Concrete Calculator
Calculate how many cubic yards of concrete you need for a slab, patio, driveway or footing, plus how many 80 lb, 60 lb, 50 lb or 40 lb bags to buy.
By CalcBuilt Editorial TeamUpdated September 10, 2026Formulas checked against manufacturer specs, see how we calculate.
Results
- Concrete needed
- 1.36 cu yd
- 36.7 cu ft including 10% waste. Ready-mix is ordered in quarter yards; round up and keep a quarter yard of cushion
- Without waste
- 1.23 cu yd
- 33.3 cu ft
- 80 lb bags to buy
- 62 bags
- Warning: past about 20 bags, earlier batches set up before the next goes down and every batch line is a cold joint; order ready-mix
- Approximate weight
- 2.5 tons
- At 150 lb per cubic foot
Concrete formula
Cubic feet = Length (ft) x Width (ft) x Thickness (in) / 12
Cubic yards = Cubic feet / 27
Order quantity = Cubic yards x (1 + waste %)
Bags = Cubic feet / bag yield (80 lb = 0.60 cu ft, 60 lb = 0.45 cu ft, 50 lb = 0.375 cu ft, 40 lb = 0.30 cu ft)Worked example
A 12 x 12 ft patio poured 4 in thick: 12 x 12 x (4 / 12) = 48 cubic feet. 48 / 27 = 1.78 cubic yards. Add 10% waste for 1.96 yards. Plants batch in quarter yards and a truck that comes up short costs a second short-load fee, so order 2.25 cubic yards of ready-mix.
In bags: 48 x 1.1 = 52.8 cubic feet. 52.8 / 0.60 = 88 bags of 80 lb mix, or 118 bags of 60 lb mix.
Quick reference
| Thickness | Cubic feet | Cubic yards | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| 4 in | 33.3 | 1.23 | 56 | 75 |
| 5 in | 41.7 | 1.54 | 70 | 93 |
| 6 in | 50.0 | 1.85 | 84 | 112 |
| 8 in | 66.7 | 2.47 | 112 | 149 |
How to calculate concrete
Concrete is sold by the cubic yard. Multiply length by width by thickness (all in feet) to get cubic feet, then divide by 27 to convert to cubic yards. A 10 ft by 10 ft slab at 4 inches thick is 10 x 10 x 0.333 = 33.3 cubic feet, or 1.23 cubic yards.
Add 5 to 10 percent for waste: subgrade is never dead level, forms bow, and some mix stays in the drum. Plants batch in 1/4 yard steps, so a 1.96 yard result becomes a 2.25 yard order. Half a yard short means a second truck at the full short-load fee plus a cold joint across the slab.
Bags vs ready-mix
Bagged mix is for fence posts, small pads and steps. An 80 lb bag yields 0.60 cubic feet, so a cubic yard takes 45 bags. Past about 20 bags the batches go down minutes apart and every batch line sets up as a cold joint. That is why pros call a truck at that size, not the price per yard.
The ticket shows a per-yard price, typically $150 to $220 delivered in 2026, plus a fuel or environmental surcharge. Orders under 4 to 6 yards pay a short-load fee, and most plants will not dispatch under 2 to 3 yards. Unloading is free for about 5 to 7 minutes per yard, then standby is billed by the minute. Have forms, rebar and crew ready before the truck rolls in. Never add water at the chute to make the mix flow; it cuts strength and cracks the slab, so order a higher slump or a plasticizer from the plant. A loaded mixer weighs 60,000 to 70,000 lb and needs about 10 ft of width and 13.5 ft of overhead clearance. The chute reaches about 16 ft; beyond that you wheelbarrow or pay for a pump truck.
Typical thickness: 4 inches for patios, walkways and shed slabs; 5 to 6 inches for driveways and garage floors; 8 inches or more where heavy trucks will park. In freeze-thaw regions order 4,000 psi air-entrained; plain 3,000 psi scales after a few winters.
