Rainwater Tank Pump Sizing Guide.
Rainwater Tank Pump Sizing Guide
Choose a rainwater pump by what the tank needs to supply — not by tank capacity alone.
A 5,000 L and a 20,000 L tank can use the same pump if the required flow, pressure and pipework are the same. Size the pump around the outlets, simultaneous demand, lift, pipe run and whether the tank is supplying the garden, household fixtures or a rain-harvesting system.
What Does Your Tank Need to Supply?
Answer three quick questions. The tool gives you a starting flow range and the pump category that best matches the job.
Answer all three questions
Your recommended starting flow range and pump type will appear here.
Tank Size Does Not Determine Pump Size
The tank stores water. The pump creates the flow and pressure needed to move that water to the outlets. A larger tank means more stored water, not automatically a larger pump.
How Much Water Is Needed at Once?
The number and type of outlets operating together determine the required flow. One hose is very different from a shower, toilet, laundry and garden outlet operating at the same time.
How Much Pressure Is Required?
Garden watering, household fixtures and irrigation can have different pressure requirements. The pump must still deliver the required flow after that pressure is added.
How Hard Is the Water Path?
Elevation, long pipe runs, small pipe diameters, filters, valves and fittings all add resistance. These conditions can increase the head the pump must overcome.
How to Size a Rainwater Tank Pump
Work through the job in this order: demand first, pressure second, then add the losses created by elevation and pipework.
List the Outlets
Identify what the rainwater will supply: hose taps, irrigation, toilets, laundry, showers or broader household use.
Estimate Simultaneous Use
Size for the outlets likely to operate at the same time, not every outlet on the property running together.
Allow for Head
Add the desired working pressure, vertical lift and friction losses through pipe, fittings and filters.
Check the Pump Curve
Select a pump that can still deliver the required litres per minute at the calculated total head.
Rainwater Pump Flow by Application
These are starting categories, not fixed pump sizes. Actual selection still depends on outlet flow, pressure and head.
| Application | Typical Demand Pattern | Selection Focus |
|---|---|---|
| Single hose or garden tap | One outlet at a time | Enough flow and pressure for the hose, sprinkler or nozzle in use. |
| Garden / irrigation | One or more sprinklers or irrigation zones | Use the irrigation system's actual L/min and operating pressure rather than the number of taps. |
| Toilets + laundry | Short overlapping demand | Automatic pump control and enough flow to handle appliances and toilet refilling without pressure collapse. |
| Tank-fed home | Several fixtures may overlap | Household flow, desired pressure, elevation and pipe losses all matter. |
| Rain harvesting with mains backup | Selected fixtures use tank water automatically | Pump sizing plus a compatible automatic rain-to-mains changeover arrangement. |
External vs Submersible Rainwater Pumps
Pump position changes installation, noise, servicing and suction conditions. It does not remove the need to match flow and pressure to the job.
External Pump
Mounted outside the tank and draws water through a suction line.
Submersible Pump
Installed inside the tank and pushes water out rather than drawing it through an external suction line.
When Do You Need a Rain Harvesting Pump Set?
A basic tank pump and a rain-harvesting system solve different problems. A harvesting setup is designed around automatic use of tank water with mains-water backup where the system and installation allow it.
Basic Rainwater Pump
Best when the job is simply to move water from the tank to garden taps, irrigation or household outlets and no automatic mains changeover is required.
Rain Harvesting Set
Designed for applications where selected fixtures use rainwater while an automatic controller manages mains-water backup when tank water is unavailable.
Household Pressure Pump
Used when the tank is the water source and the priority is delivering suitable pressure and flow to the home's plumbing system.
A 3,000 L, 5,000 L or 20,000 L tank does not tell you the pump duty. The pump is sized from required L/min at required head. Tank volume only determines how much stored water is available.
Compare Rainwater Pump Categories
Once you know what the tank must supply, compare the category that matches the installation rather than choosing from motor wattage alone.
Rainwater Tank Pump FAQs
What size pump do I need for a rainwater tank?
Size the pump from the required simultaneous flow and the total head it must overcome. Tank capacity by itself does not determine pump size.
Do I need a bigger pump for a bigger water tank?
Not necessarily. A larger tank stores more water, but the pump requirement depends on the outlets, required flow, pressure, elevation and pipework.
What pump do I need for a rainwater tank feeding a house?
Choose a pressure pump that can provide the home's expected simultaneous flow at the total head created by desired pressure, vertical lift and pipe losses.
What pump do I need for garden watering?
Use the actual water demand and operating pressure required by the hose, sprinkler or irrigation zone. Irrigation can require high flow even when only one zone operates.
Is a submersible rainwater pump better than an external pump?
Neither is universally better. Submersible pumps can be quieter externally and avoid an external suction line, while above-ground pumps can be easier to access and service. The installation determines which arrangement is more suitable.
What is the difference between a rainwater pump and a rain harvesting system?
A rainwater pump moves water from the tank. A rain-harvesting system can combine the pump with automatic controls so selected fixtures use tank water while retaining mains-water backup where designed for it.
How many outlets can a rainwater pump run?
That depends on each outlet's flow and the pump curve at the required head. The important figure is the combined litres per minute required when those outlets run together.
Can a rainwater pump be too big?
Yes. Oversizing can add unnecessary cost and may contribute to control or cycling issues. Select the pump around the required duty point rather than choosing the largest model available.
Choose the Pump for the Water Demand, Not the Tank Capacity
Start with what the tank is supplying, estimate how much water may be used at once, then allow for pressure, elevation and pipe losses before comparing pump curves.
