Concrete Calculator
Cubic yards, cubic feet and bags of concrete for slabs, footings and columns
Concrete volume (yd³) = length × width × thickness ÷ 324 with length and width in feet and thickness in inches. A 10 × 10 ft slab 4 in thick needs 1.23 yd³, or 62 bags of 80 lb mix with 10% extra.
Concrete calculator
LiveBag counts use typical yields of 0.60 ft³ (80 lb) and 0.45 ft³ (60 lb). Check the yield printed on your bag.
How to calculate concrete for a slab or column
Concrete is ordered by volume, so every estimate comes down to one number: how many cubic feet or cubic yards the form will hold. A cubic yard is a cube 3 ft on each side, which is 3 × 3 × 3 = 27 cubic feet. The only trap is depth. Slab thickness is measured in inches while length and width are in feet, so the thickness must be divided by 12 before you multiply, or the answer comes out 12 times too large. For gravel, bulk weights and volume conversions, see the other construction calculators.
Step by step
- Measure the length and width in feet and the thickness in inches. For a column, measure the inside diameter in inches and the height in feet.
- Work out the volume in cubic feet with the formula above, then multiply by the number of identical slabs or columns.
- Add the waste allowance: multiply by 1.05 for 5%, 1.10 for 10% or 1.15 for 15%.
- Divide by 27 for cubic yards of ready-mix, or divide the cubic feet by the bag yield (0.60 ft³ for an 80 lb bag, 0.45 ft³ for a 60 lb bag) and round up to whole bags.
Worked example, slab: a 10 × 10 ft patio, 4 in thick, holds 10 × 10 × 4 ÷ 12 = 33.33 ft³, which is 1.23 yd³. With 10% extra that becomes 36.67 ft³ or 1.36 yd³, so you would need 62 bags at 80 lb or 82 bags at 60 lb. These are the calculator's default inputs, so you can check the result above.
Worked example, column: a 12 in round form 4 ft tall has a radius of 12 ÷ 24 = 0.5 ft, so it holds π × 0.5² × 4 = 3.14 ft³ (0.116 yd³). With 10% extra, 3.46 ft³ fills 6 bags at 80 lb or 8 bags at 60 lb. Dividing the diameter by 24 converts inches to feet (÷ 12) and diameter to radius (÷ 2) in one move. To turn any other cubic-foot figure into yards, use the cubic feet to cubic yards converter.
What to measure
Is cement the same as concrete?
No: cement is the fine gray powder that acts as the glue, while concrete is cement mixed with water, sand and gravel, and cement typically makes up only about 10–15% of the mix by volume.
When water is added, portland cement (specified in the US by ASTM C150) reacts chemically and hardens, binding the sand and stone into a rock-like mass. The aggregates fill most of the volume and give concrete its bulk at low cost.
- Portland cement bags hold only the binder; you add your own sand, gravel and water.
- Concrete mix bags are cement, sand and gravel blended in advance, so you add only water. The bag counts on this page refer to these.
- Mortar mix is cement, sand and usually lime, with no gravel. It bonds bricks and blocks and is not meant for slabs.
So a “cement driveway” is really a concrete driveway, and a bag of plain cement will not fill a form on its own.
How do I calculate concrete for a footing or a thickened edge?
Treat it as a long, narrow slab: length × width × depth in feet, divided by 27, so 60 ft of footing 16 in wide and 8 in deep needs 53.33 ft³, or 1.98 yd³.
For a slab with a thickened edge, work out the main slab at its normal thickness, then add the extra depth around the perimeter as a separate band. A 20 × 20 ft garage slab 4 in thick holds 133.33 ft³. If the edge deepens to 12 in over a 1 ft wide band, that band covers 20 × 20 − 18 × 18 = 76 ft², not 80 ft², because the corners would otherwise be counted twice. The extra 8 in of depth adds 76 × 8 ÷ 12 = 50.67 ft³, for a total of 184.00 ft³, or 6.81 yd³, before waste.
