Work out the concrete for pad footings, water tank pads, spa pads, shed pads and small equipment bases. Get the pad size, m³, 20 kg premix bags, ready-mix order and mesh in one result.
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All your pads, with your extra allowance. For comparison only.
Concrete Needed
0m³
This concrete pad calculator is made for small, separate pours. Enter the pad, or the item that will sit on it, and it works out the concrete. For a whole floor or shed slab, use the concrete slab calculator. For long trenches, use the concrete footing calculator.
Results update as you type after the first calculation, and the Download Result PDF button saves a copy.
What this tool does not do. It works out quantities. It does not design a footing, check soil or decide how thick a pad must be. A pad that carries a roof, deck or full tank needs the right design, so follow your plans, the maker's guide, your engineer or your council. Our slab thickness guide explains the terms.
The word covers two different jobs, and they are worth keeping apart.
A pad footing is a block of concrete in the ground. It spreads the load from a post, stump or column into the soil. Decks, carports, verandahs and subfloors often use them.
A surface pad is a small, thin slab that gives something a flat, stable base. Water tanks, spas, garden sheds, bins, generators and air conditioner units sit on these.
The maths is the same for both, so one concrete pad calculator handles them. The design is not the same, because a footing is usually deep and narrow while a surface pad is wide and thin.
Square or rectangular pad (m³) = length × width × thickness
Round pad (m³) = 3.1416 × diameter² ÷ 4 × thickness
Pad around an item = item size + (2 × margin)
Premix bags = m³ with extra ÷ 0.0093, rounded up
All sizes are in metres, so change 450 mm to 0.45 m before you multiply. A pad 450 mm square and 300 mm deep is 0.45 × 0.45 × 0.3 = 0.061 m³. The 0.0093 m³ yield is the same starting figure as our concrete bag calculator. Check your bag's printed yield, and use the concrete bag yield calculator if it differs.
These tables show one pad with 10% extra. Bags are 20 kg premix at 0.0093 m³, rounded up.
| Footing Size | 300 mm Deep | 450 mm Deep | 600 mm Deep |
|---|---|---|---|
| 300 × 300 mm | 0.030 m³, 4 bags | 0.045 m³, 5 bags | 0.059 m³, 7 bags |
| 350 × 350 mm | 0.040 m³, 5 bags | 0.061 m³, 7 bags | 0.081 m³, 9 bags |
| 450 × 450 mm | 0.067 m³, 8 bags | 0.100 m³, 11 bags | 0.134 m³, 15 bags |
| 600 × 600 mm | 0.119 m³, 13 bags | 0.178 m³, 20 bags | 0.238 m³, 26 bags |
| 750 × 750 mm | 0.186 m³, 20 bags | 0.278 m³, 30 bags | 0.371 m³, 40 bags |
| Surface Pad | Area | 100 mm Thick | 150 mm Thick |
|---|---|---|---|
| 1 × 1 m | 1 m² | 0.110 m³, 12 bags | 0.165 m³, 18 bags |
| 1.2 × 0.6 m | 0.72 m² | 0.079 m³, 9 bags | 0.119 m³, 13 bags |
| 1.5 × 1.5 m | 2.25 m² | 0.248 m³, 27 bags | 0.371 m³, 40 bags |
| 2 × 2 m | 4 m² | 0.440 m³, 48 bags | 0.660 m³, 71 bags |
| 3 × 3 m | 9 m² | 0.990 m³, 107 bags | 1.485 m³, 160 bags |
| 4 × 3 m | 12 m² | 1.320 m³, 142 bags | 1.980 m³, 213 bags |
These sizes are examples of the maths, not recommendations for your job.
Pad footing sizes depend on the load, the soil and the spacing between pads. RMIT's Buildright material shows a stump pad with a 300 mm diameter and at least 150 mm of concrete. It also lists depth by site class, from 400 mm on class A and S sites to 500 mm on M and 1,000 mm on H sites, and points to AS 2870 and AS 1684 for selection.
