Concrete Calculator

A 10 ft x 10 ft slab at the standard 4-inch thickness requires approximately 1.23 cubic yards of concrete. Here is the math: convert 4 inches to feet (4 / 12 = 0.333 ft), then calculate volume as 10 x 10 x 0.333 = 33.3 cubic feet, then divide by 27 to get 1.23 cubic yards. With a recommended 10% waste factor, you should order approximately 1.36 cubic yards. In pre-mixed bags, that equals about 56 bags of 80 lb mix, 74 bags of 60 lb mix, or 111 bags of 40 lb mix. For this volume, ready-mix delivery is more practical than hand-mixing bags, though some homeowners opt for bags to work at their own pace over a weekend.

To convert cubic feet to cubic yards, divide the cubic feet value by 27 (since 1 cubic yard equals 3 ft x 3 ft x 3 ft = 27 cubic feet). For example, if your project volume is 81 cubic feet, divide 81 by 27 to get 3.0 cubic yards. For metric conversions, 1 cubic yard equals 0.7646 cubic meters, and 1 cubic meter equals 1.308 cubic yards. A quick reference: 27 cubic feet = 1 cubic yard = 0.7646 cubic meters. When ordering ready-mix concrete, always round up to the nearest quarter-yard increment, as suppliers typically batch in these increments and running short mid-pour is far worse than having a small surplus.

The number of bags per cubic yard depends on the bag size: an 80 lb bag yields approximately 0.60 cubic feet (0.022 cubic yards), so you need about 45 bags per cubic yard. A 60 lb bag yields approximately 0.45 cubic feet (0.017 cubic yards), requiring about 60 bags per cubic yard. A 40 lb bag yields approximately 0.30 cubic feet (0.011 cubic yards), requiring about 90 bags per cubic yard. For cost comparison at typical retail prices, 45 bags of 80 lb mix at $5.50 each costs about $248 per cubic yard, while ready-mix delivery averages $120–$180 per cubic yard. Bagged concrete becomes impractical and significantly more expensive above 1–2 cubic yards.

Slab thickness depends on the intended load and use. Sidewalks and garden paths require a minimum of 3–4 inches. Patios and pool decks should be at least 4 inches thick with fiber mesh or welded wire reinforcement. Residential driveways need 4–6 inches — 4 inches for passenger cars only, 5–6 inches if trucks, RVs, or heavy equipment will use the surface. Garage floors require 4–6 inches with wire mesh or rebar, with 6 inches recommended if you plan to install a vehicle lift. Commercial parking areas and loading zones typically require 6–8 inches with heavy reinforcement. Foundation footings are typically 8–12 inches deep, though exact dimensions must be engineered based on soil bearing capacity and structural loads. Always check local building codes, as many jurisdictions have specific minimum thickness requirements.

Industry best practice is to order 5–10% extra concrete beyond your calculated volume to account for waste, spillage, subgrade irregularities, and minor formwork inaccuracies. For simple rectangular slabs on level, well-compacted ground, 5% extra is usually sufficient. For projects with complex shapes, multiple corners, sloping subgrade, or stepped elevations, increase the waste factor to 10% or even 15%. Additionally, if using a concrete pump, add 0.25–0.5 cubic yards to prime the pump line. It is always better to have slightly too much concrete than to run short during a pour — cold joints from interrupted pours are structural weaknesses that are difficult and expensive to repair.

Bagged concrete is best for small projects under 0.5 cubic yards — fence post footings, small patches, mailbox bases, and minor repairs. It is readily available at hardware stores with no delivery scheduling, and you can mix only what you need at your own pace. Ready-mix truck delivery becomes more practical and cost-effective for projects above 1–2 cubic yards. At 2 cubic yards, you would need to mix approximately 90 bags of 80 lb concrete by hand — an exhausting task that takes hours and produces inconsistent results compared to plant-batched concrete. For projects between 0.5 and 2 cubic yards, consider a volumetric mixing trailer service if available in your area, as these trucks mix concrete on-site to the exact quantity needed without short-load penalties.

A cubic yard of standard concrete weighs approximately 3,900 to 4,100 pounds (1,769 to 1,860 kg), with the typical specification value being 4,050 pounds (1,837 kg) per cubic yard. This weight includes the combined mass of portland cement, aggregate (gravel and sand), and water in a standard ready-mix design. Lightweight concrete made with expanded shale or clay aggregate weighs approximately 2,400–3,000 lbs per cubic yard. Heavyweight concrete used for radiation shielding can weigh 5,000–6,000+ lbs per cubic yard. When planning your project, ensure that your subgrade, forms, and any structures receiving the concrete are designed to support this considerable weight during and after curing.

For irregular shapes, the best approach is geometric decomposition — breaking the area into standard shapes (rectangles, triangles, circles, trapezoids) and calculating each section separately, then summing the volumes. For example, an L-shaped patio can be divided into two rectangles. A curved walkway can be approximated as a series of short rectangular segments following the curve. For truly complex shapes, lay out a grid on the area and measure the thickness at each grid point to calculate an average volume. Another practical method is to use string or a measuring wheel to determine the perimeter, then measure the widest and narrowest dimensions to calculate an average area. For critical structural applications, hire a surveyor or use CAD software for precise volumetric calculations.

Standard ready-mix concrete in the United States typically costs $120 to $180 per cubic yard for a common 3,000–4,000 PSI mix, depending on your region, the supplier, and market conditions. Specialty mixes cost more: high-early-strength concrete (5,000+ PSI) ranges from $150 to $200 per cubic yard, fiber-reinforced concrete adds $5–$15 per yard, and colored or stamped-ready mixes can reach $160 to $250 per yard. Additional costs to budget for include delivery fees ($0–$50 depending on distance), short-load surcharges ($50–$100 for orders under the minimum, typically 3–5 cubic yards), Saturday or after-hours delivery premiums ($50–$150), and concrete pump rental if needed ($500–$2,000). The total installed cost of concrete — including labor, forming, finishing, and materials — typically ranges from $7 to $15 per square foot for flatwork.

Concrete curing is a gradual chemical process, not simply drying. Standard concrete reaches approximately 50% of its rated compressive strength in 3 days, 70% at 7 days, and its full design strength (typically 3,000–4,000 PSI for residential work) at 28 days under normal curing conditions of 70°F and adequate moisture. However, concrete continues to gain strength slowly for months and even years after the initial 28-day period. For practical purposes: light foot traffic is safe after 24–48 hours, furniture and light equipment after 3–7 days, vehicle traffic on driveways after 7–10 days, and full heavy loads after 28 days. Temperature significantly affects curing time — concrete cures roughly twice as fast at 90°F compared to 50°F. In cold weather (below 50°F), protect concrete from freezing for at least 48 hours and allow additional curing time before loading. Proper moist curing for the first 7 days is the single most important step for achieving maximum strength and durability.