Concrete Calculator
Work out how much concrete a pour needs in cubic yards — then see the same volume as 80, 60, 50 and 40 lb bags so you can compare bagged mix against a ready-mix delivery before you order.
How to use this calculator
- Pick the shape. Slabs and footings are both length × width × depth — the separate option just relabels the fields the way you would measure them. Round columns and post holes use diameter and depth instead.
- Measure the excavation, not the plan. A slab formed to 4 in over a subgrade that dips half an inch is really averaging 4.5 in. Take depth readings at several points and use the average.
- Set a waste allowance. 10% is a sensible default for a formed slab on prepared gravel. Use 5% for a clean form on a flat base and 15–20% for trenches, post holes, or anything dug by hand, where the sides bulge outward.
- Compare the two supply routes. The result shows both bag counts and cubic yards. Bagged mix wins on small pours; a truck wins once you pass roughly a cubic yard, provided you can place it before it sets.
The formula
Concrete is sold by volume, so every case reduces to one of two volume formulas, computed in feet and then converted:
| Shape | Volume in cubic feet |
|---|---|
| Slab, footing, curb | length (ft) × width (ft) × depth (ft) |
| Round column or post hole | π × (diameter ÷ 2)² × depth (ft) |
Depth entered in inches is divided by 12 first. The conversion to the unit suppliers quote is fixed: 27 cubic feet = 1 cubic yard, because a yard is 3 ft on every side and 3 × 3 × 3 = 27. One cubic foot is 0.0283 cubic meters.
Bag counts come from the yield printed on the bag, then round up — a partial bag is still a bag you have to buy:
bags = ceiling( total cubic feet ÷ bag yield )
Worked example
A 10 × 12 ft patio, 4 in thick, with a 10% allowance:
- Depth in feet: 4 ÷ 12 = 0.333 ft
- Volume: 10 × 12 × 0.333 = 40 cu ft
- With 10% waste: 40 × 1.10 = 44 cu ft
- Cubic yards: 44 ÷ 27 = 1.63 yd³
- 80 lb bags: 44 ÷ 0.60 = 73.3 → 74 bags
Seventy-four bags is roughly 5,900 lb of material to lift, open and mix — which is exactly the point at which most people call the ready-mix supplier instead, even with a short-load fee.
Bag yields and coverage
| Bag | Yield | Bags per cu yd | Covers at 4 in |
|---|---|---|---|
| 80 lb | 0.60 cu ft | 45 | 1.8 sq ft |
| 60 lb | 0.45 cu ft | 60 | 1.35 sq ft |
| 50 lb | 0.375 cu ft | 72 | 1.1 sq ft |
| 40 lb | 0.30 cu ft | 90 | 0.9 sq ft |
Yields are the standard figures for pre-mixed concrete; check your brand's bag, since high-strength and fast-setting mixes differ slightly.
Frequently asked questions
How many 80 lb bags of concrete are in a cubic yard?
Forty-five. An 80 lb bag of pre-mixed concrete yields about 0.60 cubic feet, and a cubic yard is 27 cubic feet, so 27 divided by 0.60 is 45 bags. For 60 lb bags it is 60 bags per yard, and for 40 lb bags it is 90.
How much concrete do I need for a 10x10 slab?
A 10 by 10 ft slab at 4 inches thick is 33.3 cubic feet, or 1.23 cubic yards. Add 10% for waste and you are at about 1.36 cubic yards, which is 62 bags of 80 lb mix. At 5 inches thick the same slab needs about 1.7 cubic yards.
Should I add a waste allowance?
Yes. Ten percent is standard for a formed slab on compacted gravel, because the subgrade is never perfectly flat and some concrete is always lost to spillage and to the mixer. For hand-dug trenches and post holes use 15 to 20 percent, since the sides of an excavation bulge outward more than you expect.
Is it cheaper to use bags or order ready-mix?
Bags win below roughly one cubic yard, ready-mix wins above three or four, and the range in between depends on the short-load fee your supplier charges. Also weigh the labour: one cubic yard is 45 bags of 80 lb mix, about 3,600 lb to move by hand, and it all has to be placed and finished before the first batches stiffen.
How many cubic feet are in a cubic yard?
Twenty-seven. A cubic yard measures 3 feet on each side, and 3 times 3 times 3 equals 27 cubic feet.
Guides
- How Many Bags of Concrete Are in a Cubic Yard?
- How Thick Should a Concrete Slab Be?
- How Many Bags of Concrete per Fence Post?
- How Long Does Concrete Take to Cure?
- What Drives the Cost of a Concrete Patio?