Concrete Calculator

How Much Concrete Do You Need?

Ordering too little concrete means an emergency second delivery (at premium cost), a cold joint where fresh concrete meets hardened concrete (a structural weakness), and frustration during what's already a physically demanding project. Ordering too much means paying for waste that gets hauled away. Our calculator finds the sweet spot — computing exact volume in cubic yards for slabs, footings, columns, and stairs, then adding an appropriate waste factor.

Enter your project type and dimensions. The calculator outputs volume in cubic yards, estimated number of pre-mixed bags (for small projects), approximate ready-mix delivery cost, and recommended reinforcement.

Quick formula: Slab volume = Length (ft) × Width (ft) × Thickness (inches) ÷ 324 = cubic yards. A standard 10' × 20' driveway at 4 inches thick: 10 × 20 × 4 ÷ 324 = 2.47 cubic yards. Always add 5–10% for waste, spillage, and subgrade irregularities.

Calculating Volume by Project Type

Slabs (Driveways, Patios, Sidewalks, Garage Floors) are the most common concrete project. The calculation is straightforward: length × width × thickness. The critical variable is getting the thickness right — too thin and the slab cracks under load; too thick and you're wasting material and money.

Standard thicknesses: Sidewalks and garden paths: 4 inches. Patios: 4 inches (6 inches if supporting hot tubs or heavy furniture). Driveways for passenger vehicles: 4 inches. Driveways for trucks, RVs, or heavy vehicles: 5–6 inches. Garage floors: 4 inches minimum, 6 inches recommended. Foundation slabs: 4–6 inches with rebar reinforcement per engineering specifications.

Example calculations: Single-car driveway (10' × 20' × 4"): 2.47 cu yd. Two-car driveway (16' × 24' × 4"): 4.74 cu yd. Patio (12' × 16' × 4"): 2.37 cu yd. Sidewalk (4' × 40' × 4"): 1.98 cu yd. Two-car garage floor (20' × 24' × 5"): 7.41 cu yd.

Footings support walls, posts, and structures by distributing weight to stable soil. They're typically rectangular in cross-section. Residential wall footings: 12–24 inches wide, 6–12 inches deep. Post footings (sonotube): 8–18 inch diameter, 36–48 inches deep (below frost line).

Columns and Posts use the cylindrical formula: π × radius² × height ÷ 27. Six 10-inch diameter sonotubes at 4 feet deep: 0.48 cubic yards.

Steps and Stairs are the most complex calculation — essentially stacked rectangles of decreasing depth. Each step typically has a 7-inch rise and 11-inch run. Our calculator automates this stacking calculation.

Ready-Mix Truck vs. Pre-Mixed Bags

Ready-mix concrete (truck delivery) is sold by the cubic yard, typically $130–$180 per cubic yard delivered in 2026. Most batch plants have a minimum order — often 1 cubic yard — with short-load fees of $30–$60 per cubic yard below their minimum truck capacity (usually 3–4 cubic yards). Use ready-mix for any project requiring 1+ cubic yards.

Pre-mixed bags (Quikrete, Sakrete) are available at any hardware store. An 80-lb bag yields approximately 0.6 cubic feet. To fill 1 cubic yard, you need 45 eighty-pound bags — 3,600 lbs of material. At $5–$7 per 80-lb bag, the material cost alone is $225–$315 per cubic yard — significantly more than ready-mix. Use bags for projects under 0.5 cubic yards (roughly 22 bags or less).

The Critical Waste Factor

Always order 5–10% more concrete than your calculated volume. Subgrade irregularities (even well-prepared ground isn't perfectly flat), form imperfections (wooden forms flex under wet concrete pressure), and spillage/over-pouring all consume extra material. The cost of running short — second delivery plus cold joint — vastly exceeds the cost of over-ordering by half a yard ($65–$90 extra vs. $300–$500+ for a short delivery).

Reinforcement: Rebar, Wire Mesh, and Fiber

Rebar (#4, 1/2-inch diameter) provides tensile strength that prevents cracking under load. Standard layout for driveways: #4 rebar on 18-inch centers in both directions. Cost: approximately $0.75–$1.25 per linear foot. Welded wire mesh (6×6 W1.4/W1.4) is an alternative for flatwork — easier to install but less effective for heavy loads. Cost: approximately $0.15–$0.25 per square foot. Fiber reinforcement mixed into concrete controls plastic shrinkage cracking but does NOT replace rebar or mesh for structural applications.

Concrete Finishing and Curing

Finishing includes screeding (leveling), bull-floating (smoothing), edging (rounding edges), and optional decorative treatments (broom finish, stamped, exposed aggregate, stained). Curing is critical — keep the surface moist for at least 7 days using curing compound, plastic sheeting, or periodic misting. Concrete reaches 70% strength in 7 days and 90%+ at 28 days. Don't pour below 40°F or above 90°F without special precautions.