Concrete is sold by the cubic yard and in bags by weight, but projects are measured in feet and inches. This calculator handles the conversion for the shapes you are most likely to pour — rectangular slabs, footings and walls, round columns, hollow tubes, curb and gutter runs, and steps — and tells you how many yards to order or how many premix bags to buy.
How to use the concrete calculator
- Choose the shape you are pouring.
- Enter each dimension and pick its unit. You can mix units freely, such as feet for length and inches for thickness.
- Enter the number of identical pours — for example, eight fence-post holes of the same size.
- Set the extra for waste; 10% is a sensible default for most DIY pours.
- Read the total in cubic yards, cubic feet and cubic meters, and compare the 40, 60 and 80 lb bag counts.
Concrete volume formulas
The calculator converts every dimension to feet, finds the volume in cubic feet, and divides by 27 for cubic yards.
For curbs, c is curb depth, h is curb height above the gutter, g is gutter width and f is the flag (gutter) thickness. Steps are treated as solid blocks down to grade: step i is one tread deep and i risers tall, so the side profile is run × rise × (1 + 2 + … + n), plus any top landing, multiplied by the stair width.
Bag counts divide the total by each bag’s yield and round up:
Worked example: a 10 × 10 ft patio
Volume = 10 ft × 10 ft × (4 ÷ 12) ft = 33.33 ft³ = 1.23 yd³
With 10% extra: 33.33 × 1.10 = 36.67 ft³ = 1.36 yd³
80 lb bags: 36.67 ÷ 0.60 = 61.1 → 62 bags (or 82 bags of 60 lb, or 123 bags of 40 lb)
Sixty-two bags weigh nearly 5,000 pounds and take hours to mix by hand, which is why many builders switch to ready-mix at around a cubic yard.
Bags or ready-mix?
| Bag size | Typical yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
Bags make sense for post holes, small pads, steps and repairs. For larger pours, a ready-mix truck delivers consistent concrete all at once, which avoids cold joints between batches. Most trucks carry up to about 9 to 11 cubic yards; suppliers often add a short-load fee for small orders, so compare the price of bags against the delivered cost. Order ready-mix in quarter- or half-yard increments, rounding up.
Typical thicknesses
| Project | Common thickness |
|---|---|
| Sidewalk or patio | 4 in |
| Residential driveway | 4–6 in (5–6 in for trucks) |
| Garage or shop floor | 4–6 in |
| Footing | at least 6 in under the IRC; width depends on load and soil |
| Post hole | 3 × the post width, below the frost line |
These are common starting points, not engineering specifications. Your local building department, the project plans or an engineer set the actual requirements.
Why order extra
Subgrade is never perfectly flat, forms bow slightly under the weight of wet concrete, and some always ends up in the wheelbarrow or on the ground. A low spot of just a quarter inch across a 10 × 10 slab adds about 2 cubic feet. For the base under the slab, use the gravel calculator or road base calculator, and for plain volume conversions, the cubic yards calculator.
Frequently asked questions
How many bags of concrete make a cubic yard?
A cubic yard is 27 cubic feet. At typical yields, that takes about 45 bags of 80 lb mix (0.60 ft³ each), 60 bags of 60 lb mix (0.45 ft³) or 90 bags of 40 lb mix (0.30 ft³), before adding any extra for waste.
How thick should a concrete slab be?
Sidewalks and patios are commonly 4 inches thick. Driveways that carry cars are usually 4 to 6 inches, with 5 to 6 inches preferred for heavier vehicles. Local codes and your soil conditions can require more.
How much extra concrete should I order?
Adding 5 to 10 percent covers uneven subgrade, slight form bulging and spillage. Running short in the middle of a pour creates a cold joint, so it is cheaper to have a little left over than to need a second delivery.
How do I calculate concrete for a round column or tube form?
Use the cylinder formula: π × radius² × height, with all measurements in feet. A 12-inch tube 4 feet deep holds π × 0.5² × 4 ≈ 3.14 ft³, or about 0.12 cubic yards, which is roughly six 80 lb bags.
How much does a cubic yard of concrete weigh?
Normal-weight concrete is about 145 to 150 pounds per cubic foot, so a cubic yard weighs roughly 3,900 to 4,050 pounds — about two tons. That matters if you plan to haul premix in a pickup.