Work out concrete for a round tank base by multiplying pi by the radius squared by the thickness, all in metres, which gives the volume in cubic metres. For a 2.7 metre base at 100 millimetres thick that is 3.14 times 1.35 squared times 0.1, which comes to 0.57 cubic metres, and you would order about 0.63 to allow for waste. Two things trip people up before they even reach the sum. The first is that the base has to be wider than the tank, so measure the base you intend to pour rather than the tank itself. The second is that almost every domestic tank base works out under one cubic metre, which is below most suppliers’ minimum load, so what you order and what you pay for are usually different numbers.
| Tank size | Tank diameter | Base diameter | Base area | Concrete at 100 mm | Order with 10% |
|---|---|---|---|---|---|
| 2,000 L | 1.4 m | 1.7 m | 2.27 m² | 0.23 m³ | 0.25 m³ |
| 5,000 L | 1.9 m | 2.2 m | 3.80 m² | 0.38 m³ | 0.42 m³ |
| 10,000 L | 2.4 m | 2.7 m | 5.73 m² | 0.57 m³ | 0.63 m³ |
| 15,000 L | 2.8 m | 3.1 m | 7.55 m² | 0.75 m³ | 0.83 m³ |
| 22,500 L | 3.2 m | 3.5 m | 9.62 m² | 0.96 m³ | 1.06 m³ |
| 30,000 L | 3.7 m | 4.0 m | 12.57 m² | 1.26 m³ | 1.38 m³ |
Tank diameters vary between manufacturers, so measure yours rather than working from the table alone. Base diameters above assume 150 mm of overhang all the way round.
The Formula
One formula covers every round base, and it is the area of a circle multiplied by how deep you are pouring. Everything else is just measuring carefully.
Volume equals pi times the radius squared times the thickness. Use 3.14 for pi, and work entirely in metres so the answer comes out in cubic metres, which is how concrete is ordered. The radius is half the diameter, so a 2.7 metre base has a radius of 1.35 metres. Thickness in millimetres converts to metres by dividing by a thousand, so 100 millimetres becomes 0.1 and 125 millimetres becomes 0.125. The two most common mistakes are using the diameter where the formula wants the radius, which gives four times too much concrete, and leaving the thickness in millimetres, which gives a thousand times too much. If your answer looks absurd, one of those two is usually why.
A Worked Example
Running through a real one makes the formula stick, and this is the size most people are pouring. A 10,000 litre rainwater tank is the standard domestic tank across regional Victoria.
The tank is 2.4 metres in diameter, and you want 150 millimetres of base showing all the way round, so the base is 2.4 plus 0.15 twice, which is 2.7 metres. Halve that for the radius and you get 1.35 metres. Square it and you get 1.82. Multiply by pi and the base area is 5.73 square metres. Multiply that by your thickness of 0.1 metres and the concrete volume is 0.57 cubic metres. Add ten percent for waste, spillage and an uneven base and you land on about 0.63 cubic metres. That is the number to give your supplier. If you would rather not do the arithmetic, our concrete calculator will do it for you.
Make the Base Bigger Than the Tank
Sizing the base to the tank is the most common planning error, and it is worth getting right because you cannot add to it later. The overhang is doing a job.
The base should extend beyond the tank wall on every side, typically by 100 to 150 millimetres, and some manufacturers ask for more. That margin keeps the tank wall clear of the slab edge where the concrete is weakest, stops the tank sitting partly on soil that will erode from under it, and gives you a clean surface to work on when connecting fittings. It also means a tank that is not positioned perfectly still has full support underneath it, which matters more than it sounds. Measure the actual tank you have bought rather than working from the nominal capacity, since two 10,000 litre tanks from different manufacturers can differ by 200 millimetres or more in diameter depending on how tall they are.
How Thick Should a Tank Base Be
One hundred millimetres suits most domestic tanks, with heavier tanks and poorer ground calling for more. The load is larger than people assume.
