How to Calculate Concrete (For Any Pour Shape)

Calculating concrete is a two-part problem. First, you find the volume of the geometry you're pouring. Second, you convert that volume to whatever unit your supplier or hardware store prices by — cubic yards in the US, cubic meters in most metric markets, or cubic feet for bag counts. Both halves are simple arithmetic, but the order in which you do them, and the unit conversions you keep straight, are what separate a confident estimate from a phone-call to the supplier two hours into the pour.

1. Identify the shape

Every residential pour reduces to one of five primitives. About 80% of jobs are slabs (patios, driveways, garage floors, sidewalks). Round footings cover deck piers, fence posts, mailbox bases, and Sonotube columns. Hollow tubes are pipes, rings, and planters. Curb-and-gutter is its own profile because the cross-section is non-rectangular. Stairs are stepped wedges with an optional landing on top.

Complex pours decompose into combinations of these primitives. An L-shaped patio is two rectangles. A stoop is a slab plus stairs. A monolithic foundation is a slab on top of a perimeter footing. Sketch the cross-section before you start measuring and number each primitive — you'll calculate each separately and sum them at the end.

2. Measure in consistent units

Pick one unit and convert everything to it before multiplying. The single most common DIY estimating error is multiplying feet by inches and forgetting the conversion. A "5 by 10 by 4" slab — feet by feet by inches — is not 200 cubic feet, it's 16.67. Either work in all-feet (4 inches becomes 0.333 ft) or all-inches and divide the final answer by 1,728 to get cubic feet.

Use a tape measure for anything that matters. Pacing off a patio is fine for a sketch, not for an order. For round footings, measure across the widest point of the hole or tube; for tubes, measure both inner and outer diameters with calipers if you can — even a quarter-inch difference at the wall changes the wall area meaningfully.

3. Apply the volume formula

• Slab: V = L × W × T
• Round footing: V = π × (d / 2)² × h
• Hollow tube: V = π × ((d₁ / 2)² − (d₂ / 2)²) × L
• Stairs: V = Σ(i × rise × run × W) + platform_depth × total_rise × W
• Curb + gutter: V = (gutter_w × flag_t + curb_d × curb_h) × L

For irregular shapes, decompose into rectangles and triangles you can measure. A truncated wedge (a tapered footing, for example) splits into a rectangular prism plus two triangular prisms. The volume of a triangular prism is ½ × base × height × length.

4. Convert to your ordering unit

Cubic feet × 0.037 = cubic yards. Cubic feet × 0.02832 = cubic meters. Bag count = cubic feet ÷ bag yield (0.30 ft³ for a 40-lb bag, 0.45 for 60-lb, 0.60 for 80-lb). Most US ready-mix suppliers price by the cubic yard with a half-yard minimum and a short-load fee under 5 yd³. In metric markets it's per cubic meter with similar minimums.

5. Add a waste margin and round up

Order more than your raw calculation. The right margin depends on conditions:

• 5% for tight forms with experienced crews on a flat sub-grade.
• 10% for typical residential pours — this is the safe default.
• 15% for irregular grades or trenches you haven't over-excavated.
• 20% for footings dug straight into rocky or root-bound ground.

Running short on a single-truck pour is far more expensive than ordering an extra quarter yard. A return trip from the plant typically costs $200–$400 plus the new short-load fee, and the cold-joint where the second batch meets the first becomes a permanent weak line in the slab.

6. Sanity-check your number

Before you call the supplier, run a back-of-envelope check. A cubic yard of concrete weighs about 4,050 lb (1,840 kg) and a standard ready-mix truck holds 8–10 yd³. If your calculator says you need 30 yards for a residential patio, you've either dropped a unit conversion or you're building something much bigger than a patio. If it says 0.05 yards for a garage floor, same problem in the other direction.

Standard sanity benchmarks: a 10 × 10 ft × 4 in patio is about 1.23 yd³. A typical 20 × 20 ft × 4 in two-car driveway is about 4.94 yd³. A 12-in diameter × 4-ft deep deck pier is 0.087 yd³ — about 4 of the 80-lb bags.

Putting it together

For most residential jobs, the workflow is: measure twice, convert to a single unit, apply the formula, divide by 27 to get yards, multiply by 1.10 for waste, round up to the supplier's nearest delivery increment. For anything more complex than rough mental math, use the concrete calculator on the home page — it handles all the unit conversions automatically and gives you bag counts for three bag sizes plus a ready-mix truck-load estimate.