What this calculator leaves out
The result is concrete only. It leaves out the 4 in compacted gravel base, the vapor retarder under interior slabs (IRC R506.2.3), rebar or mesh, forms and stakes, and haul-off of the spoil. Plan control joints at 2 to 3 times the slab thickness in feet (8 to 12 ft on a 4 in slab), sawed 1/4 deep within 6 to 18 hours. Budget the delivery and short-load fees, pump rental if the truck cannot reach, and the permit most towns require for a driveway.
Code limits that apply
- IRC R506.1checked
Slab floor at least 3.5 in thick
If you miss it: A 3 in pour cracks under the first car parked on it and the slab inspection fails on a depth probe
- IRC R506.2.2
4 in base of clean sand, gravel or crushed stone under a slab below grade
If you miss it: Concrete on wet clay wicks moisture and curls; a soft subgrade settles and the slab breaks over the low spot
- IRC R506.2.3
6 mil polyethylene vapor retarder between the base and an interior slab, joints lapped 6 in
If you miss it: Floor adhesives fail and finishes bubble as ground moisture keeps coming up through the slab for years
- IRC Table R402.2
Exterior slabs and garage floors in freeze-thaw areas air-entrained, 3,000 to 3,500 psi minimum
If you miss it: Plain 3,000 psi scales and pops within a few winters of deicing salt and freeze-thaw
- ACI 302.1R
Control joints at 2 to 3 times the slab thickness in feet, sawed 1/4 deep within 6 to 18 hours
If you miss it: The slab picks its own line and cracks across the middle of the patio within the first month
Rows marked checked produce a warning in the results when an entry crosses the limit. The others are on the plans or on site and this calculator does not test them. Local amendments can be stricter; the adopted edition and the inspector govern.
Mistakes that cost money
- Ordering 2 yards on a 1.96 yard result leaves no cushion; plants batch in quarter yards, order 2.25 or pay for a second truck.
- Adding water at the chute to make the mix flow cuts strength and causes cracking; order a plasticizer or a higher slump from the plant.
- Forms, rebar and crew not ready when the truck arrives burns the free unload window; standby is billed by the minute after that.
- The chute reaches about 16 ft; if the truck cannot get within that, budget wheelbarrows and three helpers, or a pump truck.
- Control joints cut the day after the pour are too late; saw them 1/4 of the slab depth within 6 to 18 hours.
- Plain 3,000 psi in a freeze-thaw region scales within a few winters; order 4,000 psi air-entrained for exterior slabs there.
Key facts
- One cubic yard of concrete is 27 cubic feet and weighs about 4,050 lb (2 tons).
- One cubic yard covers 81 sq ft at 4 in thick, 65 sq ft at 5 in, and 54 sq ft at 6 in.
- An 80 lb bag of concrete mix yields 0.60 cubic feet, so 45 bags make one cubic yard.
- A 60 lb bag yields 0.45 cubic feet, so 60 bags make one cubic yard.
- Standard slab thickness is 4 in for patios and walkways and 5 to 6 in for driveways.
Frequently asked questions
How many 80 lb bags of concrete make a yard?
45 bags. Each 80 lb bag yields about 0.60 cubic feet and a cubic yard is 27 cubic feet.
How much does a cubic yard of concrete cover?
At 4 inches thick, one cubic yard covers 81 square feet. At 6 inches it covers 54 square feet.
How thick should a concrete slab be?
4 inches is standard for patios, sidewalks and shed floors. Use 5 to 6 inches for driveways and anywhere vehicles will drive or park.
Should I mix bags or order a truck for a 10 x 10 slab?
Order the truck. A 10 x 10 slab at 4 in is 62 bags of 80 lb mix, three times the 20-bag point where batch lines become cold joints. At 1.36 yards you will pay a short-load fee and some plants will not dispatch that little, but one continuous pour is still the cheaper slab.
Sources and references
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The usual order for a concrete & masonry project. See all concrete & masonry calculators.
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Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.