That is training material based on an older edition, so use it as a guide to how sizes change, not a design. Your plans or engineer decide the real figure. The NCC Housing Provisions say their footing configurations only suit the soil classifications they specify.
When you dig, the hole sets the volume. A pad poured into a rough hole uses more concrete than the neat size, so measure the real hole and add extra. For a post set in concrete, the concrete post hole calculator takes off the post itself.
Tank makers set their own base rules, and warranties can depend on them. Published guidance includes:
The concrete pad calculator starts with a 300 mm margin each side. Change it to match your tank's guide.
| Tank Diameter | Square Pad | Volume At 100 mm | With 10% Extra | Premix Bags |
|---|---|---|---|---|
| 1.5 m | 2.1 × 2.1 m | 0.441 m³ | 0.485 m³ | 53 |
| 2 m | 2.6 × 2.6 m | 0.676 m³ | 0.744 m³ | 80 |
| 2.4 m | 3 × 3 m | 0.900 m³ | 0.990 m³ | 107 |
| 3 m | 3.6 × 3.6 m | 1.296 m³ | 1.426 m³ | 154 |
| 3.5 m | 4.1 × 4.1 m | 1.681 m³ | 1.849 m³ | 199 |
Water is heavy. One litre weighs about 1 kg, so a full 5,000 L tank holds about 5 tonnes of water before you count the tank. Enter the capacity, and the calculator shows that weight and the pressure across the tank's base. For a 2.4 m tank, that comes to about 10.8 kPa. It is a guide to the load, not a soil or slab check.
Because a tank is so heavy, don't place one over fill, near an unsupported edge or on a slope without advice.
Spa World gives a minimum 100 mm reinforced slab for portable spas up to 3.5 m, and recommends 150 mm with two layers of mesh for swim spas and larger spas. Its generic spa pad guide uses SL72 mesh near the top with at least 25 mm of cover, over a 0.2 mm plastic vapour barrier and 50 mm of sand. It also assumes at least 50 kPa soil bearing capacity.
Spa World also warns against more than 20 mm of fall across the length of the spa, and says weights are on each model's technical sheet. Enter your model's full weight in the options to see the pressure under the spa. Those guides are for that seller's spas, so follow the manual for yours.
The Stratco Saver shed guide gives two options for one small shed. One is a full base at least 200 mm larger than the shed. The other is separate pads at each anchor point, at least 250 mm in diameter and 400 mm deep in firm natural soil.
That is why this concrete pad calculator suits both. Choose Spa, Shed Or Unit for a full base, or Round Pad with a quantity for anchor pads. Stratco says the installer must confirm the base can support the shed and resist wind, and it says not to anchor to pavers. For a bigger shed or garage, use the concrete slab material calculator for a full list.
Outdoor air conditioner units, heat pumps and hot water units often sit on a small pad. Size and clearance rules come from the maker's installation manual, including how far the unit must sit from walls and how water drains. Follow that manual and your installer's advice, since our figures here are just examples.
You may not need to pour at all. Stratco lists precast 900 × 450 mm slabs, and yards often stock similar sizes. If you do pour, a 1.1 × 0.55 m pad at 100 mm takes 8 premix bags with 10% extra.
For most pads, it comes down to the volume and how far you must carry it.
| Volume | Usual Choice | Why |
|---|---|---|
| Under 0.1 m³ | Premix bags | Cement Australia says this is easy to mix from bags |
| 0.1 to 0.5 m³ | Bags, or mix your own | Cement Australia suggests considering your own mix up to here |
| 0.5 to 3 m³ | Compare both | Many bags, but short load fees may apply |
| Over 3 m³ | Ready-mix | Meets the usual minimum load |
iSeekPlant says most suppliers want 3 to 4 m³ per delivery, with a short load fee of around $80 per m³ or a flat $120 to $150 in some cases. The result shows the total weight of bags, because 1 m³ with 10% extra is about 119 bags, or 2.4 tonnes to carry. Check your vehicle's limit before you collect them. The concrete order calculator and concrete delivery cost calculator help you compare. To mix from cement, sand and stone, use the concrete aggregate calculator.
