How Much Concrete Do You Need?
How much concrete you need for a slab, footing or post hole: volume in cubic yards and m³, 60 lb and 80 lb bag counts, waste allowance, and ready-mix vs bags.
By Udit Agarwal · Updated October 17, 2026
To work out how much concrete you need, multiply length × width × thickness with all three in feet to get cubic feet, then divide by 27 to get cubic yards. A 10 ft by 10 ft slab 4 inches thick is 10 × 10 × (4 ÷ 12) = 33.33 cubic feet, or 1.23 cubic yards (0.94 m³). Order 5 to 10% more than that to cover spillage and uneven ground.
Volume = length × width × thickness. The one rule is to put every measurement in the same unit before you multiply. Slab thickness is usually given in inches, so divide it by 12 to get feet. Multiplying feet by feet by inches without converting gives an answer 12 times too big.
| Thickness | In feet | Area one cubic yard covers |
|---|---|---|
| 4 in | 0.333 ft | 81 sq ft |
| 5 in | 0.417 ft | 64.8 sq ft |
| 6 in | 0.5 ft | 54 sq ft |
| 8 in | 0.667 ft | 40.5 sq ft |
Then divide the cubic feet by 27 for cubic yards. A driveway slab 20 ft by 12 ft at 6 inches is 20 × 12 × 0.5 = 120 cu ft, or 4.44 cu yd (3.40 m³). The coverage column works the other way round: one cubic yard spread 4 inches thick covers 81 sq ft, so a 324 sq ft patio at that thickness needs 324 ÷ 81 = 4 cubic yards. If the area is not a simple rectangle, work it out first by splitting it into pieces, as the square footage guide shows, then multiply by the thickness.
In metric, keep everything in metres. A 4 m by 3 m slab 10 cm thick is 4 × 3 × 0.10 = 1.2 m³, and one cubic metre spread 10 cm thick covers 10 m².
A cubic yard is a cube 3 ft on each side, so it holds 3 × 3 × 3 = 27 cubic feet. Since the 1959 international agreement on units, a yard has been exactly 0.9144 m, which makes a cubic yard 0.9144³ = 0.7646 m³. Ready-mix concrete is ordered by the cubic yard in the US and by the cubic metre in most other countries.
| Unit | Cubic feet | Cubic yards | Cubic metres |
|---|---|---|---|
| 1 cubic foot | 1 | 0.037 | 0.0283 |
| 1 cubic yard | 27 | 1 | 0.7646 |
| 1 cubic metre | 35.31 | 1.308 | 1 |
A strip footing, the continuous base under a wall, is another rectangle: length × width × depth. A footing 20 ft long, 16 in wide and 8 in deep is 20 × (16 ÷ 12) × (8 ÷ 12) = 17.78 cu ft, or 0.66 cu yd.
A round column, a post hole or a cardboard tube form is a cylinder: π × radius² × height, where the radius is half the diameter. A column 12 inches across and 8 ft tall is π × 0.5² × 8 = 6.28 cu ft, or 0.23 cu yd. A round form holds π ÷ 4, about 78.5%, of a square one of the same width, so estimating round forms as boxes orders about 27% too much.
For several identical holes, multiply. Eight deck footings in 10-inch holes 3 ft deep need 8 × π × (5/12)² × 3 = 13.09 cu ft, or 0.48 cu yd. If a post will be set in the concrete, subtract its volume from each hole: a 4×4 post actually measures 3.5 × 3.5 inches and takes up about 0.26 cu ft of a 3 ft hole.
Walls and square columns are boxes too. A wall is a slab turned on its edge, so its volume is length × height × thickness: a garden wall 20 ft long, 4 ft high and 8 inches thick is 20 × 4 × (8 ÷ 12) = 53.33 cu ft, or 1.98 cu yd. For cones, spheres and other solids, the Volume Calculator has the formulas.
Bags of premixed concrete are sold by weight, but what matters is how much concrete each bag makes once mixed, its yield. The ArrayKit calculator uses typical yields of about 0.45 cu ft for a 60 lb bag and 0.6 cu ft for an 80 lb bag. Bags needed = volume in cubic feet ÷ yield per bag, rounded up to a whole bag.
| Volume | 60 lb bags (0.45 cu ft each) | 80 lb bags (0.6 cu ft each) |
|---|---|---|
| 1 cubic foot | 3 (2.2 rounded up) | 2 (1.7 rounded up) |
| 1 cubic yard (27 cu ft) | 60 | 45 |
| 1 cubic metre (35.31 cu ft) | 79 (78.5 rounded up) | 59 (58.9 rounded up) |
| 10 × 10 ft slab, 4 in thick (33.33 cu ft) | 75 | 56 |
Yields vary by brand and mix, so check the figure printed on the bag and divide by that if it differs. Bags sold by the kilogram work the same way: divide the volume by the yield stated on the bag.
