U
UseCalcForge Free Online Calculators
Tools
Publishing Science Developer Finance
 
Health Marketing Lifestyle All Tools
Contact Us
Lifestyle

Concrete Calculator.

Volume and bag count for a slab, footing, or round column, in imperial or metric, with waste allowance.

Cubic yards

including waste

Cubic metres

Cubic feet

27 ft³ = 1 yd³

Bags of pre-mixed concrete

Bag size Yield each Bags needed
80 lb 0.60 ft³
60 lb 0.45 ft³
40 lb 0.30 ft³
25 kg 0.0125 m³

Yields are the figures printed on standard pre-mixed bags. Check your own brand, as they vary by a few percent.

At cubic yards you are past the point where bags make sense. Ready-mix trucks typically carry up to 10 cubic yards and most suppliers set a minimum order of around 1 yard, so a delivery will usually be cheaper and much faster than mixing this by hand.

Runs entirely in your browser. The values you enter never leave your device — there is no request to our server and nothing is stored. How we handle data

Working Out the Volume

Concrete is sold by volume, so every calculation starts by turning your dimensions into a single consistent unit. The classic error is mixing feet and inches — a 4-inch slab is 0.333 feet thick, not 4.

Formulas

Slab: V = length × width × thickness
Column: V = π × (diameter ÷ 2)² × depth
Cubic yards = cubic feet ÷ 27
1 m³ = 35.315 ft³ = 1.308 yd³
Order quantity = V × (1 + waste %)

For an irregular pour, split it into rectangles and add the volumes, exactly as you would for area. For a footing that steps in depth, treat each depth as its own block rather than averaging, since averaging understates the deeper section when the steps are uneven.

Typical Thicknesses

Thickness drives both the cost and the load the slab can carry, and it is the dimension most often set too thin. These are common starting points; local building codes and soil conditions override them.

4 inches (10 cm) — patios, walkways, shed bases

Standard for foot traffic and light loads over a compacted sub-base. Adding reinforcement mesh matters more than adding thickness at this scale.

5-6 inches (13-15 cm) — driveways and garage floors

Needed anywhere a vehicle will park or turn. Going from 4 to 6 inches raises the concrete volume by half, which is the single biggest cost step in a typical driveway.

Footings — below the local frost line

Depth is set by code and by how deep the ground freezes, not by the load alone. Post holes are usually a third of the post's above-ground height, with a diameter around three times the post width.

Bags or Ready-Mix

Below about half a cubic yard, bags win on cost and convenience. Above one cubic yard — roughly 45 eighty-pound bags — mixing by hand becomes a serious job, and the risk of an uneven pour rises with every batch.

Ready-mix suppliers charge a short-load fee below their minimum, usually around one cubic yard, and concrete starts setting from the moment water is added, so the whole pour has to be placed and finished within a couple of hours. Order slightly over, have the forms ready and helpers on site before the truck arrives, and plan where any surplus will go.

Knowledge Base

Concrete Volume Estimation Methodology.

Concrete is ordered by volume and poured once. Getting the quantity right before the truck arrives matters more than almost any other number in a small building project, because running short mid-pour leaves a cold joint that no amount of finishing can repair.

The Calculation Branch

Slab volume = length × width × thickness (consistent units) | Column volume = π × radius² × depth × quantity | Cubic yards = cubic feet ÷ 27 | 1 m³ = 35.3147 ft³ | Bags = cubic feet ÷ bag yield, rounded up | Order = volume × (1 + waste allowance)

Industrial Standards.

All dimensions are converted to metres internally and the volume is computed in cubic metres, then expressed in cubic feet and cubic yards using exact conversion factors. Bag counts use the published yields for standard pre-mixed concrete — 0.60 ft³ for an 80 lb bag, 0.45 for 60 lb, 0.30 for 40 lb, and 0.0125 m³ for a 25 kg bag — and are always rounded up, since a part bag does not help. The waste allowance is applied before the bag count so the ordered quantity already includes it.

In-Depth Analysis & Reference Data

Sub-grade preparation affects the real quantity more than the arithmetic does. A slab poured over ground that dips half an inch across its width can take several percent more concrete than the calculation suggests, which is exactly what the waste allowance is for. Compacting and levelling the base properly is the cheapest way to keep the actual pour close to the estimate.

Concrete weighs roughly 2,400 kg per cubic metre, or about 4,050 lb per cubic yard. That figure matters for access as much as for structure: a single cubic yard is about two tonnes, which rules out moving it far by wheelbarrow and often decides whether a truck can reach the pour or a pump is needed.

Registry Questions & FAQ.

Does the calculator include reinforcement?

No. Rebar and mesh displace a negligible fraction of the volume, so the concrete quantity is unaffected. Size and space reinforcement according to your local code and the load the slab will carry.

Why does my supplier quote in cubic metres when I measured in feet?

Ready-mix is sold in cubic metres across most of the world and in cubic yards in the United States. Both figures are shown above from the same measurement, so you can order in whichever unit your supplier uses.

All metrics verified against ISO/ASTM benchmarks.