How much ag drain and stormwater pipe does a Toowoomba yard actually need?
A typical Toowoomba back yard needs 30 to 60 lineal metres of subsoil ag drain, 20 to 35 metres of stormwater line to reach the kerb, and 3 pits. The rules that get you there: in black soil one ag line draws water from 1.5 to 3 metres either side, so parallel runs go in at 4 to 6 metre centres; a 100mm stormwater line at 1 in 100 carries the runoff from about 100 to 150 square metres of roof; and one 90mm downpipe serves roughly 40 to 60 square metres. Size it below, then check your fall before you do anything else.
Size your job, then see the range
Put in the metres you measured. The estimator shows the quantity of each item and a realistic 2026 installed range for the Toowoomba region. It answers how big and how many. If you want the item by item price list instead, that lives on the pricing page and the cost guide.
What this sizing estimate assumes
- Ag drain priced as 100mm slotted pipe to AS 2439.1 in a 300mm by 500mm trench, fully lined with 140 gsm non woven geotextile and filled with 20mm washed aggregate.
- Stormwater priced as 100mm uPVC to AS/NZS 1254, bedded and haunched in aggregate, laid at a minimum 1 in 100 grade.
- Assumes a lawful point of discharge exists and is reachable. Where it is not, a charged system or a pump changes the design and the price.
- Excludes detention and retention tanks, which are sized from an approved hydraulic design, never from a calculator.
- Excludes landscaping, retaining, paving reinstatement and roof plumbing work.
- A 2026 guide only. The range is deliberately wide because rock and access are the two biggest unknowns and a site measure removes both.
How to measure it properly, with a tape and a phone.
You can get within about 10 per cent of the real numbers in half an hour with a 8 metre tape, a ball of string, a line level and your phone. Do it after rain, not in a dry week, because the wet zone is the thing you are actually measuring.
1. Map the wet zone while it is wet.
The morning after decent rain, walk the yard and push a garden stake in wherever the ground is soft enough to sink a boot heel. Photograph it from an upstairs window or hold the phone up over your head. That outline is the area the subsoil drain has to cover, and it is almost never the shape people assume from memory. Measure its length and width with the tape and write both down.
2. Count the lines and the metres.
Run the lines along the length of the wet zone, spaced 4 to 6 metres apart across its width, and start the first line about half a spacing in from the edge. A wet zone 13 metres long and 9 metres wide takes two lines at 13 metres each. Add a collector run from the ends of those lines to wherever the water is going, and add 10 per cent, because a drawn line is always shorter than a dug one.
3. Measure the fall before anything else.
This is the measurement that decides whether the job is straightforward or expensive. Tie string at ground level at the highest collection point. Run it to the kerb, hang a line level on it, pull it dead level, and tie it off. Now measure from the string down to the ground at the kerb end. That gap is your available drop. Compare it with what the grade needs: 300 millimetres for a 30 metre run at the minimum 1 in 100, 200 millimetres for a 20 metre run. If the available drop is less than the requirement plus pipe cover, you need a charged system or a pump, and the price changes before a single metre of pipe is counted.
4. Work out the roof area and the downpipes.
Pace out the footprint of the house or measure the eaves line, and count the downpipes. You can check yourself against a satellite image, and the Queensland Globe mapping service will give you a measurable aerial of your own lot for free. Divide the roof into the planes that each downpipe serves. Any downpipe carrying more than about 60 square metres of roof is working hard, and any single underground line collecting more than about 150 square metres should be 150mm rather than 100mm.
5. Photograph the kerb and the boundary.
Take a photo of the kerb in front of the house showing any existing outlet, and one of each boundary where water enters or leaves the block. Send those with your enquiry. Half the questions we would otherwise ask at the measure are answered by three photographs.
Measured it and want the numbers checked?
Send your metres and your photos and we will size it properly, free, before anyone visits.
The rules of thumb, and the allowances to add.
Every number below is the one we use when we are sizing a job in this region. Where a Standard sets a minimum, that minimum is a floor and not a target.
- Ag line spacing. 4 to 6 metre centres in Toowoomba black soil. 6 to 10 metres in lighter, sandier ground. Closer than 4 metres is money spent twice.
- Ag line depth. 400 to 600mm for a soggy lawn, 600 to 900mm for acreage or a genuine high water table. Too shallow and it never reaches the water it was meant to intercept.
- Grade. AS/NZS 3500.3 sets 1 in 100 as the usual minimum for stormwater. Subsoil lines can run flatter, and 1 in 200 is a sensible working minimum, but nothing should be dead level in reactive clay because the ground moves.
- Downpipe capacity. Roughly 40 to 60 square metres of roof per 90mm downpipe in this rainfall band.
- Buried line capacity. Roughly 100 to 150 square metres of roof per 100mm line at 1 in 100. Beyond that, step up to 150mm rather than adding a second line if you can.
- Aggregate. 0.14 cubic metres per lineal metre for a 300 by 500 trench, which is about 0.21 tonnes of 20mm washed aggregate.
- Geotextile. A 2 metre wide roll wraps a 300 by 500 trench with enough overlap at the top. Buy by the roll length, not the trench length.
- Spoil. Trench volume multiplied by about 1.25 for bulking. A 40 metre ag trench gives roughly 7 to 8 cubic metres loose.
Then add the allowances.
- Ten per cent on every pipe length, for bends, connections and the metre you always forget at the outlet.
- Fifteen per cent on aggregate, because running short mid trench costs a delivery day with the trench open.
