Concrete Calculator

Formula

Slab: L×W×T (ft) = ft³; yd³ = ft³÷27. Column: π×(d/2)²×h. Order = volume × (1+waste%).

Bags = ceil(ft³ ÷ bag-ft³). 80-lb bag ≈ 0.6 ft³ → 45 bags per yard. Weight = ft³ × 150 pcf.

How it works

Pick slabs, hole, circular tube, curb, or stairs and enter dimensions in one unit. The engine converts everything to feet, applies the shape’s geometry (length-times-width-times-thickness for slabs, pi-times-radius-squared-times-depth for round footings, annulus area for tubes), multiplies by quantity, then adds your waste factor. Cubic yards, cubic meters, weight at your density, bag count at your bag size, and both ready-mix and bag costs derive from that single cubic-feet number.

Assumptions

• Dimensions are finished in-place sizes; forms and subgrade assumed exact.
• Waste 5% default; 10–15% for hand-mix, thin, or irregular pours.
• Bag yields nominal (0.6 ft³ per 80-lb bag); ready-mix billed by the yard.

Default result

With Defaults: slab 10×10 ft × 0.333 ft, qty 1, 5% waste, 150 pcf, 80-lb bags, $150/yd: 34.97 ft³ · 1.30 yd³ · 59 × 80-lb bags · ready-mix $194.25

Worked examples

Patio slab

Slab 10×10 ft, 0.333 ft (4 in) thick, qty 1, Feet, 5% waste

10×10×0.333 ft = 33.30 ft³ → 1.30 yd³ with 5% waste · 59 bags — Example calculated with this calculator's formula.

Round footing

Hole diameter 1 ft, depth 4 ft, qty 1, Feet, 5% waste

π×0.5²×4 = 3.14 ft³ → 0.122 yd³ · 6 bags — Example calculated with this calculator's formula.

Frequently asked questions

How much concrete do I need for a slab?

To calculate concrete needed for a slab: multiply Length (ft) × Width (ft) × Thickness (ft), then divide by 27 to get cubic yards. For a 10×10 slab at 4 inches thick: 10 × 10 × 0.333 = 33.3 cubic feet = 1.23 cubic yards. Add 5–10% for waste and uneven subgrades.

How many 80lb bags of concrete do I need per cubic yard?

One 80lb bag of concrete yields approximately 0.6 cubic feet. Since there are 27 cubic feet per cubic yard, you need 27 ÷ 0.6 = 45 bags of 80lb concrete per cubic yard. For 60lb bags (0.45 ft³ each), you need 60 bags per cubic yard.

What is the standard concrete mix ratio?

The standard mix ratio for general-purpose concrete is 1 part Portland cement, 2 parts sand, and 3 parts coarse aggregate by volume. Water should be added at a ratio of approximately 0.5 parts water to 1 part cement (w/c ratio of 0.5). Adjustments may be needed based on aggregate moisture content and desired workability.

How long does it take for concrete to cure?

Concrete reaches about 70% of its design strength after 7 days and approximately 90% after 28 days under proper curing conditions. The first 7 days are the most critical. Concrete should be kept moist for at least 7 days for best results. Full curing may take up to 28 days depending on temperature, humidity, and mix design.