How to use
Enter the length, width and thickness of a slab, patio, sidewalk or footing, and add extra for spills and an uneven base. Then there are two ways to buy:
For a strip footing, choose it in “What to pour” and enter the total footing length (perimeter plus interior bearing walls), width and depth. For columns or piers, enter the diameter, height and count: each one is π × d² / 4 × height.
- bagged premix (like Quikrete or Sakrete): enter the bag size — 60 or 80 lb — to see how many bags you need;
- mixing your own: pick the mix ratio and cement bag size to see the cement, sand, gravel and water.
How it’s calculated
- Volume = length × width × thickness × (1 + extra). Concrete is ordered in cubic yards: 1 cu yd = 27 cu ft.
- An 80-lb bag of premix makes about 0.60 cu ft of concrete and a 60-lb bag about 0.45 cu ft, so one cubic yard takes about 45 bags of 80 lb or 60 bags of 60 lb.
- For a site mix, you need about 1.54 times the finished volume in dry materials — water and fines fill the gaps and the mix compacts. That’s the standard factor for mixes by volume. The dry volume is split by the ratio; cement is converted to pounds at a bulk density of about 90 lb/cu ft (1440 kg/m³).
- Water is a guide for a water-cement ratio of 0.5. Add it gradually until you reach the workability you need.
Which mix to choose
Ratios are by volume: cement : sand : gravel (for example, in buckets).
- 1 : 2 : 3 — a stronger mix for slabs and footings that carry loads.
- 1 : 2 : 4 — a general-purpose mix for small foundations, walkways and post footings.
- 1 : 3 : 5 and 1 : 3 : 6 — lean concrete for mud slabs, fill and leveling.
These are guides, not a guaranteed strength: that depends on the cement, the aggregate, the water and compaction. A 3,000 or 4,000 psi mix can’t be guaranteed from the ratio alone — for structural work, follow your plans or order ready-mix at the strength you need. Portland cement in the US comes in 94-lb bags, which is the default here.
Examples
- A 20 × 12 ft slab, 4 in thick, 5% extra: 84 cu ft = 3.11 cu yd. In premix that’s 140 bags of 80 lb (or 187 bags of 60 lb) — at this size, ready-mix delivered by the truck is usually easier. Mixing 1 : 2 : 4 yourself: about 18 bags of 94-lb cement, 1.37 cu yd of sand, 2.74 cu yd of gravel and about 100 gallons of water.
- A 10 × 10 ft patio, 4 in thick, 10% extra: 1.36 cu yd — 62 bags of 80 lb.
FAQ
How much extra should I add? 5–10%: some concrete stays in the mixer and on tools, and the subgrade is rarely perfectly flat.
When is ready-mix worth it? For about a cubic yard or more. Many suppliers have a minimum order, and short-load fees apply below it.
Sand and gravel are sold by the ton — how do I convert? Roughly 1.3–1.4 tons per cubic yard for dry sand and about 1.4 tons for gravel. Check with your supplier.
For walls, use the brick calculator.