How Much Concrete Do I Need? Cubic Yards, Bags and Waste
Work out how much concrete a slab or footing needs in cubic yards and bags, add the right waste allowance, and round up to a delivery load.
October 20, 2026 · 7 min read
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Quick answer: multiply the length, width and thickness of the pour in feet to get cubic feet, divide by 27 to get cubic yards, then add 5% to 10% for waste and round up to the next delivery increment. Most concrete is bought and priced by the cubic yard, so this is the number you need before you call a ready-mix supplier.
The formula for concrete volume
Concrete volume is a simple box calculation. A cubic yard is 3 feet by 3 feet by 3 feet, which is 27 cubic feet, so any volume you work out in cubic feet has to be divided by 27 to become cubic yards. The concrete calculator formula most estimating guides use is the same one:
length (ft) × width (ft) × thickness (ft) ÷ 27 = cubic yards
The catch is units. Thickness is almost always written in inches on a plan, so convert it to feet by dividing by 12 before you multiply. A 4 inch slab is 4 ÷ 12 = 0.33 feet, and a 6 inch slab is 0.5 feet. Mixing inches and feet in the same multiplication is the most common reason a concrete order is wrong by a factor of ten.
Worked example: a 20 by 20 foot slab
Take a patio slab 20 feet long, 20 feet wide and 4 inches thick. The crew works through it in four steps, and the table below shows the figures for this pour alongside two other common jobs.
- Convert the thickness: 4 inches ÷ 12 = 0.33 feet.
- Multiply the three dimensions: 20 × 20 × 0.33 = 133.3 cubic feet.
- Divide by 27: 133.3 ÷ 27 = 4.94 cubic yards of finished concrete.
- Add 10% for waste: 4.94 × 1.10 = 5.43 cubic yards, which rounds up to 5.5 cubic yards on a quarter-yard delivery.
| Pour | Dimensions | Volume (cu yd) | With 10% waste | Order (quarter-yard steps) |
|---|---|---|---|---|
| Patio slab | 20 ft × 20 ft × 4 in | 4.94 | 5.43 | 5.5 |
| Footing | 40 ft × 1.5 ft × 1 ft | 2.22 | 2.44 | 2.5 |
| Small pad | 10 ft × 10 ft × 4 in | 1.23 | 1.36 | 1.5 |
How much concrete a footing needs
Footings are long, narrow and deep, which makes them easy to under-measure. Multiply the run length by the width and then by the depth, all in feet. A footing 40 feet long, 1.5 feet wide and 1 foot deep is 60 cubic feet, or 2.22 cubic yards. Measure the full run along every corner, not the straight-line distance between the two ends.
Where the ground is uneven, measure the depth at several points and use the average. Measuring only the deepest or shallowest point will push the order a long way out, and on a footing that is a second delivery you do not want to pay for.
Estimating bags for a small pour
For a job too small to justify a truck, such as a fence post base or a short step, count bags instead. A 60 lb bag yields about 0.45 cubic feet of mixed concrete, so a cubic yard takes about 60 bags. An 80 lb bag yields about 0.60 cubic feet, or about 45 bags per cubic yard. Those figures come from published yield tables, and the manufacturer's printed yield on the bag is the number to trust.
Take a 3 foot by 3 foot patch 4 inches thick. That is 3 × 3 × 0.33 = 3.0 cubic feet. With 10% waste it is 3.3 cubic feet, and 3.3 ÷ 0.45 = 7.3, so buy 8 bags of 60 lb mix. Adding water too early or too late changes the yield slightly, so measure the water as the bag instructs.
How much waste to add
Concrete is lost to over-excavated trenches, uneven subgrade, spillage and the material left stuck in the truck or the wheelbarrow. Published estimating guides suggest adding roughly 4% to 10% as a general allowance, with 10% as a reasonable default. Vertigraph's concrete yardage guide and a slab estimating walkthrough both put complex or irregular pours at the higher end.
- Large, simple slabs: 5% to 8%, because the shape is predictable and there is less to cut around.
- Standard slabs and footings: about 10%, the usual default for a rectangular pour on reasonably level ground.
- Complex or irregular forms: 12% to 15%, for stepped pours, curved edges and anything with a lot of corners.
- Small, awkward pours: 15% to 20%, because a short-load fee can cost more than the extra concrete.
These are working ranges, not a code requirement. For a structural pour, the engineer's drawings and your supplier's advice take priority over any percentage in an article.
Rounding up to a delivery load
Ready-mix is delivered in fixed increments, and the increment depends on your supplier. Some suppliers deliver in quarter-yard steps and others in half-yard steps. Always round up, not to the nearest figure, because running short on a placed pour is far more expensive than a small amount left over. Ask the supplier about their minimum load and any short-load fee before you book the truck.
JobPlumb's takeoff tools trace the slab or footing on your plan and work out cubic yards, waste and order quantity for you, so the arithmetic is checked every time you change a dimension.
Start freeWhere to go next
If the pour is a shape rather than a simple rectangle, measure it first with the method in how to calculate square footage, splitting irregular areas into rectangles. For a full plan set with several slabs and footings, the rectangle-by-rectangle approach becomes a construction takeoff, and the concrete takeoff guide walks through a plan-based example. If you are setting up your first estimate, construction estimating for beginners shows where the concrete line fits in a full bid.
For a quick check on a single pour, the concrete calculator returns cubic feet, cubic yards and bag counts from the same three dimensions.
Frequently asked questions
How many cubic yards of concrete do I need for a 10 by 10 slab?
A 10 ft by 10 ft slab 4 inches thick is 33.3 cubic feet, or 1.23 cubic yards. Add 10% for waste to get 1.36 cubic yards, then order 1.5 cubic yards if your supplier delivers in quarter-yard steps.
How many bags of concrete make one cubic yard?
About 60 bags of a 60 lb mix, or about 45 bags of an 80 lb mix, if each bag yields 0.45 or 0.60 cubic feet. Those yields are typical rather than exact, so check the printed yield on the bag you buy before you order.
How much extra concrete should I order for waste?
Most estimators allow 5% to 10% on a simple, large slab and 10% to 15% on a complex or irregular pour. Small, awkward pours can need 15% to 20%, because short-load fees make over-ordering the cheaper mistake.
Is a concrete calculator accurate enough for a real job?
A calculator is accurate for simple rectangles, and the arithmetic is the same every time. For L-shaped or stepped pours, split the shape into rectangles first, add the volumes, and then apply your waste allowance.