A 20 × 12 ft patio slab
Slab, length 20 ft, width 12 ft, thickness 4 in.
Concrete needed, including waste
3.26 cubic yards
- Total required: 3.26 yd³ (88 ft³)
Concrete Calculator
Use this concrete calculator to work out how much concrete a slab, round slab, footing, wall, column or set of stairs needs, in cubic yards or cubic metres, and how many premixed bags that is.
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How much concrete do I need for a 12 × 12 ft patio?
A 12 × 12 ft slab that is 4 inches thick needs 1.78 yd³ (48 ft³) of concrete, or 1.96 yd³ with a 10% waste allowance. That is about 88 bags of 80 lb mix, or a 2 yd³ ready-mix order. A 4 in gravel base under it takes about 1.96 yd³, including waste.
Concrete needed, including waste
3.26 cubic yards
- Base volume
- 2.96 yd³
- Waste allowance (10%)
- + 0.30 yd³
- Total required
- 3.26 yd³ (88 ft³)
- Recommended orderRounded up for ready-mix delivery.
- 3.50 yd³
- Premixed 60 lb (27 kg) bags≈ 0.45 ft³ (13 L) per bag
- 196
- Premixed 80 lb (36 kg) bags≈ 0.60 ft³ (17 L) per bag
- 147
- Slab areaArea for reinforcing mesh, vapour barrier and forms.
- 240 ft²
- Gravel base neededAbout 4.39 short tons of crushed stone, incl. waste.
- 3.26 yd³ (88 ft³)
Inputs
- Project type: Slab
- How do you want to measure?: Length × width
- Length: 20 ft
- Width: 12 ft
- Thickness: 4 in
- Gravel base depth: 4 in
- Waste / overage: 10 %
- • 1 cubic yard = 27 cubic feet = 0.7646 m³. Your total is 3.26 yd³.
- • Above roughly 1–2 cubic yards, ready-mix delivery is usually easier than mixing bags. Ask your supplier about minimum loads.
- • The gravel base figure assumes crushed stone at about 2,700 lb/yd³. For other densities or depths, use the gravel calculator.
- • Most slabs also include steel mesh or rebar. Check your local code and your plans for the right type and spacing, and size the bars with the rebar calculator.
Results are estimates for planning only. Supplier specifications, product yields, site conditions and local codes vary. Confirm quantities with your supplier or a qualified professional before you buy or build.
How this is calculated
Formulas
- Slab: volume = length × width × thickness.
- Footing: volume = total footing length × width × depth.
- Round slab: volume = π × (diameter ÷ 2)² × thickness.
- Wall: volume = length × height × thickness.
- Column (round): volume = π × (diameter ÷ 2)² × height × number of columns.
- Stairs (solid): volume = width × run × rise × n(n+1) ÷ 2, where n is the number of steps.
- Total required = volume × (1 + waste %).
- Bags = total required ÷ yield per bag, rounded up. 1 cubic yard = 27 cubic feet.
Assumptions
- Slabs, footings and walls are simple rectangular prisms with constant thickness. Wall openings are not subtracted. Stairs are solid, poured down to the ground, with no landing.
- Premixed bag yields are typical manufacturer figures: about 0.45 ft³ for a 60 lb bag and 0.60 ft³ for an 80 lb bag. Check the bag you buy.
- The recommended order is rounded up to 0.25 yd³ or 0.1 m³.
Worked examples: common projects
A 12 m strip footing
Footing, total footing length 12 m, footing width 40 cm, footing depth 20 cm.
Concrete needed, including waste
1.06 cubic metres
- Total required: 1.06 m³ (1,056 L)
A two-car driveway, 20 × 20 ft at 5 in
Slab, length 20 ft, width 20 ft, thickness 5 in.
Concrete needed, including waste
6.79 cubic yards
- Total required: 6.79 yd³ (183.3 ft³)
A 4 × 3 m shed slab, 10 cm thick
Slab, length 4 m, width 3 m, thickness 10 cm.