The weight adds up quickly. The 56 bags of 80 lb mix for that 10 × 10 slab weigh 4,480 lb, more than two US tons, and every bag has to be lifted, carried and mixed.
Order 5 to 10% more than the calculated volume. The extra covers concrete left in the mixer and wheelbarrow, spillage, ground that is a little low in places and forms that bow outward under the weight. Running short partway through a pour is worse than having a little left over, because fresh concrete placed against concrete that has already started to set does not bond fully and leaves a weak joint.
To add the allowance, multiply by 1.05 for 5% or 1.10 for 10%. For the 10 × 10 ft slab, 1.2346 cu yd × 1.10 ≈ 1.36 cu yd. In bags, 33.33 cu ft × 1.10 ≈ 36.67 cu ft, which is 82 bags of 60 lb mix or 62 bags of 80 lb mix. The percentage guide explains why adding 10% is the same as multiplying by 1.10.
Bagged concrete suits small jobs that you can mix a bag at a time: setting posts, a small pad or step, or repairs. A cubic yard takes 45 bags of 80 lb mix, 3,600 lb in all, so beyond about that size the bag count and the mixing time climb steeply. For larger pours, ready-mix delivered by truck arrives already mixed to a specified strength and can be placed in one go, which matters for a slab that has to be finished before it sets.
Before ordering ready-mix, ask the supplier about minimum orders and small-load charges, check that the truck can reach the forms or whether you need a pump, and line up enough people to place and finish the concrete. A slab is often poured on a compacted gravel base, and the Gravel Calculator works out its volume and weight. If the slab is reinforced, the Rebar Calculator counts the bars at your chosen spacing.
The Concrete Calculator has three shapes and two unit systems. Pick Slab, Footing or Column, then US (ft / in) or Metric (m / cm). Each field shows its unit, and they are not all the same:
| Shape | Feet (or metres) | Inches (or centimetres) | Starting values |
|---|---|---|---|
| Slab | length, width | thickness | 10 ft × 10 ft × 4 in |
| Footing | length | width, depth | 20 ft × 16 in × 8 in |
| Column | height | diameter | 12 in diameter, 8 ft high |
Quantity multiplies the volume for several identical pours, such as eight post holes. The result shows the volume in cubic yards and cubic metres to two decimal places and the number of 60 lb and 80 lb bags, rounded up once for the whole quantity, and the copy button copies a plain-text summary. Slabs and footings use length × width × thickness (or depth), and columns use π × r² × height, the formulas above. The starting slab gives 1.23 cu yd, 0.94 m³, 75 bags of 60 lb mix and 56 bags of 80 lb mix.
Its limits:
It does not price concrete. Everything is calculated in your browser, and the results are planning estimates to confirm with your supplier before you order.
How many bags of concrete do I need for a 10x10 slab? At 4 inches thick, a 10 × 10 ft slab is 33.33 cu ft, which is 75 bags of 60 lb mix or 56 bags of 80 lb mix at typical yields. With 10% extra, plan on 82 or 62 bags.
How many 80 lb bags of concrete make a yard? About 45, because a cubic yard is 27 cu ft and an 80 lb bag yields about 0.6 cu ft. With 60 lb bags at about 0.45 cu ft each, it takes 60.
How many cubic yards are in a cubic metre? About 1.308. One cubic yard is 0.7646 m³, so multiply cubic yards by 0.7646 to get cubic metres, and multiply cubic metres by 1.308 to get cubic yards.
How do I calculate concrete for a post hole? Use π × radius² × depth with everything in feet, where the radius is half the hole's width. A 10-inch hole 3 ft deep holds about 1.64 cu ft, which is 4 bags of 60 lb mix or 3 bags of 80 lb mix. Subtract the post's volume if it will be set in the concrete.
How thick should a concrete slab be? Four inches is the usual thickness for patios, walkways and shed bases, and driveways are commonly 4 to 6 inches. Local building codes and any engineer's design take precedence for structural work.
How much extra concrete should I order? Add 5 to 10% to the calculated volume to cover spillage, uneven ground and forms that bulge. Running short is worse than a small surplus, because a second batch placed against setting concrete leaves a weak joint.
Should I use bags or ready-mix concrete? Bags suit small jobs such as posts, steps and small pads. A cubic yard is already 45 bags of 80 lb mix weighing 3,600 lb, so for bigger pours a ready-mix delivery is usually more practical. Ask the supplier about minimum orders and small-load charges.
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