Stormwater Pit Sizing for Australian Homes
Stormwater pit sizing, written for Australian homeowners, builders and plumbers dealing with a low point that will not drain by gravity.
Sizing a stormwater pit? Our stormwater pump sizing calculator works out the peak runoff for your area and postcode, the pit size and the pump you need.
Stormwater pit sizing is decided by how much water arrives during a storm, measured in litres per minute, not by how many litres the pit holds. Work out the catchment area feeding the pit, convert it to a flow rate, pick a pump that clears that flow at your discharge height, then choose a pit big enough that the pump is not switching on every thirty seconds. For a typical Australian house, that lands on a 40L to 180L pit for a courtyard or a driveway grate, and a 250L station once a garage or basement is involved.
That is the answer. Below is why it works that way, and where people get it wrong.
The short version
Most residential stormwater pits fall into three brackets. A small poly pit of 40L to 180L handles a courtyard, a patio, a single driveway grate or a subsoil line. A 250L station handles a garage, a basement, or a roof and driveway combined. Anything above that is a whole property, a shared driveway on a unit block, or light commercial.
The bracket is set by inflow. A pit that is too small for the inflow does not overflow, it just makes the pump cycle constantly and burn out early. A pit that is too big for the inflow wastes excavation money and lets water sit and go stale.
Sizing starts with inflow, not litres
Inflow is the number that decides everything, and it comes from two figures: the area draining to the pit, and the rainfall intensity you design for.
Step one: measure the catchment
Catchment is every surface that sheds water into the pit, added together in square metres. Roof sections that feed the downpipe running to that pit. The driveway slab. The paved courtyard. Any lawn or garden that slopes toward it, though grass absorbs some, so it counts for less than concrete.
Measure the plan area, not the sloped surface. A pitched roof sheds the water that falls on its footprint, so the footprint is what you use.
Step two: turn square metres into litres per minute
The conversion is one line of arithmetic: catchment area in square metres, multiplied by rainfall intensity in millimetres per hour, divided by 60, gives litres per minute.
Take a 120 square metre catchment, a roof section plus a driveway, in a storm running at 100mm per hour. That is 120 x 100 / 60, which is 200 litres per minute arriving at the pit. A REEFE pit and pump kit handles that comfortably at low head. A 300 square metre catchment in the same storm produces 500 litres per minute, and now you are in pump station territory.
Design rainfall intensity is not something to guess. It varies enormously across Australia and by storm duration, and your hydraulic consultant or licensed plumber pulls it from the Bureau of Meteorology intensity data for the postcode. A figure around 100mm per hour is a reasonable back of envelope number for a quick sanity check. It is not a design figure.
What pit volume actually buys you
Pit volume buys you run time between pump starts, and nothing else. It is a buffer, not a storage tank.
Here is the mechanism. The float switch turns the pump on at a high level and off at a low level. The volume between those two levels is the working volume, which is always less than the rated volume of the pit. Divide that working volume by the net rate the pump is emptying it, and you get the run time. Too little working volume and the pump short cycles, starting and stopping every few seconds during heavy rain, which is the fastest way to kill a motor and a float.
A practical target is fewer than about ten to fifteen starts per hour during a sustained storm. That is why a 40L pit is fine on a small courtyard and hopeless on a whole roof, even though the pump might be identical.
The sizes, and the homes that suit them
40L to 180L: courtyards, driveway grates and subsoil
The small poly pits cover light domestic drainage where the catchment is a single surface. The REEFE Stormwater Pit and Pump comes pre-assembled in three sizes: 300mm diameter for 40L from $799, 450mm for 85L at $1,099, and 600mm for 180L at $1,250. All three run an automatic pump with a float, so there is no control panel to wire.
Pick these for a sunken courtyard, a paved entertaining area, a single driveway grate, a lift pit, or a subsoil line that has nowhere to fall to. They drop into a hole a shovel can dig. They will not cope with a full roof.
250L: garages, basements and combined catchments
The 250L station is the step up when a garage or basement is involved, or when a roof and a driveway both feed the same low point. The Kwikflo 250L Stormwater Pump Station is $2,607 pre-assembled, with an Aline Sabre AL750-1A 0.75kW drainage pump, a gastight 600mm poly lid, and the valves and pipework already fitted.
It measures 880mm deep and 720mm across, which fits through a standard doorway. That matters on a retrofit at the back of an existing house with no side access, and it is the single most common reason people choose the 250L over something larger.
900L and 1200L: whole property and light commercial
The 900L and 1200L stations are for whole property drainage, shared driveways on unit blocks, and light commercial sites. The Kwikflo 900L Stormwater Pump Station with the AL750 pump is $3,206 and sits 1200mm deep. Step to the 900L with the AL1500 1.5kW pump at $4,039 when the head is high or the inflow is heavy, because the tank is the same and the pump is doing the extra work.
The Kwikflo 1200L Stormwater Pump Station is $5,199 at 1600mm deep and 1075mm across. Note the depth before you commit. At 1.6m the excavation moves into shoring and confined space work, and the labour cost moves with it.
Where a flooded basement means real damage, the Kwikflo Dual Stormwater Pump Station from $5,297 runs two drainage pumps that alternate, so a single failure does not put water through the garage.