Concrete needed, including waste
1.32 cubic metres
- Total required: 1.32 m³ (1,320 L)
A 20 ft garden wall, 3 ft high and 8 in thick
Wall, wall length 20 ft, wall height 3 ft, wall thickness 8 in.
Concrete needed, including waste
1.63 cubic yards
- Total required: 1.63 yd³ (44 ft³)
Three solid concrete steps, 4 ft wide
Stairs, number of steps 3 pcs, rise (height of each step) 7 in, run (depth of each tread) 11 in, stair width 4 ft.
Concrete needed, including waste
0.52 cubic yards
- Total required: 0.52 yd³ (14.1 ft³)
Ten fence posts in 10 × 24 in holes
Post holes, hole diameter 10 in, hole depth 24 in, post width (square) 4 in, number of posts 10 pcs.
Concrete needed, including waste
0.35 cubic yards
- Total required: 0.35 yd³ (9.6 ft³)
- Concrete per post: 0.96 ft³ (incl. waste)
Quick reference
Cubic yards to bags of concrete
Using typical yields of 0.45 ft³ (60 lb bag) and 0.60 ft³ (80 lb bag). Round up and add your waste allowance.
| Cubic yards | Cubic feet | 60 lb bags | 80 lb bags | Cubic metres |
|---|---|---|---|---|
| 0.25 | 6.8 | 15 | 12 | 0.19 |
| 0.5 | 13.5 | 30 | 23 | 0.38 |
| 1 | 27 | 60 | 45 | 0.76 |
| 2 | 54 | 120 | 90 | 1.53 |
| 3 | 81 | 180 | 135 | 2.29 |
| 5 | 135 | 300 | 225 | 3.82 |
| 10 | 270 | 600 | 450 | 7.65 |
Slab thickness to concrete per 100 sq ft
Per 100 square feet of slab, before and after a 10% waste allowance.
| Thickness | Cubic yards | With 10% waste | 80 lb bags (with waste) |
|---|---|---|---|
| 3 in | 0.93 | 1.02 | 46 |
| 4 in | 1.23 | 1.36 | 62 |
| 5 in | 1.54 | 1.70 | 77 |
| 6 in | 1.85 | 2.04 | 92 |
Concrete per fence post
One post, no waste. The post's own volume is subtracted. 80 lb bags yield about 0.60 ft³.
| Hole and post | Concrete | 80 lb bags |
|---|---|---|
| 8 in wide × 24 in deep, 4×4 post | 0.48 ft³ | 0.79 |
| 10 in wide × 24 in deep, 4×4 post | 0.87 ft³ | 1.45 |
| 10 in wide × 30 in deep, 4×4 post | 1.09 ft³ | 1.81 |
| 12 in wide × 30 in deep, 4×4 post | 1.69 ft³ | 2.81 |
| 12 in wide × 36 in deep, 6×6 post | 1.61 ft³ | 2.68 |
Before you order concrete
Measure the finished dimensions of the pour, then add a waste allowance. Uneven subgrade, spillage and slightly over-dug trenches all use more concrete than the drawing says.
Small jobs are often poured from premixed bags. Larger pours are usually ordered as ready-mix by the cubic yard or cubic metre. Suppliers set their own minimum load sizes and short-load fees, so ask before you finalise the quantity. Pouring under pavers or a fence? Plan the gravel base, the paver layout or the fence posts too.
How to measure each type of pour
Slab: measure length and width at the outside edge of the forms, and thickness at the thinnest point. If the subgrade is uneven, measure thickness in several places and use the average. Round slab: measure the diameter at the widest point of the form.
Footing: add up the total length of every trench run, including corners, then enter the trench width and depth. Wall: enter the length, the height of the poured section and the thickness. The calculator does not subtract door or window openings, so for walls with large openings, calculate the pieces separately.
Stairs: count the steps, then measure the rise (the height of each step) and the run (the depth of each tread). The calculator assumes solid stairs poured down to the ground, where each step is one rise taller than the one below it. Landings and side walls are separate pours. Column: enter the diameter of the form tube, its height, and how many you are pouring.
