Box / room
Also: rectangular prism, bed, trench, room
V = L × W × H
L = length, W = width, H = height or depth.
Example: A 10 × 10 ft patio slab, 4 in thick: 10 × 10 × 0.333 = 33.3 cu ft = 1.23 cu yd, or 56 bags of 80 lb before waste.
Quick answer: cubic yards of concrete = length × width × thickness (all in feet) ÷ 27. One cubic yard is 45 bags of 80 lb or 60 bags of 60 lb, and it covers 81 sq ft at 4 in thick. Add 5–10% extra.
How to calculate how much concrete you need
- Measure the length and width of the area in feet. For a round slab or a post hole, measure the diameter and use the Cylinder shape.
- Choose the thickness in inches and divide by 12 to get feet: 4 in = 0.333 ft, 5 in = 0.417 ft, 6 in = 0.5 ft.
- Multiply length × width × thickness to get cubic feet.
- Divide by 27 to get cubic yards, the unit ready-mix suppliers sell by.
- Add 5–10% for waste, spillage and a subgrade that is not perfectly level.
- Convert to bags if you are mixing it yourself: cubic feet ÷ 0.60 for 80 lb bags, ÷ 0.45 for 60 lb bags.
Cubic yards = L (ft) × W (ft) × T (in) ÷ 12 ÷ 27
A 12 × 16 ft patio, 4 in thick.
12 × 16 × (4 ÷ 12) = 64 cu ft. 64 ÷ 27 = 2.37 cu yd.
With 10% extra: 2.61 cu yd. As bags: 70.4 cu ft ÷ 0.60 = 118 bags of 80 lb, so this one is a job for a ready-mix truck.
How many bags of concrete do you need?
Bagged premix is sold by weight, but what matters is how much concrete each bag makes once mixed. These yields are typical for standard concrete mix; high-strength and fast-setting mixes can differ a little, so check the bag.
| Bag size | Makes | Bags per cu ft | Bags per cu yd |
|---|---|---|---|
| 80 lb | 0.60 cu ft | 1.67 | 45 |
| 60 lb | 0.45 cu ft | 2.22 | 60 |
| 50 lb | 0.375 cu ft | 2.67 | 72 |
| 40 lb | 0.30 cu ft | 3.33 | 90 |
And for every 100 sq ft of slab, by thickness (no waste added):
| Thickness | Cu yd per 100 sq ft | 80 lb bags | 60 lb bags | 1 cu yd covers |
|---|---|---|---|---|
| 3 in | 0.93 | 42 | 56 | 108 sq ft |
| 4 in | 1.23 | 56 | 75 | 81 sq ft |
| 5 in | 1.54 | 70 | 93 | 65 sq ft |
| 6 in | 1.85 | 84 | 112 | 54 sq ft |
| 8 in | 2.47 | 112 | 149 | 40.5 sq ft |
Concrete for common slab sizes
Quick figures for popular patio, shed and garage sizes. Volumes are exact; add your waste allowance before you order.
| Slab | Thickness | Cubic yards | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| 10 × 10 ft | 4 in | 1.23 | 56 | 75 |
| 10 × 12 ft | 4 in | 1.48 | 67 | 89 |
| 12 × 12 ft | 4 in | 1.78 | 80 | 107 |
| 12 × 16 ft | 4 in | 2.37 | 107 | 143 |
| 20 × 20 ft (2-car garage) | 4 in | 4.94 | 223 | 297 |
| 20 × 20 ft | 6 in | 7.41 | 334 | 445 |
| 24 × 24 ft | 4 in | 7.11 | 320 | 427 |
| 24 × 24 ft | 5 in | 8.89 | 400 | 534 |
Formulas for slabs, footings, columns, steps and curbs
Pick the matching shape in the calculator. For a project with several parts, such as a slab with a thickened edge, or steps plus a landing, work out each part and press Add this pour to a project total.
Cylinder
Also: round hole, post hole, tank, planter
V = π × r² × h
r = radius (half the diameter), h = height.
Example: Ten deck footings in 10 in tubes, 4 ft deep: π × 0.417² × 4 = 2.18 cu ft each, 21.8 cu ft in all = 0.81 cu yd, or 37 bags of 80 lb.
Steps / stairs
Also: concrete steps, stoop, solid stairs
V = W × R × H × n(n + 1) ÷ 2
n = number of steps, W = width, R = tread depth, H = riser height (solid steps built on the ground).
Example: Four steps, 4 ft wide, 11 in treads and 7 in risers: 4 × 0.917 × 0.583 × (4 × 5 ÷ 2) = 21.4 cu ft = 0.79 cu yd.
Curb and gutter
Also: curb & gutter, roll curb, concrete curb
V = L × [c × (h + t) + g × t]
L = length, c = curb depth, h = curb height above the gutter, g = gutter width, t = gutter thickness.
Example: 40 ft of curb and gutter, with a 6 × 6 in curb on an 18 in gutter 6 in thick: 40 × (0.5 × 1 + 1.5 × 0.5) = 50 cu ft = 1.85 cu yd.
