection is critical to ensure good flow and to make sure your pump is not restricted. Compare 60L/min through 20mm poly pipe (128.97m head loss per 100m) versus 32mm poly pipe (12.78m head loss per 100m) — that’s 10x less. This dramatic difference shows why even a small upgrade in pipe diameter can significantly improve your system’s overall performance.
Choosing the correct diameter isn’t just about maximizing flow, either — it’s also about protecting your pump. An undersized pipe forces your pump to work harder than necessary, generating excess heat and placing additional strain on the motor. Over time, this can shorten the lifespan of the unit and increase your energy costs. As a general rule, it’s better to slightly oversize your pipework than to undersize it, especially for longer pipe runs or systems that operate for extended periods.
Velocity for Sewage and Wastewater
While size matters when pumping all liquids, when pumping raw sewage, special attention also needs to be given to the velocity — how fast the liquid is travelling in the pipe. The larger the pipe, the less resistance, and hence the slower it moves in terms of metres per second. The issue if sewage is pumped too slowly is that solids will drop out of suspension and block the pipework over time. A rule of thumb is 1 metre/second, and this value is shown on the relevant reference table at any given flow rate.
Choosing the Right Pipe for Your Application
When selecting pipework for a garden irrigation system, pond, or wastewater setup, it helps to think about three things: the distance water needs to travel, the flow rate your pump is rated for, and whether solids need to stay suspended (as with sewage). Shorter, wider runs will always perform better than long, narrow ones. If you’re unsure which diameter suits your setup, it’s worth checking your pump’s specifications against a friction loss table before finalising your pipework, as an incorrectly sized pipe can undo much of the performance your pump is designed to deliver.
Worked Example: Calculating Total Head Loss
Let’s put the numbers into practice. Say you’re running a pump at 60L/min through a 50-metre length of 20mm poly pipe. Using the earlier figure of 128.97m of head loss per 100 metres for that pipe size, we can scale it down proportionally:
- Head loss per 100m = 128.97m
- Head loss per 50m = 128.97m ÷ 2 = 64.5m
Now compare that to running the same 60L/min through 50 metres of 32mm poly pipe, which has a head loss of 12.78m per 100 metres:
- Head loss per 50m = 12.78m ÷ 2 = 6.4m
The difference is dramatic — the larger pipe results in roughly 10 times less head loss over the same distance. In practical terms, this means your pump needs far less effort to push water through the wider pipe, which translates into stronger flow at the outlet, lower running costs, and less wear on the pump itself. When you’re planning a run of any real length, even a modest increase in pipe diameter can make the difference between a system that performs well and one that constantly struggles to keep up.
Summary
In general, larger pipes have lower resistance than smaller diameter pipes, and getting this balance right is one of the simplest ways to improve the performance and longevity of your whole water system.
