Concrete Calculator

Calculate concrete for slabs, footings, columns, piers and steps. Get the volume in cubic yards and m³, how many bags to buy, and whether bagged or ready-mix is the better choice.

Results update as you type.

Shape

Length

Width

Thickness

4 in for patios, 5–6 in for driveways

Quantity

Identical pours (e.g. 6 piers)

Waste

5–10% typical

Bags & pricing

How will you make it?

Pre-mixed bagsMix on site

Bag size

Price per bag

Ready-mix price

$/yd³

Concrete needed

1.36yd³

Cubic feet: 36.7; 1.038 m³
Bags (80 lb bag): 62; 4,960 lb
Ready-mix order: 1.5 yd³; Rounded to ¼ yd³
Bagged cost: —; Add a bag price

Mix yourself or order ready-mix

Borderline volume. Mixing is doable with a mixer and a crew; ready-mix saves labour and gives a more consistent pour. Compare quotes, including short-load fees.

Shopping list

6280 lb bag pre-mixed concrete
4,960 lb total
1.5 yd³Alternative: ready-mix concrete
Pick one option
—#4 rebar or wire mesh, chairs
Per your engineer / local code

Add prices below to see a cost estimate.

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How we calculate

We compute the volume of the shape, multiply by the quantity and add your waste allowance:

slab / footing = length × width × thickness
square column = side A × side B × height
round column = π × (diameter ÷ 2)² × height
stairs = width × run × rise × n(n+1)/2 + landing depth × width × n × rise
volume = shape × quantity × (1 + waste %)
bags = ⌈volume ÷ bag yield⌉ · 1 yd³ = 27 ft³ = 0.7646 m³

Bag yields (standard pre-mixed concrete): 40 lb = 0.30 ft³, 60 lb = 0.45 ft³, 80 lb = 0.60 ft³. Metric bags use the same density (~2,130 kg of dry mix per m³), so a 25 kg bag yields ≈ 11.7 L.

Ready-mix is rounded up to the next ¼ yd³ (0.25 m³), which is how most plants dispatch.

Stairs are treated as solid steps poured on grade: step k is k rises tall, so the volume grows with the triangular number n(n+1)/2.

How much concrete do I need?

Concrete is sold by volume: cubic yards in the US and cubic metres elsewhere. Measure your forms in feet (or metres), convert thickness from inches to feet by dividing by 12, multiply the three dimensions, and divide by 27 to get cubic yards.

Slab (4 in thick)	Volume	80 lb bags
4 × 4 ft	0.20 yd³	9
8 × 8 ft	0.79 yd³	36
10 × 10 ft	1.23 yd³	56
12 × 12 ft	1.78 yd³	80
20 × 20 ft	4.94 yd³	223

Bag counts above are before waste. Remember that 56 bags × 80 lb is over two tonnes of material to lift, mix and place — the hidden cost of bagged concrete.

Frequently asked questions

How do I calculate cubic yards of concrete?

Multiply length × width × thickness in feet to get cubic feet, then divide by 27. A 10 × 10 ft slab 4 in (0.333 ft) thick is 33.3 ft³ ÷ 27 = 1.23 yd³. Add 5–10% for waste.

How many 80 lb bags of concrete make a yard?

An 80 lb bag yields about 0.60 ft³, so one cubic yard (27 ft³) needs 45 bags. With 60 lb bags (0.45 ft³) it’s 60 bags, and with 40 lb bags (0.30 ft³) it’s 90 bags.

How many bags of concrete do I need for a 10x10 slab?

At 4 in thick, a 10 × 10 ft slab is 1.23 yd³ — about 56 × 80 lb bags before waste, or 62 bags with 10% waste. At that size ready-mix is often worth pricing.

When should I order ready-mix instead of bags?

As a rule of thumb, mix bags yourself under 1 yd³ (0.75 m³) and order ready-mix above 2 yd³ (1.5 m³). In between, compare the cost of bags plus labour against the truck price including short-load fees.

How much extra concrete should I order?

Order 5–10% extra. Subgrade is never perfectly level, forms bow slightly and some concrete is always lost. Running short mid-pour creates a cold joint, which is far worse than a little leftover.

How thick should a concrete slab be?

4 in (10 cm) for patios, walkways and shed bases; 5–6 in (12–15 cm) for driveways and garage floors carrying vehicles. Heavy equipment pads need an engineer’s spec.