Concrete Calculator
Enter each pour below — a slab, a footing, a round column or a flight of steps — and get the exact volume in cubic yards, feet and metres, how many premix bags it takes, and what it costs either way.
Bag counts use the waste-adjusted volume and always round up — you can’t buy part of a bag. Ready-mix is usually ordered to the nearest quarter yard; your plant may add short-load or delivery fees.
The short answer: concrete volume is length × width × thickness. A 10 × 10 ft slab poured 4 inches thick is 33.33 ft³ — that’s 1.23 cubic yards, or 62 bags of 80 lb premix once you add the usual 10% waste allowance. Divide cubic feet by 27 for yards; divide by a bag’s yield (0.60 ft³ for 80 lb) and round up for bags.
How the calculation works
Every shape reduces to a volume in cubic feet. A slab or footing is a flat box: length × width × thickness, with the thickness converted from inches to feet (divide by 12). A column is a cylinder: π × radius² × height. A flight of steps is a stack of boxes — the bottom step is one rise tall, the next two rises, and so on — so its volume is width × tread depth × rise × n(n+1)/2, where n is the number of steps. Add up every pour, apply your waste allowance, then convert: yd³ = ft³ ÷ 27 and m³ = ft³ × 0.028317.
Bag counts use the yield printed on standard premix bags: an 80 lb bag makes about 0.60 ft³ of concrete, a 60 lb bag 0.45 ft³, a 50 lb bag 0.375 ft³ and a 40 lb bag 0.30 ft³. Counts always round up, and they’re figured on the waste-adjusted volume — running one bag short mid-pour is far worse than having one left over. Yields are the figures major manufacturers print on the bag; check yours, because brands vary slightly.
Worked examples
| Pour | Volume | Cubic yards | 80 lb bags (no waste) |
|---|---|---|---|
| 10 × 10 ft slab, 4 in | 33.33 ft³ | 1.23 yd³ | 56 |
| 12 × 12 ft slab, 4 in | 48.00 ft³ | 1.78 yd³ | 80 |
| 20 × 20 ft slab, 4 in | 133.33 ft³ | 4.94 yd³ | 223 |
| 24 × 24 ft slab, 4 in | 192.00 ft³ | 7.11 yd³ | 320 |
| Column, 12 in diameter × 8 ft tall | 6.28 ft³ | 0.23 yd³ | 11 |
| Steps, 5 steps · 12 in tread · 7 in rise · 4 ft wide | 35.00 ft³ | 1.30 yd³ | 59 |
Bag counts above exclude waste so you can see the raw geometry; the calculator adds your allowance on top. Past roughly one cubic yard, bags stop being practical — one yard is 45 bags of 80 lb mix, about 3,600 lb to haul, mix and place — and a ready-mix truck usually wins on both price and finish quality.
Popular slab sizes
Frequently asked questions
How many bags of concrete do I need for a 10 × 10 slab?
At the standard 4 inch thickness, a 10 × 10 ft slab is 33.33 ft³. That’s 56 bags of 80 lb premix with no allowance, or 62 bags with the usual 10% waste added. At 6 inches thick the same slab needs 92 bags with waste — well into ready-mix territory.
How much waste should I add?
Ten percent is the standard planning allowance. It covers uneven subgrade, spillage and forms that aren’t perfectly level. Go to 15% for hand-mixed bag work or rough, hand-dug footings; a precisely graded, formed slab can run 5%.
When should I order ready-mix instead of bags?
Around one cubic yard is the usual crossover. Below that, bags are cheaper and avoid truck minimums and short-load fees. Above it, the bag count — 45 bags per yard for 80 lb mix — makes ready-mix cheaper per yard and far less labour, with a more consistent mix.
Does the calculator handle round post holes and sonotubes?
Yes — use the Column shape with the tube’s diameter in inches and the depth as the height. A 12 inch diameter hole 8 ft deep takes 6.28 ft³, about 0.23 yd³.
How accurate are the bag yields?
They’re the mixed-volume figures printed on standard premix bags (80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45 ft³, 50 lb ≈ 0.375 ft³, 40 lb ≈ 0.30 ft³). Individual brands vary a little, so confirm the yield on the label before a big order.