Concrete & Masonry
Concrete Calculator
Cubic yards for slabs, footings and posts — plus the 80 lb bag count if you are mixing it yourself.
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About this calculator
Ready-mix concrete is ordered by the cubic yard, and short loads cost extra — so the estimate matters. This calculator converts slab, footing, column and post-hole dimensions into cubic yards with an overage allowance, and into pre-mix bags for small jobs.
What this number means
Concrete is unforgiving about quantity in both directions. Order short and the truck's second trip arrives with a short-load fee while your first pour is already setting; order long and you pay to dispose of the surplus. The result above gives you three numbers for three decisions: the exact volume in your forms, the volume with a sensible overage allowance, and the order quantity rounded up to the nearest quarter yard — the increment ready-mix plants actually sell in.
The bag count is there for the crossover decision. As a rule, anything much over a cubic yard (45 × 80 lb bags) is a ready-mix job for most people: a yard is roughly two tons of dry mix to move, batch and place before it sets.
The formula
- Slab / footing volume = Length × Width × Thickness (all in feet)
- Round volume = π × (Diameter ÷ 2)² × Height
- Cubic yards = Cubic feet ÷ 27
- Order quantity = Volume × (1 + overage %), rounded up to the nearest ¼ yard
- Bags = Volume with overage (cu ft) ÷ yield per bag (80 lb = 0.60, 60 lb = 0.45, 40 lb = 0.30 cu ft), rounded up
Worked example
A 10 × 12 ft patio slab, 4 in thick.
- Volume = 10 × 12 × (4 ÷ 12) = 40 cu ft = 1.48 cu yd
- With 10% overage = 1.63 cu yd → order 1.75 cu yd (rounded to the quarter yard)
- Mixing it yourself instead: 44 cu ft with overage ÷ 0.60 = 74 × 80 lb bags — a strong argument for the truck
Assumptions and limitations
- Volumes assume forms built exactly to dimension on a level, compacted subgrade. A wavy base can add 10–20% to a thin slab's real consumption — that is what the overage is for, and thin slabs deserve the full 10%.
- Bag yields are the manufacturers' standard mixed yields. Hand mixing in a wheelbarrow consistently yields slightly less per bag than a drum mixer.
- Post-hole volume is the whole hole. A post sitting in it displaces some concrete (a 4×4 post in a 3 ft hole displaces about 0.3 cu ft) — the overage absorbs this at normal post counts.
- Reinforcement (rebar, mesh, fibre) does not change the volume maths and is not priced here.
Frequently asked questions
At 4 in thick a 10×10 slab is 33.3 cu ft. In 80 lb bags (0.60 cu ft each) that is 56 bags before overage, about 62 with 10% added. At that point most people order ready-mix: the same slab is 1.25 cu yd, or 1.5 cu yd ordered with overage.
45 bags. An 80 lb bag yields 0.60 cu ft of mixed concrete and a cubic yard is 27 cu ft, so 27 ÷ 0.60 = 45. For 60 lb bags the figure is 60 per yard, and for 40 lb bags it is 90 per yard.
Ready-mix plants dispatch and bill in quarter-yard (sometimes half-yard) increments, and the truck cannot carry fractional precision anyway. Rounding the overage-adjusted volume up to the next quarter yard matches what you will actually be quoted and charged.
Four inches is standard for patios, walkways and shed bases. Driveways carrying cars want 5–6 in, and anything carrying trucks or RVs more, usually with reinforcement. Footings must reach below the frost line and meet local code widths — check your jurisdiction before digging.
Yes. Forms bow, subgrade is never perfectly flat, and spillage is real. Running a wheelbarrow short on the last corner is far more expensive — a second delivery carries a short-load fee and the first pour may set before it arrives. Pros order 10% over as a matter of routine.