Trench / trapezoid
Also: ditch, berm, swale, trapezoidal prism
V = (a + b) ÷ 2 × h × L
a = top width, b = bottom width, h = depth, L = length.
Example: A footing trench 40 ft long, 20 in wide at the top, 16 in at the bottom and 12 in deep: (1.667 + 1.333) ÷ 2 × 1 × 40 = 60 cu ft = 2.22 cu yd.
Hollow cylinder
Also: pipe, tube, ring, annulus
V = π × (R² − r²) × h
R = outer radius, r = inner radius, h = height.
Example: A ring footing for a round tank, 12 ft outside, 10 ft inside, 1 ft deep: π × (6² − 5²) × 1 = 34.6 cu ft = 1.28 cu yd.
Concrete for fence and deck posts
A common rule is a hole about three times the width of the post and a third to a half as deep as the post stands above ground, and always below the frost line where you live. The post itself takes up space, so you need a little less concrete than the hole’s volume. These figures already subtract the post.
| Post | Hole | Concrete per hole | 80 lb bags | 60 lb bags | 50 lb bags |
|---|---|---|---|---|---|
| 4 × 4 | 8 in × 2 ft | 0.53 cu ft | 1 | 2 | 2 |
| 4 × 4 | 10 in × 2 ft | 0.92 cu ft | 2 | 3 | 3 |
| 4 × 4 | 10 in × 3 ft | 1.38 cu ft | 3 | 4 | 4 |
| 4 × 4 | 12 in × 3 ft | 2.10 cu ft | 4 | 5 | 6 |
| 6 × 6 | 12 in × 3 ft | 1.73 cu ft | 3 | 4 | 5 |
| 6 × 6 | 12 in × 4 ft | 2.30 cu ft | 4 | 6 | 7 |
| 6 × 6 | 16 in × 4 ft | 4.74 cu ft | 8 | 11 | 13 |
Building a deck? The deck calculator counts the footings for you along with the joists, beams and boards.
How thick should the concrete be?
Typical thicknesses for residential work. Your local building code, the soil and the loads come first, so treat these as a starting point and check before you pour.
| Project | Typical thickness |
|---|---|
| Sidewalk or path | 4 in |
| Patio | 4 in |
| Shed floor | 4 in |
| Hot tub pad | 4–6 in, check the maker’s instructions |
| Driveway, cars | 4–5 in |
| Driveway, trucks and RVs | 5–6 in or more |
| Garage floor | 4–6 in |
| Thickened slab edge | 8–12 in at the edge, tapering to the slab |
| Wall footings | At least 6 in thick, often 8 in or more, below the frost line |
Bags or ready-mix?
The right choice depends mostly on size. Mixing 45 bags by hand to make one yard is a long, heavy day, and a big slab mixed in batches can show cold joints where one batch meets the next.
| Job size | Usually best | Why |
|---|---|---|
| Under ½ cu yd (posts, small pads, repairs) | Bags | No delivery fee; mix as you go |
| ½–1½ cu yd | Bags with a rented mixer, or a short load | Compare a short-load fee against 25–70 bags |
| Over about 1½ cu yd | Ready-mix truck | Cheaper per yard, one even pour, far less labor |
| Over 10 cu yd | Ready-mix, more than one truck | Schedule trucks so the pour doesn’t stall |
Ready-mix is ordered in cubic yards, often in steps of a quarter or half yard. The calculator rounds your order up to the next half yard and shows how many trucks it takes.
How much does concrete cost?
Prices vary a lot by region, season and supplier, so the calculator lets you enter your own: per cubic yard for ready-mix, or per bag for premix. To compare, divide the bag price by the yield. For example, an 80 lb bag at $6.50 is $6.50 ÷ 0.60 = $10.83 per cu ft, or about $292 per cubic yard of material, before tools and labor.
When you get ready-mix quotes, ask about extra charges that often apply: short-load fees for small orders, fuel or environmental fees, extra time on site, pump rental if the truck cannot reach the pour, and additives such as fiber, accelerator in cold weather or retarder in hot weather.
Concrete strength and mix
Concrete is specified by its compressive strength after 28 days, in pounds per square inch (psi). Higher strength costs a little more per yard. Codes set minimums, especially for concrete exposed to freezing and de-icing salts, so check what your area requires.
| Strength | Common uses |
|---|---|
| 2,500 psi | Footings and slabs not exposed to weather, where code allows |
| 3,000 psi | General slabs, patios and walks in mild climates |
| 3,500 psi | Driveways, garage floors and outdoor flatwork in freeze-thaw areas (usually air-entrained) |
| 4,000 psi and up | Heavy-duty slabs, high traffic, structural work as specified |
If you mix from scratch, a traditional general-purpose mix is 1 part cement, 2 parts sand and 3 parts gravel by volume. Use only as much water as you need to make it workable: extra water makes concrete easier to place but weaker and more likely to crack.
Rebar, wire mesh and the base under the slab
Reinforcement does not change how much concrete you order, but plan it at the same time. Many residential slabs use #3 or #4 rebar in a grid about 18–24 in on center, or welded wire mesh, supported on chairs so it sits in the middle of the slab rather than on the ground. Fiber-reinforced mixes are another option for some flatwork.
