Concrete Calculator

How to measure for concrete

Concrete math is simple — it's the measuring that trips people up. Measure length and width in feet, right at the inside of your forms. For thickness, measure in inches and convert: 4 inches is 0.33 feet, 6 inches is 0.5 feet. The core formula never changes: length × width × thickness (in feet), divided by 27, gives you cubic yards.

For footings and other trenches, measure the trench after digging, not before. Over-digging is normal, and it's the number-one reason footing pours come up short. If the trench walls are irregular, take several width measurements and use the largest.

Odd shapes? Break them into rectangles, calculate each, and add them together. For circles (like a round patio or a sauna tube), use 3.14 × radius × radius × thickness, then divide by 27.

Ready-mix vs. bags: the real decision

This is the question that matters more than the exact yardage. Bagged concrete makes sense up to about half a cubic yard — roughly 20 to 25 eighty-pound bags. Beyond that, the labor of mixing, the inconsistency between batches, and the sheer physical toll make ready-mix the obvious choice.

Ready-mix suppliers typically have a 1-yard minimum and charge short-load fees for orders under 4–5 yards, so small pours carry a premium either way. If your pour lands between half a yard and two yards, get quotes both ways: sometimes two yards of ready-mix with a short-load fee still beats 90 bags and a weekend of mixing.

One more consideration: time. A truck pours a driveway in under an hour. The same driveway in bags is a two-day project for most DIYers, and every batch is a cold joint risk if you fall behind.

Material and prep tips

• Base first. 4 inches of compacted gravel under slabs prevents settling and cracking. Skipping the base to save $100 is the most expensive mistake in concrete work.
• Thickness by use: 4" for sidewalks, patios, and shed floors; 5–6" for driveways and garage floors; 8–12" for footings per local code.
• Reinforcement: wire mesh or rebar doesn't add strength the way people think — it holds cracks tight. Use it in anything that carries a load or spans more than a few feet.
• Weather matters. Don't pour below 40°F without cold-weather measures, or above 90°F without planning for rapid set. Rain within 4–6 hours of finishing ruins surfaces.
• Order a touch heavy. Running short mid-pour is a disaster; having half a yard left over is a minor cost. The 10% default waste factor exists for this reason.

What affects concrete cost

Material is only part of the bill. Ready-mix pricing runs roughly $125–$175 per yard in most markets, but short-load fees ($50–$150), Saturday delivery surcharges, and pump fees (if the truck can't reach the forms) add up fast. Bagged concrete runs $4–$7 per 80-lb bag, which works out to $180–$315 per yard — expensive per unit, but no delivery minimums.

Labor is the bigger variable: a simple flat slab might run $6–$10 per square foot installed, while stamped or exposed-aggregate finishes push $12–$18. Excavation, base gravel, forming, and finishing are where contractor quotes diverge — always compare itemized quotes, not just the bottom line.