Answer: Enter your values and the Concrete Calculator returns the exact result instantly — formula, worked example, and a plain-English explanation are included below the tool.
Cubic yards, 80lb bags, and cost estimate
Concrete in the US is sold by the cubic yard — 27 cubic feet — and ready-mix trucks typically carry 9 to 11 yards per load. Calculating the volume you need before ordering matters because ordering short leaves you with a cold joint or a short pour, while ordering long wastes money on a minimum-load charge. This page computes volume for slabs, footings, and columns in cubic yards, converts to 80-lb bag equivalents for small jobs, and applies your unit price.
The basic math is simple: volume = length × width × depth, with all dimensions in the same unit, then divided by 27 to get cubic yards from cubic feet. For columns, volume = π × radius² × height. The classic example: a 10 ft × 10 ft slab at 4 inches thick is 100 × 0.333 = 33.3 cubic feet = 1.23 cubic yards.
Common practice, reflected in building codes like the IRC, sets minimum slab thickness by use: 4 inches for patios, walkways, and floors with light loads; 6 inches for residential driveways that will see vehicles. Footings under walls are commonly 12 × 12 to 16 × 16 inches depending on soil and load. Depth drives volume linearly: going from 4 to 6 inches on a slab increases concrete needs by 50%.
Ready-mix suppliers often require a minimum order of about 1 cubic yard and may charge for partial loads. For jobs under roughly 1 yard, bagged concrete mixed on site is usually more economical: an 80-lb bag yields about 0.60 cubic feet of mixed concrete, so 45 bags make one yard.
The table below shows concrete volume for common project sizes, plus the equivalent 80-lb bag count, using the standard 0.60 cu ft yield per bag.
| Project | Dimensions | Volume (cu yd) | 80-lb bags |
|---|---|---|---|
| Patio slab | 10 ft × 10 ft × 4 in | 1.23 | ~56 |
| Driveway section | 10 ft × 20 ft × 6 in | 3.70 | ~167 |
| Sidewalk | 3 ft × 40 ft × 4 in | 1.48 | ~67 |
| Footing | 12 in × 12 in × 20 ft | 0.74 | ~33 |
| Column | 12 in dia × 4 ft | 0.12 | ~5 |
Add 5–10% for spillage, uneven subgrade, and over-excavation — the calculator's waste field does this for you. Schedule the pour when weather is moderate; ACI guidance addresses hot-weather and cold-weather concreting because both extremes weaken curing concrete. Have enough help and tools ready, since ready-mix concrete typically must be placed within 60 to 90 minutes of batching, and finish before it sets.
For slabs that will carry loads, reinforcement is standard: wire mesh or #3/#4 rebar at 12–24 inch spacing. Reinforcement does not change the volume calculation but belongs in the plan. Always check local code minimums for your structure.
Bagged concrete comes in 40, 60, and 80 lb sizes, all yielding concrete in proportion to weight: an 80-lb bag makes about 0.60 cubic feet, a 60-lb bag about 0.45, and a 40-lb bag about 0.30. Water matters more than most DIY builders expect — the bag's printed water ratio is calibrated for full strength, and adding extra water for easier mixing measurably weakens the result. ACI guidance treats water-cement ratio as the single most influential mix parameter.
Curing is the continuation of the chemical reaction after placement. Concrete typically reaches design strength at 28 days, gains its first useful strength within 24 to 48 hours, and benefits from kept-moist curing for at least the first week. In hot or freezing weather, ACI 305 and ACI 306 practices — shading, evaporation control, insulating blankets — protect the young slab.
Columns and footings round out the shapes most calculators handle: a round column's volume is pi times the radius squared times height, so a 12-inch diameter column 4 feet tall holds about 3.14 cubic feet — roughly 0.12 cubic yards, or a little over five 80-lb bags. A rectangular footing of the same height and a 12x12 cross-section holds 4 cubic feet. Pouring columns and footings with the slab in one visit avoids cold joints between pours, which is one more reason to compute the total volume for the whole job at once.
How many cubic feet are in a cubic yard of concrete?
27. One cubic yard is 3 ft × 3 ft × 3 ft. Divide your volume in cubic feet by 27 to get cubic yards.
How many 80-lb bags of concrete make a cubic yard?
An 80-lb bag yields about 0.60 cubic feet of mixed concrete, so a full cubic yard takes about 45 bags (27 ÷ 0.60).
How thick should a concrete slab be?
Common practice: 4 inches for patios and walkways, 6 inches for residential driveways carrying vehicles. Your local building code sets the actual minimum for your structure.
How much extra concrete should I order?
Add 5–10% for spillage and uneven subgrade. For small slabs, round up to the nearest quarter or half yard since suppliers batch in increments.
Why does the calculator divide by 27?
Dimensions entered in feet and inches are converted to cubic feet, and there are 27 cubic feet (3³) in one cubic yard — the unit ready-mix plants sell by.