Concrete & Masonry
Concrete Calculator
Find out how much concrete you need for a slab, wall, footing, column or post holes. This concrete calculator gives the volume in cubic yards, cubic feet and cubic meters, adds a waste allowance, and tells you what to buy: ready-mix by the quarter yard, or the number of 40, 50, 60 or 80 lb bags, with an optional cost estimate.
Result
Enter your measurements and press Calculate. What to buy, the working and the cost will appear here.
How to use the concrete calculator
- Pick the shape of each pour: a rectangle for slabs, patios, walls and footings; a circle for columns, piers and post holes; a ring for hollow forms; or Known volume if you already have a figure.
- Enter the dimensions and thickness. Slab thickness is usually in inches; switch the unit to feet for column height or wall height.
- Add more pours for a job with several parts, such as a slab plus its footings, and use How many for repeated post holes.
- Check the allowance (10% is a safe default), choose the bag size you plan to buy, and add prices if you want a cost.
- Press Calculate to see cubic yards, the ready-mix order, bags of every size and the working.
How much concrete do I need? The formula
Concrete is ordered by volume. Work out the volume in cubic feet, then divide by 27 to get cubic yards:
cubic yards = length (ft) × width (ft) × thickness (ft) ÷ 27
Thickness is usually measured in inches, so divide it by 12 first: 4 in = 0.333 ft, 6 in = 0.5 ft. For round shapes, the area is π × radius²:
- Slab, wall or footing: length × width × thickness.
- Column or post hole: 3.1416 × (diameter ÷ 2)² × depth.
- Hollow tube: the outer circle minus the inner circle, times the height.
Then add an allowance for uneven ground and spillage, and round up to what you can buy.
Worked example: a patio slab and four fence posts
A 12 ft × 10 ft patio slab is 4 inches thick, and four fence posts need 10 in diameter holes 3 ft deep. With a 10% allowance, ready-mix at $175 per cubic yard and 80 lb bags at $6.50:
- Pour 1: 12 ft × 10 ft = 120 ft²; × 4 in = 40 ft³
- Pour 2: π × (5 in)² = 0.55 ft²; × 3 ft = 1.64 ft³ × 4 = 6.54 ft³
- Total: 46.54 ft³ = 1.724 yd³
- With allowance: 1.724 yd³ × 1.1 = 1.896 yd³
- Bags: 51.2 ft³ ÷ 0.6 ft³ per bag = 85.33 → buy 86 × 80 lb bags
You need 1.9 yd³ of concrete: order 2 yd³ of ready-mix, or buy 86 × 80 lb bags. Ready-mix cost: $350.00. 80 lb bags cost: $559.00. Open this example in the calculator.
How many bags of concrete are in a yard?
Bagged concrete mix is sold by weight, and each bag makes a set volume once mixed. These are typical yields printed on standard concrete mix bags:
| Bag size | Yield per bag | Bags per cubic yard | Total weight |
|---|---|---|---|
| 40 lb bag | 0.3 ft³ | 90 | 3,600 lb |
| 50 lb bag | 0.375 ft³ | 72 | 3,600 lb |
| 60 lb bag | 0.45 ft³ | 60 | 3,600 lb |
| 80 lb bag | 0.6 ft³ | 45 | 3,600 lb |
High-strength and fast-setting mixes can yield less per bag. If your bag lists a different yield, enter it in Yield per bag and the calculator uses your figure. For bag-only projects, the concrete bag calculator compares sizes side by side.
Bags or ready-mix?
- Under about 1 cubic yard (post holes, small pads, steps), bags are practical. Mixing 45 bags of 80 lb for one yard by hand is hard work, so rent a mixer for anything over a few dozen bags.
- From about 1 cubic yard, ready-mix is usually easier and often cheaper, and the whole pour is one consistent batch. Many suppliers have a minimum order and charge a short-load fee for small deliveries, so ask for a quote.
- Order in quarter yards. Suppliers typically deliver in ¼ or ½ yard steps. Running short mid-pour leaves a cold joint, so it is better to have a little left over.
- Plan access. A loaded mixer truck is very heavy. If it cannot reach the forms, budget for a pump, a line of wheelbarrows or a conveyor.
Common concrete thickness
| Project | Typical thickness |
|---|---|
| Walkways, patios, shed bases | 4 in (10 cm) |
| Residential driveways | 4–6 in (10–15 cm) |
| Garage floors and slabs for heavier loads | 5–6 in (12–15 cm) or more |
| Fence post holes | depth commonly one third to one half of the post’s height above ground, and below the frost line where the ground freezes |
These are common starting points, not a design. Loads, soil, reinforcement and your local building code decide the right thickness; the concrete slab calculator covers slabs and their gravel base in more detail.
How much extra concrete should I order?
Order 5–10% more than the calculated volume. Subgrades are never perfectly flat, forms bow slightly under the weight of wet concrete, and some is always lost in the truck chute, the wheelbarrow and the mixer. Use the higher end for rough or dug ground, footings and post holes; the lower end for slabs over a well-compacted, level base.
Assumptions and limitations
- This is a volume and quantity estimate. It does not size footings, specify reinforcement or check loads; follow your plans, the mix manufacturer and local building codes.
- Each pour is treated as a regular shape with an even thickness. For a sloping slab, use the average thickness.
- Rebar, mesh and embedded items take up very little volume and are ignored. Use the rebar calculator for steel quantities.
- The placed weight uses about 150 lb per cubic foot (2,400 kg/m³) for normal-weight concrete.
Concrete questions
How much concrete do I need for a 10 × 10 slab?
At 4 inches thick, a 10 ft × 10 ft slab is 10 × 10 × 0.333 = 33.3 cubic feet, or 1.23 cubic yards. With a 10% allowance, order about 1.36 cubic yards, which is 1.5 yards of ready-mix or 62 bags of 80 lb mix.
How many 80 lb bags of concrete make a yard?
45. An 80 lb bag yields about 0.6 cubic feet, and a cubic yard is 27 cubic feet: 27 ÷ 0.6 = 45. You would need 60 bags of 60 lb or 90 bags of 40 lb.
How much does a cubic yard of concrete weigh?
About 4,000 lb (roughly 2 tons). Normal-weight concrete is around 150 lb per cubic foot, and 150 × 27 = 4,050 lb.
How do I calculate concrete for post holes?
Multiply 3.1416 × radius² × depth for one hole, then by the number of holes. A 10 in hole is 5 in (0.417 ft) in radius: 3.1416 × 0.417² × 3 ft = 1.64 cubic feet each. Many people subtract the post itself; the calculator does not, which leaves a small built-in margin.
Is it cheaper to mix concrete myself or order ready-mix?
For small jobs, bags are cheaper because you avoid delivery and short-load fees. As the volume grows past about a cubic yard, ready-mix usually wins on price and certainly on labor. Enter both prices in the calculator to compare.
Formulas, unit conversions and default values are explained on our methodology page. Results are estimates; check manufacturer data and local codes before you buy or build.