Footing width and depth are structural decisions, not estimating ones. Many US jurisdictions follow the International Residential Code (section R403) for minimum footing sizes, and ACI 332 covers residential concrete, so take the dimensions from your plans or building department.
Concrete for common slab sizes
Volumes are exact. Bag counts include 10% extra and use typical yields of 0.60 ft³ per 80 lb bag and 0.45 ft³ per 60 lb bag.
| Slab | Cubic feet | Cubic yards | With 10% extra | 80 lb bags | 60 lb bags |
|---|---|---|---|---|---|
| 4 × 4 ft, 4 in thick | 5.33 ft³ | 0.20 yd³ | 0.22 yd³ | 10 | 14 |
| 10 × 10 ft, 4 in thick | 33.33 ft³ | 1.23 yd³ | 1.36 yd³ | 62 | 82 |
| 10 × 12 ft, 4 in thick | 40.00 ft³ | 1.48 yd³ | 1.63 yd³ | 74 | 98 |
| 12 × 12 ft, 4 in thick | 48.00 ft³ | 1.78 yd³ | 1.96 yd³ | 89 | 118 |
| 10 × 20 ft, 4 in thick | 66.67 ft³ | 2.47 yd³ | 2.72 yd³ | 123 | 163 |
| 12 × 20 ft, 4 in thick | 80.00 ft³ | 2.96 yd³ | 3.26 yd³ | 147 | 196 |
| 20 × 20 ft, 4 in thick | 133.33 ft³ | 4.94 yd³ | 5.43 yd³ | 245 | 326 |
| 24 × 24 ft, 4 in thick | 192.00 ft³ | 7.11 yd³ | 7.82 yd³ | 353 | 470 |
| 10 × 10 ft, 6 in thick | 50.00 ft³ | 1.85 yd³ | 2.04 yd³ | 92 | 123 |
| 20 × 20 ft, 6 in thick | 200.00 ft³ | 7.41 yd³ | 8.15 yd³ | 367 | 489 |
80 lb vs 60 lb bags of concrete: which should you buy?
Both bag sizes hold the same kind of premixed concrete; they differ in how much each bag yields and how heavy it is to handle. The figures use typical yields, so check the number printed on your bag.
| Attribute | 80 lb bag | 60 lb bag |
|---|---|---|
| Typical yield | 0.60 ft³ (0.022 yd³) | 0.45 ft³ (0.017 yd³) |
| Bags per cubic yard | 45 | 60 |
| Dry mix per cubic yard | 3,600 lb | 3,600 lb |
| 10 × 10 ft slab, 4 in thick, +10% | 62 bags | 82 bags |
| 12 in round form, 4 ft deep, +10% | 6 bags | 8 bags |
| Handling | Heavier lift, fewer bags to open and mix | Easier for one person to carry and pour, more bags |
4 key differences
- Per pound, both sizes make the same amount of concrete: 45 bags at 80 lb and 60 bags at 60 lb each weigh 3,600 lb and fill one cubic yard at typical yields.
- An 80 lb bag yields about 33% more volume than a 60 lb bag, so a pour that takes 60 small bags takes only 45 large ones.
- Compare prices per cubic foot rather than per bag: divide the bag price by 0.60 for an 80 lb bag or by 0.45 for a 60 lb bag.
- The yield printed on the bag overrides these typical figures, so recalculate with it before you buy.
How much concrete does a sonotube need?
A 12 in sonotube holds 0.785 ft³ per foot of height, so a 4 ft column takes 3.14 ft³ (0.116 yd³), or 6 bags of 80 lb mix with 10% extra. The table gives the volume per foot for common round form diameters and the total for a 4 ft column; bag counts include 10% extra at 0.60 ft³ per 80 lb bag and 0.45 ft³ per 60 lb bag.