Thickness, base and reinforcement
Four inches (10 cm) is a common thickness for patios, walkways and shed slabs. Driveways and anywhere that carries heavy loads are usually poured thicker, and the right figure depends on soil, climate and the vehicles involved. Ask your local building department or a contractor.
Most slabs sit on a compacted gravel base, and many include steel mesh or rebar. Footings, walls and stairs can be structural, so they are often governed by building code and may need inspection. Use this calculator for quantities and follow your local requirements for design.
Bags or ready-mix?
A cubic yard needs about 45 bags of 80 lb mix, or 60 bags of 60 lb mix. Mixing that by hand or with a drum mixer is slow, and a pour that stops partway can leave a visible joint. For anything beyond a small pad, ready-mix delivered by truck is usually the practical choice. Suppliers set minimum loads and charge a short-load fee below them, so ask before you decide.
Tell the supplier your dimensions and the volume you calculated, and let them advise on the mix strength for your use and climate. Concrete keeps gaining strength for weeks after the pour, so follow their guidance on curing and on when you can walk, drive or build on it.
Concrete calculator in square feet or square metres
Already know the slab's area from a plan? Choose Slab, then I know the area, and enter square feet or square metres plus the thickness. The result is the same as length × width × thickness. For a footing, wall, stairs or post holes, pick that type instead.
Frequently asked questions
How many 80 lb bags of concrete make a cubic yard?
About 45. A cubic yard is 27 cubic feet and an 80 lb bag yields roughly 0.60 cubic feet, so 27 ÷ 0.60 = 45 bags. Add your waste allowance on top.
How much extra concrete should I order?
5–10% is a common allowance. Use more if the ground is uneven or the forms are rough. Running short mid-pour is usually more costly than having a little left over.
How thick should a concrete slab be?
Requirements depend on use, soil and local code. Patios and walkways are commonly poured around 4 inches (10 cm); driveways and garage floors are often thicker. Confirm with your local code or a professional.
When should I use ready-mix instead of bags?
As a rough guide, once a job passes one to two cubic yards, delivered ready-mix is easier than mixing bags. Your supplier can tell you their minimum load.
How do I convert cubic yards to cubic metres?
Multiply cubic yards by 0.7646. This calculator also shows the other unit system when you switch between imperial and metric.
How much concrete do I need for a 10 × 10 slab?
At 4 inches thick it is 33.3 cubic feet, or 1.23 cubic yards. With a 10% waste allowance that becomes about 1.36 cubic yards, or 62 bags of 80 lb mix.
How many bags of concrete for a 12 × 12 patio?
At 4 inches thick the slab is 48 cubic feet. With 10% waste you need about 52.8 cubic feet, which is 88 bags of 80 lb mix or 118 bags of 60 lb mix.
How do I calculate concrete for steps?
For solid stairs, multiply width × run × rise × n(n+1)/2, where n is the number of steps. Each step is poured one rise taller than the step below it, so a three-step stair stacks 1, 2 and 3 risers. The calculator does this for you when you choose Stairs.
How much does a cubic yard of concrete weigh?
Roughly 4,000 pounds, or about 2 short tons. That matters for the load on trucks and for anything that is moving the material by hand.
Should I use 60 lb or 80 lb bags?
80 lb bags need fewer trips and fewer bags to open, so they are usual for bigger jobs. 60 lb bags are easier to lift and carry. Total concrete is the same either way. Compare the bag counts above.
How much concrete do I need for a fence post?
For a 10 inch wide, 24 inch deep hole around a 4 × 4 post, about 0.87 cubic feet, or roughly 1.5 bags of 80 lb mix. A 12 inch hole 30 inches deep takes about 1.7 cubic feet, or 2.8 bags. Choose Post holes in the calculator to enter your own sizes.
How much concrete does a concrete truck hold?
Ready-mix trucks commonly carry about 8 to 10 cubic yards. Your supplier can tell you the load size and the charge for loads under their minimum.
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Results are estimates for planning only. Supplier specifications, product yields, site conditions and local codes vary. Confirm quantities with your supplier or a qualified professional before you buy or build.