Concrete calculator
Work out concrete volume in cubic yards, cubic metres and bags for slabs, footings and post holes, with a waste allowance.
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Area
240 sq ft
Area
4.71 sq ft
Measured area
244.71 sq ft
With 5% waste
256.95 sq ft
In square metres
23.87 m²
In square yards
28.55 sq yd
Bags of concrete
165
0.6 cu ft each
Estimated cost
$0.00
enter a price to see this
| Area | Shape | Dimensions | Sq ft | Cu ft |
|---|---|---|---|---|
| Patio slab | Slab / footing | 20' 0" × 12' 0" | 240 | 80 |
| Post holes | Round column / post hole | 1' 0" | 4.71 | 14.14 |
Formula and working
Area of each shape
Slab / footing: length × width × thickness · Round column / post hole: π × (diameter ÷ 2)² × depth Patio slab: 20.00 × 12.00 ft = 240.00 sq ft Post holes: 1.00 ft × 6 = 4.71 sq ft = 244.71 sq ft total
Volume
cubic feet = area × thickness ÷ 12 total = 94.14 cu ft, plus 5% waste = 98.84 cu ft cubic yards = 98.84 ÷ 27 = 3.661 cu yd (2.799 m³)
Bags
bags = cubic feet ÷ yield per bag, rounded up = 98.84 ÷ 0.6 = 165 bags of 80 lb
Metric conversion
1 sq ft = 0.09290304 m² exactly (1 ft = 0.3048 m) 244.71 × 0.09290304 = 22.735 m²
Feet-and-inches entries such as 12′ 6″ are read exactly, and the conversion to metres uses the international foot of 0.3048 m, which is exact by definition.
Assumptions
- Measure the longest and widest points of each space, and split awkward rooms into two rectangles rather than trying to describe them with one shape.
- Waste allowance covers off-cuts, breakages and mistakes. Ten per cent is the usual figure; go to fifteen for diagonal layouts, herringbone patterns or a room with many angles.
- Quantities are always rounded up to whole units — cans, boxes and bags are not sold in fractions.
- Bag yields are the manufacturer-stated figures for standard ready-mix: 0.30, 0.375, 0.45 and 0.60 cubic feet for 40, 50, 60 and 80 lb bags. Ready-mix delivered by truck is ordered in cubic yards or cubic metres, usually with a quarter-yard minimum.
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Volume, not area
Concrete is the one job on this site where the third dimension does the work. Everything follows from it:
slab: length × width × (thickness ÷ 12) [cubic feet]
round hole: π × (diameter ÷ 2)² × (depth ÷ 12) [cubic feet]
cubic yards: cubic feet ÷ 27
cubic metres: cubic feet × 0.0283168A 20 × 12 ft patio slab at 4 inches thick is 20 × 12 × 0.333 = 80 cubic feet, or 2.96 cubic yards. Add a 10% allowance and you are ordering 3.26 — which in practice means 3.5 yards, because trucks deliver in quarter-yard increments.
Bags or a truck?
| Bag | Yield | Bags per cubic yard | Bags per cubic metre |
|---|---|---|---|
| 40 lb | 0.30 cu ft | 90 | 118 |
| 50 lb | 0.375 cu ft | 72 | 94 |
| 60 lb | 0.45 cu ft | 60 | 79 |
| 80 lb | 0.60 cu ft | 45 | 59 |
The practical crossover is around one cubic yard. Below that, bags are simpler and there is no truck minimum to pay. Above it you are mixing forty-five 80 lb bags by hand, which is roughly two tonnes of material and most of a day — a ready-mix truck is cheaper as well as faster.
Most suppliers have a minimum load (often 1 cubic yard) and a short-load surcharge below their full-truck quantity, so ask before assuming. Trucks also have a limited discharge window before the mix goes off; have everything formed, reinforced and staffed before it arrives.
Typical thicknesses
| Application | Thickness |
|---|---|
| Footpath, patio, shed base | 4 in / 100 mm |
| Driveway (cars) | 4–5 in / 100–125 mm |
| Driveway (heavy vehicles) | 6 in / 150 mm |
| Garage floor | 5–6 in / 125–150 mm |
| Fence post hole depth | ⅓ of post height, min 24 in |
Local building codes and frost-depth requirements override any rule of thumb, especially for footings — in cold climates footings must sit below the frost line.
Post holes
Use the round column shape: enter the hole diameter and the depth in inches, and set the quantity to the number of identical holes. A 12 inch diameter hole 36 inches deep is 2.36 cubic feet, so six of them is 14.1 cubic feet — twenty-four 80 lb bags.
The post itself displaces some of that. A 4 × 4 in post in a 12 in hole occupies about 11% of the volume, which the waste allowance comfortably covers. A larger post in a tight hole is worth subtracting properly.
Why to over-order, not under-order
Running out mid-pour is the one mistake in concrete work that cannot be fixed later. The first pour begins curing immediately, and topping it up half an hour later creates a cold joint — a permanent plane of weakness and a visible line. Five to ten per cent over is standard, and there are three reasons for it:
- Sub-base is never perfectly level, and low spots swallow surprising quantities.
- Timber forms bow outwards under the weight of wet concrete.
- Spillage, and whatever is left in the barrow and the mixer.
What the volume does not tell you
- Strength. Mix specification (3,000 psi / C20-25 and up) is chosen for the application and the climate, not the quantity.
- Reinforcement. Rebar, mesh, fibre and control joints are separate decisions, and the joints in particular decide whether the slab cracks in a straight line or randomly.
- Sub-base. Most slabs need a compacted hardcore or gravel layer beneath, which is its own volume calculation.
- Weight. Concrete is roughly 150 lb per cubic foot, or 2,400 kg per cubic metre. A cubic yard is about two tonnes — worth knowing before you plan the access.
Mixed jobs
Add a slab row for each flat pour and a round column row for each set of holes, set quantities where they repeat, and the total covers the whole job. That is the number to give the supplier, and it exports to CSV, Excel or PDF along with the individual pours so the breakdown is checkable.
Works on site, with no signal
Nothing here is uploaded and there is no server behind the page — once loaded it runs entirely on your device, which is the point when you are standing in a garden with a tape measure and no reception. Your list is stored only if you switch saving on, only in this browser.
Questions
How do I calculate cubic yards of concrete?
Multiply length by width by thickness in feet, then divide by 27. A 20 × 12 foot slab at 4 inches thick is 80 cubic feet, which is 2.96 cubic yards before any waste allowance.
How many bags of concrete are in a cubic yard?
Forty-five 80 lb bags, sixty 60 lb bags, or ninety 40 lb bags. Those bag yields are 0.60, 0.45 and 0.30 cubic feet respectively, and this page rounds up to whole bags.
When should I order ready-mix instead of bags?
Around a cubic yard is the practical crossover. Above that you are mixing forty-five bags by hand, and a truck is cheaper as well as faster — though most suppliers have a minimum load and a short-load charge.
How much extra should I order?
Five to ten per cent. Sub-base is never perfectly level, forms bulge slightly, and running out mid-pour leaves a cold joint you cannot undo.
Can I mix slabs and post holes in one job?
Yes. Add a slab row for each flat pour and a round column row for each hole, set a quantity for repeated holes, and the volume totals across everything.