Quick comparison
| Pit size | Typical catchment | Suits | Pump supplied | From |
|---|---|---|---|---|
| 40L (300mm) | Up to about 40 sqm | Courtyard, lift pit, single grate | REEFE automatic pump | $799 |
| 85L (450mm) | Up to about 80 sqm | Patio, small driveway, subsoil line | REEFE automatic pump | $1,099 |
| 180L (600mm) | Up to about 150 sqm | Driveway plus a roof section | REEFE automatic pump | $1,250 |
| 250L | Up to about 300 sqm | Garage, basement, combined catchment | Aline Sabre AL750-1A 0.75kW | $2,607 |
| 900L | Whole property | Full site drainage, unit driveway | AL750-1A or AL1500-1A | $3,206 |
| 1200L | Large or commercial | Light commercial, high inflow | Aline Sabre AL1500-1A 1.5kW | $5,199 |
Catchment figures are indicative only, at roughly 100mm per hour rainfall intensity and low discharge head. Your plumber or hydraulic consultant should confirm against local intensity data. Prices are Waternex list prices in AUD including GST, current as of September 2026.
The full range sits in Stormwater Pit and Pump Systems and the broader Packaged Pump Stations collection.
Discharge head decides the pump, every time
Head is the vertical lift plus friction loss in the pipe, and it cuts the flow a pump can deliver. Every pump has a curve, and the higher it pushes, the less it moves. A pump rated at 12,000 litres per hour on the box might deliver half that at 6 metres of lift.
So the sizing check is a pair, not a single number. The pump has to clear the calculated inflow at the actual head, not at zero head. Measure from the bottom of the pit to the highest point the discharge line reaches, then add roughly a metre of head for every 10 metres of horizontal run, plus a bit for each bend.
That arithmetic is worked through properly in How to Calculate Head Height for a Pump Station. It is the same for stormwater as it is for sewer.
If the pit is already in the ground and only the pump needs replacing, the vortex sump pumps suit stormwater duty: the REEFE RVS200VF at 12,000 litres per hour for $674, or the RVS300VF at 18,000 litres per hour for $939. Both take soft solids and run a vertical float, which is what you want in a narrow pit where a tethered float has nowhere to swing.
Do not put a drainage pump on sewage
A stormwater pit runs a drainage pump, which has no cutter and will not survive sewage. The tanks look identical, the pumps are completely different machines, and swapping them is an expensive mistake. If the pit is taking anything from a toilet, a kitchen or a laundry, it is a sewer job and it needs a grinder. The difference is set out in Grinder Pump vs Drainage Pump, and the sewer packages live in Sewer Pump Stations.
The lid is part of the sizing decision
Access covers are rated by what can drive over them, Class A through G, and the rating has to be chosen before the concrete goes down. Pedestrian rated covers come standard on most packages, which is fine in a garden bed and wrong under a driveway.
If the pit ends up in a trafficable area, specify a load rated cover at order: a 600 x 600mm Class B Composite Pit Cover at $530 for light vehicle areas, or a 600 x 600mm Class D Cast Iron Trafficable Pit Cover at $1,186 where trucks and heavy vehicles cross it. Cheap at order. Expensive to retrofit.
Four mistakes we see over and over
Sizing on pit volume alone. Volume sets cycle frequency. Inflow and head set the pump. Get the pump right first.
Measuring the sloped roof area. Use the plan footprint. Measuring the pitch inflates the catchment and oversizes the whole job.
Forgetting the discharge climbs before it falls. People measure to the legal point of discharge and miss the hump over the retaining wall in between. The hump is the number.
Single pump under a basement. If a failed pump means water through a garage full of stuff, the second pump pays for itself the first time it runs.
Frequently asked questions
What size stormwater pit do I need for a house in Australia?
Most Australian homes need a 40L to 180L pump out pit for a courtyard, patio or single driveway grate, and a 250L station once a garage, a basement or a combined roof and driveway catchment is involved. The deciding figure is the inflow in litres per minute, calculated from catchment area multiplied by design rainfall intensity, not the size of the yard.
How do you calculate stormwater flow into a pit?
Multiply the catchment area in square metres by the design rainfall intensity in millimetres per hour, then divide by 60 to get litres per minute. A 120 square metre catchment at 100mm per hour gives 200 litres per minute. Your plumber or hydraulic consultant should confirm the intensity figure from Bureau of Meteorology data for the site.
Can a stormwater pump handle sewage?
No. A stormwater pit runs a drainage pump with no cutter, and it will fail quickly if fed sewage or anything containing wipes and solids. Sewage needs a grinder or cutter pump in a sewer rated station.
How deep does a stormwater pit need to be?
Deep enough that the inlet pipe enters above the pump start level, which in practice means the pit depth plus bedding and cover. A 40L REEFE pit is a shovel job, a 250L station at 880mm wants a metre or so of excavation, and a 1200L station at 1600mm is machine work with shoring.
Do I need council approval for a stormwater pump out pit?
Usually yes, because the legal point of discharge and the connection are controlled by your council or water authority, and requirements differ by state. Your licensed plumber handles the approval as part of the connection. Confirm what your authority requires before you order.
How long does a stormwater pump last?
The pit outlives everything and the pump is the wear item. Life depends almost entirely on how often it cycles and what grit passes through it, which is exactly why correct pit sizing matters. A short cycling pump in an undersized pit can fail in a couple of seasons.
Send us your numbers before you dig
Give us the catchment area, the pit depth and the run to the point of discharge, and we will tell you which pit and pump combination fits. It takes a minute, it costs nothing, and it is a lot cheaper than putting the wrong unit in the ground. Builders and plumbers doing volume should also look at trade pricing.
Shop all sewer and stormwater pump stations
Talk to the Waternex team about your site
This guide is general information for Australian conditions and is not a substitute for advice from a licensed plumber or hydraulic consultant. Stormwater drainage design, pump station installation and connection to a legal point of discharge are regulated work, and requirements differ between states, councils and water authorities. Have your plumber confirm catchment, sizing, approvals and compliance before you order.