Calculate-Compare-Decide
Work out exactly how much concrete you need — in cubic yards, cubic feet, cubic meters or bags — for any slab, footing, wall, column, patio or set of stairs. Includes weight, waste factor, rebar and a ready-mix vs bagged cost comparison.
A concrete calculator estimates how much concrete a project needs — in cubic yards, cubic feet, cubic meters or bags — based on the dimensions of the pour. It multiplies length by width by thickness, converts the volume into the units concrete is sold in, and also estimates weight, bag count, a waste allowance and cost.
Multiply length × width × thickness in feet to get cubic feet, then divide by 27 (one cubic yard = 27 cubic feet). A 10 × 10 ft slab at 4 in thick is 10 × 10 × 0.333 = 33.3 ft³ ÷ 27 ≈ 1.24 cubic yards. Add 5–10% for waste.
Worked example — a 10 × 10 ft patio at 4 inches. Convert the thickness to feet: 4 ÷ 12 = 0.333 ft. Volume = 10 × 10 × 0.333 = 33.3 ft³. Divide by 27 → 1.24 yd³. That's about 56 × 80 lb bags (33.3 ÷ 0.60), weighs roughly 5,000 lb, and with a 10% waste factor you'd order 1.36 yd³.
Why 27? A cubic yard is 3 ft × 3 ft × 3 ft, and 3 × 3 × 3 = 27 cubic feet. Concrete is sold by the cubic yard in the US, so every calculation ends with that division.
| Thickness | Coverage per yd³ | yd³ per 100 ft² |
|---|---|---|
| 2 in | 162 ft² | 0.62 |
| 3 in | 108 ft² | 0.93 |
| 4 in (standard) | 81 ft² | 1.24 |
| 5 in | 65 ft² | 1.54 |
| 6 in | 54 ft² | 1.85 |
| 8 in | 41 ft² | 2.47 |
| Bag | Yield | Bags / yd³ |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 50 lb | 0.375 ft³ | 72 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
| Project | Thickness | PSI |
|---|---|---|
| Sidewalk / patio | 4 in | 3,000 |
| Car driveway | 4–5 in | 3,500 |
| Truck / RV driveway | 6 in | 4,000 |
| Garage floor | 4–6 in | 3,500 |
| Shed / hot-tub base | 4–6 in | 3,000 |
| Footings | 8–12 in | 3,000 |
| 1 cubic yard | 1 cubic meter | 1 cubic foot |
|---|---|---|
| 27 ft³ | 35.31 ft³ | 0.037 yd³ |
| 0.7646 m³ | 1.308 yd³ | 0.0283 m³ |
| 764.6 liters | 1,000 liters | 28.32 liters |
| ≈ 4,050 lb (2.03 t) | ≈ 2,400 kg | ≈ 150 lb |
1.24 yd³ · 33.3 ft³ · 56 × 80 lb bags · ~5,000 lb
4.94 yd³ · 133 ft³ · order ~5.5 yd³ with waste
8.89 yd³ · ready-mix territory — bags would be ~400
9.88 yd³ · 267 ft³ · ~20 tons of concrete
1.85 yd³ · 50 ft³ · 84 × 80 lb bags
1.32 yd³ · 35.6 ft³ · typical for a small addition
0.23 yd³ · 6.28 ft³ · 11 × 80 lb bags
0.04 yd³ · 1.09 ft³ · 2 × 80 lb bags per post
In the United States concrete is sold by the cubic yard — a block three feet on every side, which works out to 27 cubic feet. Most of the world orders in cubic meters. Bagged concrete, by contrast, is sold by weight, and each bag lists the volume of wet concrete it yields. That's the single most common source of confusion: the bag says "80 lb," but what matters for your slab is that it makes about 0.60 cubic feet.
Every concrete calculation is the same idea: find the volume of the space you're filling. For a slab that's length × width × thickness. For a round column or post hole it's π × radius² × depth. The one step people fumble is units — a 4-inch slab is 0.333 feet, not 4. Convert the thickness to feet first, and the rest of the arithmetic falls into place.
The crossover is around half a cubic yard. Below that, bags win: no delivery fee, no short-load charge, and you can mix at your own pace. Above roughly one cubic yard, bagged concrete becomes punishing — a single yard is about 45 × 80 lb bags, over 4,000 lb to haul, open and mix — and ready-mix is both cheaper per yard and dramatically less work. Suppliers usually charge a short-load fee below about 3 yards, so factor that in; the calculator's cost panel shows both routes side by side.
Order exactly what the math says and you will come up short. Subgrades are never perfectly level, forms bow slightly, footing trenches over-excavate, and some concrete is simply lost to the chute and the wheelbarrow. Add 5–10% — 10% on small or irregular jobs. Running out mid-pour is the worst outcome on a concrete job: the first batch starts curing while you wait, and you end up with a cold joint, a permanent weak seam in the slab.
Four inches is the workhorse thickness for patios, sidewalks, shed bases and residential floors. Driveways carrying only cars are fine at 4–5 inches; if a truck or RV will sit on it, go to 6 inches. Strength is specified in PSI: 2,500–3,000 for general residential flatwork, 3,500–4,000 for driveways and garage floors that see vehicle loads, and 4,000+ where freeze-thaw or heavy traffic is a factor. Higher PSI costs a little more per yard and is almost always worth it on a driveway.
