Concrete Calculator
Work out the concrete volume and number of bags for a slab or footing.
Results update as you type.
About this calculator
A concrete calculator works out how much concrete a slab, footing or path needs, and how many bags of pre-mix that is. It multiplies length × width × thickness to get the volume — in cubic yards or cubic metres — using consistent units (feet with inch thickness in imperial, metres with centimetre thickness in metric). Set a quantity to size several identical pours at once, add a waste allowance for spillage and over-excavation, choose the bag size, and enter a price per bag for a cost estimate.
Worked example: a 10 × 10 ft slab, 4 inches thick, is 10 × 10 × (4 ÷ 12) = 33.3 cubic feet, about 1.23 cubic yards. Since an 80 lb bag yields roughly 0.6 cubic feet, that is 33.3 ÷ 0.6 ≈ 56 bags — a lot of mixing, which is why ordering ready-mix by the yard is usually cheaper past about a third of a yard.
Use it before pouring a patio, shed base, fence-post footing or path so you order the right amount and neither run short mid-pour nor pay for wasted concrete.
Frequently asked questions
How much concrete do I need for a slab?
Multiply length × width × thickness in consistent units. A 10×10 ft slab, 4 in thick is 10 × 10 × (4 ÷ 12) = 33.3 cubic feet, which is about 1.23 cubic yards.
How many bags of concrete per cubic yard?
An 80 lb bag yields roughly 0.6 cubic feet, so a cubic yard (27 cu ft) takes about 45 bags. Ordering ready-mix by the yard is usually cheaper once you pass a third of a cubic yard.
Should I add extra for waste?
Yes — a 5–10% allowance covers spillage, uneven sub-base and over-excavation so you do not run short mid-pour. Set it under advanced options and the volume, bags and cost all include it.
How many bags of concrete make a cubic yard?
It depends on bag size: a cubic yard is 27 cubic feet, so at 0.6 cu ft per 80 lb bag it takes about 45 bags, at 0.45 cu ft per 60 lb bag about 60, and at 0.30 cu ft per 40 lb bag about 90. The calculator uses the bag size you select.
When should I order ready-mix instead of bags?
Bags suit small jobs up to roughly half a cubic yard; beyond that the mixing effort and per-unit cost climb fast, and a ready-mix truck delivered by the yard is cheaper and far less work. Most suppliers have a minimum load of around one cubic yard.
How thick should a concrete slab be?
Four inches is standard for patios, walkways and shed bases; driveways and areas taking vehicles are usually 5–6 inches, often with reinforcement. Enter the thickness your project needs — it directly scales the volume and bag count.
API — use this calculator from code
Call this calculator as a free JSON endpoint — no key required. Send the field values below as query parameters or JSON. Read the full API docs →
Endpoint
GET https://calculator.free/api/v1/concrete/curl
curl "https://calculator.free/api/v1/concrete/?units=imperial&length=10&width=10&thickness=4"JavaScript fetch()
const r = await fetch(
"https://calculator.free/api/v1/concrete/?" + new URLSearchParams({
"units": "imperial",
"length": "10",
"width": "10",
"thickness": "4"
}));
const data = await r.json();
console.log(data.results);Results are estimates for general guidance only, not financial, medical or tax advice.