How do I measure for a concrete slab?

Measure the length and width of your slab area in feet. Decide on thickness (typically 4 inches for patios/sidewalks, 6 inches for driveways). Multiply length × width × thickness (in feet) and divide by 27 to get cubic yards. Add 10% for waste.

Is it cheaper to use bags or ready-mix concrete?

For projects under about 1 cubic yard (27 cubic feet), bagged concrete is usually more economical since ready-mix trucks have minimum delivery charges. For projects over 1–2 cubic yards, ready-mix concrete is generally cheaper when you factor in the labor saved from not mixing bags by hand.

How much does concrete cost per yard?

Ready-mix concrete typically costs $125–$175 per cubic yard depending on your location, the PSI specification, and any additives like fiber reinforcement or accelerants. Short-load fees for small orders can add $15–$25 per yard below the minimum delivery amount.

How long does concrete take to set?

Concrete reaches initial set (firm enough to walk on carefully) in 24–48 hours under normal conditions. It reaches about 70% of its design strength at 7 days and full strength at 28 days. Avoid heavy loads for at least 7 days, and keep the surface moist during curing for best results.

Can I mix concrete by hand?

Yes, but it's physically demanding. Hand-mixing works well for projects under half a cubic yard. Use a large mixing tub or wheelbarrow, add water gradually, and mix until a uniform consistency is achieved with no dry pockets. For more than a few bags, renting a portable concrete mixer is worth the cost.

How Much Concrete Do You Need? Complete Guide (2026)

The Basic Concrete Formula

Calculating concrete is straightforward once you understand the core formula. Concrete volume is measured in cubic yards in the United States. To find cubic yards, measure your project's length, width, and thickness in feet, multiply them together, and divide by 27 (since there are 27 cubic feet in one cubic yard).

Formula: (Length × Width × Thickness in feet) ÷ 27 = Cubic Yards

For example, a 10×12 foot slab that is 4 inches (0.33 feet) thick requires: 10 × 12 × 0.33 ÷ 27 = 1.47 cubic yards.

How to Measure Different Concrete Projects

Concrete Slabs (Patios, Driveways, Floors)

Slabs are the simplest to calculate. Measure the length and width of the area, then decide on thickness. Residential driveways typically use 4 inches of thickness; structural slabs may need 6 inches. Always measure in feet and convert inches to a decimal (4 inches = 0.333 feet, 6 inches = 0.5 feet).

Concrete Footings

Footings are rectangular trenches poured around the perimeter of a foundation. Measure the total linear feet of footing, then multiply by the footing width and depth. A 12-inch wide by 8-inch deep footing running 40 linear feet requires: 40 × 1.0 × 0.667 ÷ 27 = 0.99 cubic yards.

Round Columns and Piers

For cylindrical shapes, use the formula: π × radius² × height ÷ 27. A 12-inch diameter column (6-inch radius = 0.5 feet) that is 4 feet tall needs: 3.14159 × 0.25 × 4 ÷ 27 = 0.116 cubic yards. Always calculate each column individually and multiply by the number of columns.

Bag Sizes and Coverage

Pre-mixed concrete bags are available in three common sizes. Knowing the yield of each bag helps you plan for smaller projects where ready-mix delivery isn't cost-effective:

40 lb bag — yields approximately 0.30 cubic feet
60 lb bag — yields approximately 0.45 cubic feet
80 lb bag — yields approximately 0.60 cubic feet

To convert cubic yards to bags: multiply cubic yards by 27 (to get cubic feet), then divide by the bag yield. A 1-yard pour would require about 45 bags of 80 lb mix, making bags impractical for large jobs.

The 10% Waste Factor

Always add 10% to your calculated volume to account for spillage, uneven subgrade, and slight miscalculations. For a 2-yard pour, order 2.2 yards. For a 10-yard pour, order 11 yards. Running short of concrete mid-pour creates cold joints that weaken the structure and are difficult to fix.

Ready-Mix vs. Bagged Concrete

The break-even point is roughly 1 cubic yard. Below that, bagged concrete from a home center is typically more economical. Above 1 yard, ready-mix delivery becomes competitive, and above 3–4 yards it is almost always cheaper and saves significant labor time.

Bagged concrete: Best for repairs, fence posts, small slabs under 1 yard. No minimum order, mix on-site. Labor-intensive for large pours.
Ready-mix (truck delivery): Most economical for projects over 1 yard. Minimum delivery charges typically apply (often 1 cubic yard minimum). Consistent mix quality.

Concrete Costs

Ready-mix concrete typically costs $125–$175 per cubic yard depending on region, PSI strength, and additives. Short-load fees apply for orders under 5 yards (typically $15–$25 per yard under the minimum). Bagged 80 lb concrete runs about $5–7 per bag at retail, making it roughly $200–250 per cubic yard in material costs alone — before labor.

Concrete Curing and Setting Time

Concrete reaches initial set in 2–4 hours under normal conditions (70°F). It achieves roughly 70% of its design strength at 7 days, and full design strength (typically 2,500–4,000 PSI for residential) at 28 days. Keep concrete moist for at least 7 days by misting or covering with plastic sheeting to ensure proper curing.