Pick the mesh on your plans in the options, and the concrete pad calculator counts whole 6 × 2.4 m sheets. Small pads are cut from one sheet where they fit, with a 50 mm clearance from the edge. Larger pads allow a 225 mm lap. Sheet size, lap and clearance are the same figures as our concrete slab material calculator.
Enter a base depth if your job has one, such as the 50 mm of sand in Spa World's guide. The tool adds 30% for compaction, based on Cement Australia's advice to place 60 to 70 mm and tamp it to 50 mm. Mesh needs support to stay inside the concrete, so use bar chairs. Cement Australia says to keep mesh at least 5 cm above the base and about 4 cm below the top. On a thin pad those two figures won't both fit, so follow your plans.
These examples use the same maths as the concrete pad calculator, so you can enter them and see matching results.
Each pad is 450 × 450 mm and 300 mm deep.
The tank is 2.4 m across, with a 300 mm margin and a 100 mm square pad.
The spa is 2.2 × 2.2 m with a 200 mm margin, on a 100 mm pad with 50 mm of sand.
Each pad is 300 mm across and 400 mm deep.
Tank and spa guides want a flat pad, while most outdoor concrete needs to shed water. Check your maker's guide before adding a fall, and see the concrete slope calculator for the numbers. Cement Australia says to allow at least 7 days before heavy loads, and Water Tank Factory says 5 to 7 days before installing a tank. The guide to pouring a concrete slab has more. Wet cement can burn skin, and cutting concrete creates silica dust, so read Safe Work Australia's silica guidance.
Enter your bag price and ready-mix rate in the concrete pad calculator options to compare them. For ready-mix, Yellow Pages lists about $400 to $450 per m³, and iSeekPlant gives $200 to $350 per m³. Short load fees can matter more than the rate on a small pad. Also allow for formwork, mesh, base, tools and any labour. The concrete cost calculator takes that further.
Multiply length, width and thickness in metres, then add about 10% extra. For a round pad, use 3.1416 × diameter² ÷ 4 × thickness.
At 100 mm thick, about 12 × 20 kg premix bags with 10% extra. At 150 mm, about 18.
A 450 × 450 × 300 mm pad takes about 8 bags with 10% extra. Check the bag's yield.
It depends on the load and maker's guide. Published tank and portable spa guides start at 100 mm. Pad footings come from your plans.
Published guides range from 100 mm larger overall to at least 300 mm past the tank. Follow your tank's guide.
Water Tank Factory and Kingspan both include mesh in their concrete slab guidance. Check your tank's guide.
Water weighs about 1 kg per litre, so 5,000 L is about 5 tonnes, plus the tank.
Spa World gives at least 100 mm reinforced for portable spas up to 3.5 m, and 150 mm with two layers of mesh for swim spas and larger spas.
Some can. The Stratco Saver guide allows pads at each anchor point. Check your shed's instructions and wind rating.
A block of concrete in the ground that spreads a post or stump load into the soil.
A pad is a small, separate piece of concrete. A slab usually covers a larger floor or yard area.
Bags usually suit small pads. Near 1 m³ or more, compare them with ready-mix, including short load fees.
Water Tank Factory says at least 5 to 7 days. Follow your tank maker's guide.
No. Your numbers stay in your browser and nothing is uploaded. Use Download Result PDF to keep a copy.
Figures reviewed November 2026. Sources: Cement Australia, Water Tank Factory, Kingspan, Spa World, Stratco, RMIT, iSeekPlant, Yellow Pages and the other sources linked in the sections above. Maker guides, prices and rules change, so treat this page as a planning guide. Read our Disclaimer and Calculator Methodology.