Concrete Calculator
Figure out exactly how much concrete a slab, patio, or footing needs — in cubic feet, cubic yards, and bags — plus a cost estimate.
80-lb bag ≈ 0.60 ft³ (0.022 yd³) · 60-lb ≈ 0.45 ft³ · 40-lb ≈ 0.30 ft³. For ready-mix, order by cubic yard and add 10% for grade variance.
How the volume formula works
Concrete is sold by volume, so the first job is turning a footprint and a thickness into cubic feet. Every slab, patio, and footing is a box: length × width × depth. Measure the footprint in feet and the depth in inches, then divide the depth by 12 to bring it into feet.
Volume (ft³) = Length (ft) × Width (ft) × Depth (in) ÷ 12
The ÷ 12 is there because there are 12 inches in a foot: a 4″ slab is 4 ÷ 12 = 0.333 ft thick. Skipping that step — multiplying the footprint by 4 instead of 0.333 — is the quickest way to oversize an order. Keep the units on the label straight and the rest of the math is multiplication.
From cubic feet to bags
Bagged concrete is sold by weight, but each bag carries a published yield — the volume of wet mix it produces:
- 80-lb bag ≈ 0.60 ft³
- 60-lb bag ≈ 0.45 ft³
- 40-lb bag ≈ 0.30 ft³
Divide the volume you need by the bag yield, then round up to the next whole bag. You can't buy two-thirds of a bag, and coming up short mid-pour is worse than having one left on the pallet — the first batch stiffens before the next one lands and you get a cold joint where the two meet.
Cubic yards and ready-mix
Ready-mix is ordered by the cubic yard. One yard is 27 cubic feet (3 ft × 3 ft × 3 ft), so convert by dividing cubic feet by 27. Most plants also have a delivery minimum — often somewhere around 1 to 3 yd³ — so on a small pour the short-load fee can outweigh the cost of the concrete itself. That minimum is the real dividing line between buying bags and calling the plant.
Waste — why you add it, and how much
The theoretical volume describes a perfect box. In the field the subgrade isn't dead level, wood forms flex under the weight of wet concrete, and mix sticks to the wheelbarrow, the chute, and the tools. That's what the waste allowance covers, and it's why the calculator defaults to 10%. Leave it in for a formed slab on compacted grade; raise it for uneven or sloped ground, or for a first pour where you don't know yet how much the forms will move. Ordering the theoretical number and hoping is not a plan — the difference is one more trip to the truck or a cold joint.
What the code says — slab thickness and footings
Two parts of the IRC 2021 govern residential flatwork, and both are worth knowing before you order:
- Slab thickness. R506.1 sets the minimum for slab-on-ground at 3.5″. A 4″ slab clears that and is the ordinary choice for a patio or shed floor. Where reinforcement is used, R506.2.4 requires it to sit in the middle-to-upper third of the slab — wire on the ground does nothing for cracks.
- Footing size. R403.1(1) gives the minimum width and thickness for light-frame footings by stories and soil bearing capacity. The floor of that table is 12″ wide × 6″ thick (R403.1.1) — that's the starting point, not a universal answer. Sizing a footing is a table lookup, not a guess.
- Footing depth. R403.1.4 requires exterior footings to bear at least 12″ below undisturbed grade, and R403.1.4.1 sends the bottom below the local frost line. Frost depth, not volume, is the first thing to check before you dig.
- Mix strength. The R402.2 table sets minimum compressive strengths: 2,500 psi for foundation and interior slabs, 3,000 psi for porch and garage slabs in moderate exposure, and 3,500 psi in severe. Air entrainment of 5–7% is required where the slab sees moderate or severe freeze-thaw.
One honest note: the IRC does not require steel in a slab-on-ground. Reinforcement in a residential slab is spec-driven, not code-driven, unless an engineer calls for it.
Worked example — 20 ft × 15 ft slab at 4″
Say you're pouring a 20-ft × 15-ft driveway slab, 4″ thick, with a 10% waste allowance. Work it in the same order the calculator does:
- Footprint: 20 × 15 = 300 ft²
- Depth in feet: 4 ÷ 12 = 0.333 ft
- Raw volume: 20 × 15 × 4 ÷ 12 = 100 ft³
- Add 10% waste: 100 × 1.10 = 110 ft³
- Cubic yards: 110 ÷ 27 = 4.07 yd³
Bag counts, dividing 110 ft³ and rounding up:
- 80-lb: 110 ÷ 0.60 = 183.3 → 184 bags
- 60-lb: 110 ÷ 0.45 = 244.4 → 245 bags
- 40-lb: 110 ÷ 0.30 = 366.7 → 367 bags
At 4.07 yd³ this is a ready-mix pour. 184 eighty-pound bags is about 14,720 lb of mix to carry, open, and mix one wheelbarrow at a time — the bags only make sense once you've checked them against the plant's minimum and your own back.
Common mistakes
- Entering depth in feet instead of inches. A 4″ slab is 0.333 ft. Entering 4 in the depth field computes a slab 12× too thick. Match the units to the label.
- Buying bags for a large pour. Past about a cubic yard, bag counts climb into the hundreds and the mixing time climbs with them. That's the point to call ready-mix.
- Forgetting waste. The theoretical volume assumes a perfect box on perfect grade. Order 10% more.
- Rounding down on bags. You can't buy a partial bag. Round up every time; a leftover bag is cheap, a short pour is not.
- Pouring a footing without checking frost depth. R403.1.4.1 puts the bottom of the footing below the frost line. Get it wrong and the footing heaves. Depth is a code question, not a volume question.
- Treating bag yield as exact. The label yield assumes a full bag mixed consistently. Mixing by hand, you lose some to the tools and the ground — the waste allowance is what covers that.
Concrete calculator FAQ
How do I calculate how much concrete I need?
Measure the footprint in feet and the depth in inches, then multiply Length × Width × Depth ÷ 12 for cubic feet. Add your waste allowance, then divide by 27 for cubic yards or by the bag yield (0.60 ft³ for an 80-lb bag) for a bag count. A 20-ft × 15-ft slab at 4″ thick is 100 ft³ before waste.
How many 80-lb bags of concrete are in a cubic yard?
45. A cubic yard is 27 ft³ and an 80-lb bag yields about 0.60 ft³, so 27 ÷ 0.60 = 45 bags. In practice you'd order a little more to cover waste.
Why add waste to a concrete order, and how much?
Because the subgrade isn't perfectly level, forms flex, and some mix is lost to tools and spillage. A 10% allowance is the calculator default; raise it for uneven or sloped ground. Coming up short means a cold joint where one batch stiffens before the next arrives.
What thickness should a slab be, and how big does a footing need to be?
IRC 2021 R506.1 sets the minimum slab-on-ground thickness at 3.5″. Footings are governed by R403.1(1), which gives minimum width and thickness for light-frame construction; the minimum is 12″ wide × 6″ thick (R403.1.1), and exterior footings must bear at least 12″ below grade (R403.1.4) and below the local frost line (R403.1.4.1).