Most slab failures aren't concrete failures — they're base failures. A properly compacted 4–6 inch layer of gravel gives the slab uniform support and lets water drain; without it, the ground settles unevenly and the slab cracks. Get the base right, keep the thickness consistent, and use control joints (cut at roughly 2–3 times the slab thickness in feet, so every 8–12 feet on a 4-inch slab) so the inevitable shrinkage cracks happen where you chose.
Rebar or wire mesh doesn't stop concrete from cracking — it holds the cracks tight so the slab keeps acting as one piece. A common residential pattern is #3 or #4 bar on a 16 or 18 inch grid, kept in the middle third of the slab depth on chairs (rebar lying on the ground does nothing). The advanced mode estimates the linear footage a grid will need so you can price it.
The recurring ones: forgetting to convert inches to feet, ordering with no waste allowance, ignoring the short-load fee when pricing ready-mix, choosing bags for a job that clearly needs a truck, skipping the compacted base, and placing reinforcement on the subgrade instead of on chairs. Every one of them is cheap to avoid at the planning stage and expensive to fix in cured concrete.
Call the supplier with your volume including waste, your PSI, and the placement method. Have the forms braced, the base compacted, your control-joint plan set, and enough hands ready — a truck typically allows a limited free unloading window before charging waiting time. Concrete is unforgiving of hesitation: once it's in the chute, the clock is running.
A concrete calculator estimates how much concrete a project needs, in cubic yards, cubic feet, cubic meters or bags, based on the dimensions of the pour. It multiplies length by width by thickness to get volume, converts to the units concrete is sold in, and can also estimate weight, bag count, waste allowance and cost.
Multiply length by width by thickness, all in feet, to get cubic feet, then divide by 27 because one cubic yard equals 27 cubic feet. For example, a 10 by 10 foot slab at 4 inches thick is 10 x 10 x 0.333 = 33.3 cubic feet, which is 33.3 divided by 27, or about 1.24 cubic yards.
One cubic yard is 27 cubic feet. An 80 lb bag yields about 0.60 cubic feet, so you need about 45 bags per cubic yard. A 60 lb bag yields about 0.45 cubic feet (60 bags per yard), a 50 lb bag about 0.375 (72 bags), and a 40 lb bag about 0.30 (90 bags per cubic yard).
Normal-weight concrete weighs roughly 150 pounds per cubic foot, so one cubic yard weighs about 4,050 pounds, or just over 2 tons. That is about 1,840 kilograms. Weight varies with the mix, aggregate and moisture, so treat it as an estimate when planning transport or structural loads.
Four inches is the standard for patios, sidewalks and residential floors. Driveways for cars are typically 4 inches, but 5 to 6 inches is recommended where trucks or heavy vehicles will park. Garage floors are commonly 4 to 6 inches. Always follow local building codes and a proper compacted base.
Add 5 to 10 percent extra. Spillage, an uneven subgrade, over-excavated footings and slight thickness variations all consume more concrete than the math predicts. Running short mid-pour causes a cold joint, which is far worse than paying for a little extra, so most contractors order about 10 percent over on small jobs.
Bagged concrete is usually cheaper for very small jobs, under roughly half a cubic yard, because ready-mix carries delivery fees and short-load charges. Beyond about one cubic yard, ready-mix is normally cheaper per yard and vastly less labor. This calculator compares both so you can see the crossover point for your project.
No. It provides estimates based on the dimensions you enter and standard bag yields and densities. Actual quantities vary with subgrade, formwork, compaction, mix design and site conditions. Always confirm with your supplier or a qualified contractor, and add a waste factor, before placing a concrete order.
A 10 by 10 foot slab at 4 inches thick is 33.3 cubic feet, or about 1.24 cubic yards. At 0.60 cubic feet per 80 lb bag that is about 56 bags, and roughly 62 with a 10 percent waste allowance. That is well over two tons to mix by hand, so most people order ready-mix at this size.
Ready-mix commonly runs somewhere around 140 to 200 dollars per cubic yard in the US before delivery, with regional variation and often a short-load fee under about 3 yards. Bagged concrete typically works out more expensive per yard once you pass a very small pour. Enter your local prices in the calculator for an accurate comparison.
For patios, sidewalks and general residential flatwork, 3,000 PSI is the common choice. Driveways and garage floors that carry vehicles are typically 3,500 to 4,000 PSI, and 4,000 or higher suits heavy traffic or harsh freeze-thaw climates. Footings are often 3,000 PSI. Local building codes may set a minimum, so check first.
Use pi times the radius squared times the depth. The radius is half the diameter. A 12 inch diameter column 8 feet tall is 3.1416 x 0.5 x 0.5 x 8 = 6.28 cubic feet, about 0.23 cubic yards, or roughly 11 bags of 80 lb mix. Select the Column shape and the calculator does this for you.
Reinforcement does not prevent cracks, it holds them tightly together so the slab keeps acting as one piece. Most residential slabs use rebar or wire mesh, commonly #3 or #4 bar on a 16 to 18 inch grid, supported on chairs in the middle of the slab depth. Rebar resting on the ground provides no benefit.
Concrete is typically walkable in 24 to 48 hours, ready for vehicles after about 7 days, and reaches most of its design strength at 28 days, which is when test cylinders are measured. Keeping it moist during the first days makes it stronger. Cold weather slows curing considerably; hot, windy weather can cause surface cracking.
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