- One more pit than you drew. It is a few hundred dollars now and a full excavation later.
- A spare 300mm of trench depth capacity if you might ever extend the system, because retrofitting depth is impossible.
The two sizing mistakes that cost the most here.
Running out of fall.
This is the expensive one, and it is specific to the flat western side of Toowoomba, out through Glenvale, Wilsonton and Westbrook. Thirty metres of pipe at the minimum 1 in 100 grade consumes 300 millimetres of height. Add 100 millimetres of cover over the pipe and the pipe diameter itself, and you need most of half a metre of drop between the collection point and the kerb invert. Plenty of flat blocks simply do not have it. If you discover that after the trench is open, the options are a charged system at $900 to $2,400 extra, a pump with an ongoing power and maintenance cost, or a redesign. Measure the drop with a string and a line level before you count a single metre of pipe, and if it is marginal, say so at the quote stage so it gets designed rather than discovered.
Sizing a subsoil drain for a surface water problem.
An ag drain relieves a saturated soil profile. It does nothing at all for water running across the surface, because by the time surface water has soaked in far enough to reach a slotted pipe at 500 millimetres, the flood has already crossed your patio. We are regularly asked to quote 60 metres of ag line for a yard whose real problem is 40 metres of sheet flow coming off a neighbouring driveway, which needs a spoon drain or a swale at a third of the cost. Work out which one you have before you size anything: the symptoms guide and the flooding backyard page both separate the two, and the surface drainage page covers the other half of the answer.
A worked example on a typical Kearneys Spring house.
An early 1980s brick veneer on a 700 square metre block, roughly 165 square metres of roof, four downpipes discharging onto the lawn, deep black soil, a flat back yard that stays soft for a week and water creeping towards the garage. Here is the whole sizing exercise with the numbers added up.
- Fall check first. String line from the rear collection point to the kerb: 30 metres horizontal, 520 millimetres of available drop. The grade needs 300 millimetres, leaving 220 millimetres for cover and pipe. Tight but workable, so no charged system and no pump.
- Roof and pipe size. The rear pair of downpipes serves about 95 square metres, the front pair about 70. Both sit under the 100 to 150 square metre limit for a 100mm line, so 100mm throughout and no upsize needed.
- Stormwater metres. Rear collection to the kerb 18 metres, cross connection between the two rear downpipes 7 metres, front pair into the same line 4 metres. That is 29 metres, plus 10 per cent, so 32 metres.
- Wet zone and ag metres. Measured after rain at 13 metres by 9 metres. Two lines at 5 metre centres, 13 metres each, is 26 metres, plus a 9 metre collector to the silt pit is 35 metres. Plus 10 per cent, so 40 metres.
- Pits. One rear junction, one change of direction, one silt pit where the ag line meets the stormwater. Three, with a fourth carried as provisional.
- Materials. Aggregate 40 metres at 0.21 tonnes is 8.4 tonnes, plus 15 per cent means ordering 10 tonnes. Geotextile 40 metres plus 10 per cent is 44 metres, so one 50 metre roll of 2 metre wide fabric. Spoil across both trenches is about 11 to 13 cubic metres loose, which is two to three tandem tipper loads.
| Item | Quantity | Rate | 2026 range |
|---|---|---|---|
| Subsoil ag drain, wrapped | 40 m | $80 - $140 per m | $3,200 - $5,600 |
| Stormwater line to kerb | 32 m | $110 - $190 per m | $3,520 - $6,080 |
| Pits and silt pit | 3 | $280 - $520 each | $840 - $1,560 |
| Turf lifted and relaid | 1 job | lump | $350 - $1,400 |
| Spoil carted off site | 11 - 13 m³ | lump | $250 - $600 |
| Estimator total | $8,160 - $15,240 |
A fixed written quote on a job like that normally lands between $9,500 and $11,000. The estimator range is wider on purpose, because it carries the full spread for rock, access and depth. A site measure removes those unknowns, which is the entire reason the measure is free.
When the honest answer is that you need less than you think.
Three situations where the right quantity is a lot smaller than the one people arrive with.
- The downpipes are the whole problem. Four downpipes discharging onto a black soil lawn put an enormous volume into a very small area. Piping them away is often the entire fix, and it needs zero metres of ag drain. Start there, wait one wet season, and size the subsoil work afterwards if you still need it. The budget page sets out how to stage it.
- Only one strip stays wet. If the soft ground is a two metre band along a fence or a path, that is one line and a collector, not a grid. Twelve to eighteen metres, not sixty.
- It is a surface problem with a surface answer. A formed spoon drain at $60 to $130 per metre often beats forty metres of buried pipe for water crossing the block from next door.
And the limit on what a tool like this can do: it cannot size a detention or retention tank. On site detention volume and the discharge orifice come from an approved hydraulic design against the conditions on your development approval, and guessing them means an inspection that fails. If your approval mentions OSD or a permissible site discharge, the tanks page and the council approval page are the right starting points, not a slider.
Sizing questions we get asked.
How many metres of ag drain does a Toowoomba backyard need?
How far apart should ag drain lines be in black soil?
What size stormwater pipe do I need for my roof?
How much gravel does an ag drain need?
How do I check I have enough fall to the street?
How many pits will a drainage job need?
Send us your metres and we will check the sizing free
Send this with your measurements and we reply within 1 business day, then book a free on site measure and send a fixed written quote within 7 days.
Know the quantities? Now pick the specification.
The materials page turns metres into a spec sheet, and the installation guide turns the spec sheet into a week.