| Form diameter | Per foot of height | 4 ft column | 4 ft column (yd³) | 80 lb bags | 60 lb bags |
|---|---|---|---|---|---|
| 6 in | 0.196 ft³ | 0.79 ft³ | 0.029 yd³ | 2 | 2 |
| 8 in | 0.349 ft³ | 1.40 ft³ | 0.052 yd³ | 3 | 4 |
| 10 in | 0.545 ft³ | 2.18 ft³ | 0.081 yd³ | 4 | 6 |
| 12 in | 0.785 ft³ | 3.14 ft³ | 0.116 yd³ | 6 | 8 |
| 16 in | 1.396 ft³ | 5.59 ft³ | 0.207 yd³ | 11 | 14 |
| 18 in | 1.767 ft³ | 7.07 ft³ | 0.262 yd³ | 13 | 18 |
| 24 in | 3.142 ft³ | 12.57 ft³ | 0.465 yd³ | 24 | 31 |
How many square feet does a yard of concrete cover?
One cubic yard of concrete covers 81 ft² at 4 in thick, 108 ft² at 3 in and 54 ft² at 6 in, because coverage in square feet is 324 ÷ thickness in inches. A bag covers its yield divided by the thickness in feet. Need the area in square meters instead? Use the square feet to square meters converter.
| Thickness | 1 yd³ covers | 1 yd³ covers (yd²) | One 80 lb bag covers | One 60 lb bag covers |
|---|---|---|---|---|
| 3 in | 108.0 ft² | 12.0 yd² | 2.40 ft² | 1.80 ft² |
| 3.5 in | 92.6 ft² | 10.3 yd² | 2.06 ft² | 1.54 ft² |
| 4 in | 81.0 ft² | 9.0 yd² | 1.80 ft² | 1.35 ft² |
| 5 in | 64.8 ft² | 7.2 yd² | 1.44 ft² | 1.08 ft² |
| 6 in | 54.0 ft² | 6.0 yd² | 1.20 ft² | 0.90 ft² |
| 8 in | 40.5 ft² | 4.5 yd² | 0.90 ft² | 0.68 ft² |
How the waste allowance changes the order
The same 12 × 12 ft slab, 4 in thick, with each waste option from the calculator. The exact volume is 48 ft³; bag counts are rounded up.
| Extra for waste | Cubic feet | Cubic yards | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| None | 48.00 ft³ | 1.78 yd³ | 80 | 107 |
| 5% | 50.40 ft³ | 1.87 yd³ | 85 | 113 |
| 10% | 52.80 ft³ | 1.96 yd³ | 89 | 118 |
| 15% | 55.20 ft³ | 2.04 yd³ | 92 | 123 |
Should you use bagged concrete or ready-mix?
Bags suit pours under about one cubic yard; above that, a ready-mix truck is usually the practical choice. Premixed bags work well for fence posts, a few deck footings, a mailbox post or a small pad, but they become hard work quickly: one cubic yard takes 27 ÷ 0.60 = 45 bags at 80 lb, or 27 ÷ 0.45 = 60 bags at 60 lb, and both add up to 3,600 lb of dry mix that has to be lifted, mixed and placed before the first batch starts to set.
Questions to ask a ready-mix supplier
- Minimum order and short-load fees. Many plants charge extra for small loads, so compare that fee with the cost and labor of bags.
- Increments. Plants usually batch in fractions of a yard, such as quarter or half yards. Round your calculated figure up to the next increment, never down.
- Mix strength. Residential slabs are commonly specified in psi, for example 3,000 to 4,000 psi. Follow the plans or your local building department.
- Truck access. If the chute cannot reach the forms, you may need a pump, wheelbarrows or a conveyor, which affects how much extra to order.
Ordering in cubic meters? One cubic yard is about 0.7646 m³, and the calculator shows the metric figure under the cubic-yard result. The cubic yards to tons calculator estimates the weight of the load, which matters for trailers and suspended floors: placed concrete weighs roughly 2 tons per cubic yard.
Common concrete estimating mistakes
- Forgetting to convert inches. A 4 in slab is 0.333 ft thick, not 4 ft. Multiplying 10 × 10 × 4 gives 400 ft³ instead of 33.33 ft³.