Water weighs a tonne per thousand litres, so a full 10,000 litre tank puts ten tonnes on that slab, and a 22,500 litre tank puts 22.5 tonnes on it. That load sits there permanently rather than passing over like a vehicle, which is why the ground underneath matters as much as the concrete. For tanks up to around 10,000 litres, 100 millimetres over a well compacted base is standard. For larger tanks, reactive clay, or fill that has not settled, go to 125 or 150 millimetres and consider mesh. SL72 mesh in the upper third is typical, and 20 to 25 MPa concrete is normal for domestic tank bases. Our driveway MPa guide explains what the strength grades mean if you are unsure.
Level Matters More Here Than Almost Anywhere Else
A tank base has to be genuinely flat and level, not close enough. This is the single most important thing on the page and the one that causes the most expensive failures.
Poly tanks are designed to be supported evenly across their whole floor. Put one on a base that dips, tilts or has a high spot and the tank wall carries load it was never designed to take, which stresses the seams and can split the tank. On a concrete tank the same problem cracks the base or the tank wall. A full tank is far too heavy to lift and correct afterwards, so the base has to be right before the tank goes on. Screed the surface flat, check it in several directions with a long level rather than by eye, and give it a smooth trowel or light broom finish. Do not build in a fall for drainage the way you would on a driveway, because a tank base is meant to be level.
Check What the Tank Manufacturer Requires
Before you pour anything, read the tank’s installation instructions, because the warranty usually depends on them. This is a five minute job that protects a large purchase.
Most tank manufacturers publish a base specification covering minimum thickness, concrete strength, overhang, surface finish and level tolerance, and installing on a base that does not meet it can void the warranty on a tank worth thousands. Some specify sand bedding or crushed rock instead of concrete for smaller tanks, and some require a smooth finish with no sharp edges or protrusions that could wear the tank floor. If the tank feeds a house supply or sits near a boundary or structure, check whether your council has anything to say as well. Where the specification is unclear, ring the manufacturer rather than assuming, since they would rather answer than replace a tank.
Order the Right Amount and Know About Short Loads
This is where the arithmetic meets what you will actually be charged, and it surprises people. Almost every domestic tank base is a small pour.
As the table shows, tank bases from 2,000 to 30,000 litres land between roughly 0.25 and 1.4 cubic metres. Most suppliers, including us, have a minimum load, and orders below it attract a short load charge because the truck and driver cost the same whether they carry half a metre or five. That is not a reason to under-order. Coming up short mid pour on a tank base is far worse than paying a short load fee, because a cold joint through a slab that has to be flat and watertight is a genuine problem rather than a cosmetic one. Always add ten percent, and if you have another small job on site such as post footings, a step or a shed pad, pour them the same day and put the concrete to use. Our note on what size truck turns up for a small residential pour covers how small deliveries work.
Getting It Delivered
Tell us the volume and what it is for, and we will sort the rest. Tank bases are a routine job for us across the region.
Give us your base diameter, your thickness and the date you want to pour, and we will confirm the volume, the mix and what it will cost including any short load charge, so there are no surprises. We supply pre-mix concrete for tank bases and small pours across Broadford, Kilmore, Wallan, Seymour and the surrounding area. Let the base cure properly before the tank goes on and well before you fill it, and our guide on how long concrete takes to cure covers the timeline. Call +61 3 5784 1500 or send your details through our contact page.
Frequently Asked Questions
How do I calculate concrete for a round slab?
Multiply pi by the radius squared by the thickness, working in metres. For a 3 metre round slab at 100 millimetres, that is 3.14 times 1.5 squared times 0.1, which is 0.71 cubic metres. Add ten percent for waste.
How much bigger than the tank should the base be?
Extend the base 100 to 150 millimetres beyond the tank wall all the way round, or more if the manufacturer specifies it. That keeps the tank off the weak slab edge and gives full support even if the tank is not positioned perfectly.