Under the slab, most outdoor projects need a compacted base of gravel, often about 4 in. Use the gravel calculator for that layer: it uses the same shapes, so you can enter the same slab size.
Worked examples
Driveway
A 12 × 40 ft driveway, 5 in thick: 12 × 40 × 0.417 = 200 cu ft = 7.41 cu yd. With 10% extra, 8.15 cu yd, so order 8.5 yards: one truck.
Shed foundation with a thickened edge
A 10 × 12 ft shed slab, 4 in thick, is 1.48 cu yd. The perimeter edge is thickened to 12 in deep and 12 in wide, which adds an extra 8 in × 12 in along 44 ft: 44 × 0.667 × 1 = 29.3 cu ft = 1.09 cu yd. Total 2.57 cu yd before waste. Enter both parts and add them to the project total.
Front steps
Three steps, 5 ft wide, 12 in treads and 7 in risers: 5 × 1 × 0.583 × 6 = 17.5 cu ft = 0.65 cu yd, or 30 bags of 80 lb. A landing in front of the door, 5 × 4 ft at the height of the top step (21 in), adds 5 × 4 × 1.75 = 35 cu ft. Together 52.5 cu ft = 1.94 cu yd.
Notes for pros
Check the subgrade. A base that is ½ in low over a 20 × 20 ft slab swallows another 0.6 cu yd. Shoot the grade before you call the plant.
Order for the last load. Ask the dispatcher how they handle a short final load, and have a place ready for leftover concrete, such as a small pad or forms for blocks.
Steps are solid here. The steps shape assumes solid concrete built on the ground. If you fill the core with compacted fill or rubble, subtract that volume.
Weight matters. A yard weighs about 2 tons. Check what a pickup or trailer can carry before hauling a mixer load or a pallet of bags; a pallet of 42 bags of 80 lb is over 3,300 lb.
Tips
- Don’t pour on frozen ground or in freezing weather without protection; ask the supplier about cold-weather mixes.
- Keep it damp while it cures. Concrete gains strength for weeks; curing compound, wet burlap or plastic sheeting helps in hot, dry or windy weather.
- Wear gloves and eye protection. Wet concrete is caustic and can burn skin.
- Planning a whole project? Press Add to my shopping list under the result, then do the same for the gravel base and the lumber for the forms.
Frequently asked questions
How do I calculate how much concrete I need?
Multiply length × width × thickness, all in feet, to get cubic feet, then divide by 27 to get cubic yards. A 10 × 10 ft slab 4 in thick is 10 × 10 × 0.333 = 33.3 cu ft, which is 1.23 cubic yards. Add 5–10% for spillage and uneven ground.
How many bags of concrete are in a yard?
One cubic yard takes 45 bags of 80 lb, 60 bags of 60 lb, 72 bags of 50 lb or 90 bags of 40 lb. An 80 lb bag of standard premix makes about 0.60 cu ft of concrete.
How much area does a yard of concrete cover?
One cubic yard covers 81 sq ft at 4 in thick, about 65 sq ft at 5 in and 54 sq ft at 6 in. At 3 in it covers 108 sq ft.
How much does a yard of concrete weigh?
Normal concrete weighs about 4,000 lb per cubic yard (around 145–150 lb per cubic foot), so a yard is roughly 2 tons. Lightweight concrete is closer to 3,100 lb per yard.
Is it cheaper to use bags or ready-mix?
For small jobs under about 1 cubic yard, bags are usually cheaper and easier. Above 1–2 yards, ready-mix delivered by truck is normally cheaper per yard and gives a more even slab, though many suppliers add a short-load fee for small orders. Enter both prices in the calculator to compare.
How thick should a concrete slab be?
Patios, sidewalks and shed floors are usually 4 in thick. Driveways are commonly 4–6 in, with 5–6 in for heavier vehicles. Garage floors are typically 4–6 in. Always follow your local building code and the plans for your project.
How much concrete is in a truck?
A standard ready-mix truck carries up to about 10 cubic yards; some carry 8 or 11. The calculator shows how many truckloads your pour needs.
How many bags of concrete do I need for a fence post?
For a 4 × 4 post in a 10 in hole 3 ft deep, you need about 1.4 cu ft of concrete after the post itself is taken away: 3 bags of 80 lb or 4 bags of 60 lb. Use the Cylinder shape for the hole and subtract the post if you want to be exact.
What is the mix ratio for concrete?
A common general-purpose mix is 1 part cement, 2 parts sand and 3 parts gravel by volume, with just enough water to make it workable. Bagged premix already has the right blend; you only add water as the bag says.
How much extra concrete should I order?
Most contractors add 5–10%. Subgrades are never perfectly flat, forms bow a little and some concrete is left in the truck chute or wheelbarrow. Running short in the middle of a pour is far worse than having a little left over.
Is this concrete calculator free to use online?
Yes. The CalcWall Concrete Calculator is completely free and works online in any browser, on a phone, tablet or computer. There is nothing to download and no account to create, and the math runs right in your browser, so results appear as you type.