- Using nominal lumber size for form depth. A 2 × 4 form board is actually about 3.5 in tall, so a slab formed with 2 × 4s is closer to 3.5 in than 4 in unless the subgrade is dug lower. Measure the real depth at several points.
- Ignoring uneven subgrade. A base that dips half an inch over a large slab adds more volume than most people expect. At 4 in, an extra 0.5 in of depth is 12.5% more concrete.
- Skipping thickened edges and footings. Garage slabs and sheds often have a deeper perimeter. Calculate the edge as a separate long, narrow slab and add it to the total.
- Trusting a generic bag yield. The 0.60 ft³ and 0.45 ft³ figures are typical, but yields vary by product. Read the bag and recalculate if it differs.
- Mixing up measurements. If your plans are metric, convert lengths first with the feet to meters converter rather than mixing units in one formula.
Thickness, reinforcement, base preparation and footing depth depend on soil, climate and load. This calculator estimates material only; follow your plans and local building code for the structural design.
Sources and further reading
The standards and references behind the numbers on this page. Links open in a new tab.
Standards and official sources
Concrete calculator FAQs
Short, exact answers to what people ask most.
How much concrete do I need for a 10x10 slab?
A 10 × 10 ft slab 4 in thick needs 33.33 ft³, or 1.23 cubic yards. With 10% extra for waste, plan on about 1.36 yd³, which is 62 bags of 80 lb mix.
How many 80 lb bags of concrete are in a yard?
At a typical yield of 0.60 ft³ per bag, one cubic yard takes 27 ÷ 0.60 = 45 bags of 80 lb concrete. Add your waste allowance on top of that.
How many 60 lb bags of concrete do I need for a cubic yard?
A 60 lb bag typically yields 0.45 ft³, so one cubic yard takes 27 ÷ 0.45 = 60 bags. Check the yield printed on the bag, as products differ.
How many bags of concrete do I need for a fence post?
It depends on the hole size. A 12 in diameter hole 4 ft deep holds about 3.14 ft³, which is 6 bags of 80 lb mix with 10% extra, less the small volume taken by the post itself.
How thick should a concrete slab be?
Patios, walkways and shed bases are commonly poured 4 in thick, while driveways and garage floors are often 5 to 6 in. Your plans and local building code set the actual requirement.
How much extra concrete should I order?
Most estimators add 5 to 10% to the exact volume to cover uneven ground, form movement and spillage. Use 10 to 15% when the subgrade is rough or the shape is irregular.
How many yards of concrete do I need for a 20x20 slab?
A 20 × 20 ft slab 4 in thick needs 133.33 ft³, or 4.94 cubic yards. With 10% extra, order about 5.43 yd³.
How many bags of concrete do I need for a 4x4 pad?
A 4 × 4 ft pad 4 in thick holds 5.33 ft³. With 10% extra that is 10 bags of 80 lb mix or 14 bags of 60 lb mix at typical yields.
How much concrete does a 24x24 garage slab need?
At 4 in thick, a 24 × 24 ft slab needs 192.00 ft³, or 7.11 yd³; at 5 in it needs 8.89 yd³, and at 6 in 10.67 yd³, before any allowance for waste.
How many cubic feet does an 80 lb bag of concrete make?
A typical 80 lb bag makes about 0.60 ft³ of concrete, and a 60 lb bag about 0.45 ft³. The exact yield is printed on the bag and varies by product.
How do I convert cubic yards of concrete to cubic meters?
Multiply cubic yards by 0.764555. For example, 5 yd³ of ready-mix is 3.82 m³; to go the other way, multiply cubic meters by 1.308.
Related conversions
Volume math from the exact definitions in NIST Handbook 44 (1 yd³ = 27 ft³); bag yields are the typical figures printed on premixed concrete bags, which vary by product; cement proportions from the Portland Cement Association, with ASTM C150 for portland cement and ACI 332 and the International Residential Code (R403) for residential footings. Last updated . How we verify our numbers.