Concrete Calculator
Calculate exactly how many cubic yards of concrete you need for any slab, footing, or pour.
This concrete calculator estimates the volume of concrete and the number of premixed bags needed for a slab, footing, or pour based on length, width, and thickness.
Ready-mix suppliers price concrete by the cubic yard, but most people only have square footage and a thickness in mind when they start. A concrete calculator — whether you’re running it as a concrete slab calculator for a patio or a concrete yard calculator for a bigger pour — bridges that gap, and the formula behind it is short enough to run by hand if you’d rather check the math yourself.
The concrete volume formula
Cubic yards = (Length × Width × Thickness in feet) ÷ 27. Thickness has to convert to feet first — a 4-inch slab is 0.333 ft, a 6-inch slab is 0.5 ft — since the rest of the measurements come in feet and 27 cubic feet make up one cubic yard.
Always add 10% on top of the raw result before ordering. Forms aren’t perfectly square, sub-base grading dips and rises slightly across a pour, and running short mid-pour creates a cold joint — a weak seam where an interrupted pour meets fresh concrete — that’s a real structural problem, not just a cosmetic one.
Worked example: a 12×12 patio slab
144 sq ft at 4 inches thick: 144 × 0.333 = 48 cubic feet, or 48 ÷ 27 = 1.78 cubic yards. Add 10% and that becomes 1.96 cubic yards — round up to 2 cubic yards when placing the ready-mix order, since suppliers won’t deliver a fraction of a yard.
In bags instead: 48 cubic feet ÷ 0.6 (the yield of an 80 lb bag) = 80 bags before waste, or 88 bags once the same 10% allowance is added — the same math a concrete bag calculator runs, just without the ready-mix conversion step.
Bags vs. ready-mix: when each one makes sense
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
Bagged mix makes sense up to roughly 1 cubic yard — fence posts, small footings, a single step. Past that, the labor of mixing dozens of bags by hand stops being worth the money saved over a ready-mix delivery, and most suppliers charge a short-load fee anyway for orders under 3–5 cubic yards, narrowing the gap further.
Slab thickness by project type
| Project | Typical thickness |
|---|---|
| Sidewalks, patios, interior flatwork | 4 in |
| Driveways, garage floors | 6 in |
| RV pads, heavy vehicle areas | 8 in or more |
Thicker isn’t automatically better past what the application calls for — it adds material cost and weight without a corresponding benefit. Matching thickness to actual load is more useful than defaulting to the thickest option available.
Choosing the right PSI
PSI measures compressive strength at 28 days, and it scales with how much load the slab will actually bear:
| PSI range | Typical use |
|---|---|
| 2,500 PSI | Sidewalks, patios, steps |
| 3,000–4,000 PSI | Driveways, garage floors, footings |
| 4,000–5,000 PSI | Commercial floors, parking structures |
In climates with regular freeze-thaw cycling, ask for an air-entrained mix regardless of PSI — the microscopic air bubbles it introduces give water room to expand when it freezes inside the slab, instead of cracking the surface from the inside out.
What concrete actually costs in 2026
Ready-mix runs $125–$175 per cubic yard delivered for a standard 3,000–4,000 PSI mix, though pricing swings by region and rises toward $195 in higher-cost markets. That price covers material and a standard delivery radius — distance beyond it, weekend pours, and winter surcharges all add on top.
The number that catches people off guard is the short-load fee: order less than a full truck — typically 8–10 cubic yards — and suppliers charge an extra $50–$150 per yard under that minimum. A small 2-yard patio order can end up costing nearly as much per yard as a 4-yard driveway pour once that fee lands. Running the numbers through a concrete cost calculator before calling a supplier at least tells you which side of that line your project falls on.
Bagged concrete works out to roughly $150–$290 per cubic-yard equivalent depending on bag price, which is why it only beats ready-mix on small jobs under about 1 cubic yard. Past that, the labor of mixing dozens of bags stops paying for itself.
Building with block instead of pouring a slab
Block walls run on a completely different formula from a poured slab — there’s no thickness or cubic yardage involved, just wall area divided by the face size of one block. A concrete block calculator for standard 8×8×16 CMU uses Blocks = Wall area (sq ft) × 1.125, since each block’s nominal face — including its mortar joint — covers about 0.889 sq ft (144 ÷ 128).
Worked example: a 50 ft long, 8 ft high wall is 400 sq ft. 400 × 1.125 = 450 blocks, plus a 10% waste allowance brings the order to 495. Openings for doors and windows get subtracted from the wall area before that multiplication, not after.
How long concrete actually takes to cure
Concrete sets within a day but keeps gaining strength for weeks. The practical schedule: 24–48 hours before light foot traffic is safe, about 7 days to reach roughly 70% of rated strength — enough for passenger vehicles on a driveway — and the full 28 days to hit 90%+ of the design PSI used to size the slab in the first place.
Cold weather slows every stage of that timeline; concrete cured below 55°F should get the full 28 days before any vehicle load, regardless of how solid it looks or feels on the surface days earlier.
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A 20×20 foot slab at 4 inches thick requires approximately 4.9 cubic yards of concrete. At 6 inches thick, the same area needs about 7.4 cubic yards. Always add 5–10% to the calculated volume for waste, over-excavation, and form imperfections — so plan to order approximately 5.5 yards for a standard 4-inch pour.
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One 80-pound bag of concrete mix yields approximately 0.60 cubic feet of mixed concrete. Since one cubic yard equals 27 cubic feet, you need roughly 45 bags per cubic yard. For small projects under half a yard, bags are practical. For larger pours, ready-mix is typically more cost-effective once labor is factored in.
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The standard thickness for a residential driveway handling passenger vehicles is 4 inches. For heavier use — RVs, trailers, or frequent delivery trucks — 5 to 6 inches is recommended, with rebar or wire mesh reinforcement. Going below 4 inches significantly reduces load capacity and increases the risk of cracking under vehicle weight.
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Concrete should be kept free of vehicle traffic for a minimum of 7 days after pouring. Foot traffic is generally safe after 24–48 hours. Concrete reaches its full design strength at 28 days — heavy vehicles like trucks or RVs should not use a new slab until the full 28-day cure is complete. Curing too quickly in hot or dry conditions weakens the final strength; keep the slab moist for the first 7 days.