How Much Concrete Do I Need

Quick answer

How Much Concrete Do I Need: multiply length × width × thickness in feet, divide by 27 for cubic yards, add 5–10% waste, then round a truck up to the next quarter yard. A 10×10 slab at 4 inches is 1.23 yd³ before waste — about 1.50 ordered from most plants after waste and rounding.

How Much Concrete Do I Need: Measure the shape you are pouring

Rectangles cover most residential work. Split L-shapes into two pads. Thickness is the side people leave in inches — 4 inches is 0.333 feet, not 4 feet. Measure inside the forms when they are set; measuring the gravel pile understates the pour if the stone is crowned.

Cylinders (sonotubes, post holes) need diameter and height. Stairs need width, tread, riser, and step count. Curb needs a cross section. If the shape is odd, sketch it, split it, and add the pieces in Concrete Calculator’s job list.

Common mistakes with a how much concrete do i need

Cubic feet ÷ 27 = cubic yards. Plants batch yards. Bags are just that volume divided by yield. The classic mistake is multiplying length in feet × width in feet × thickness in inches. That number is not cubic feet. Convert thickness first, or leave each field on its own unit in the concrete calculator.

• 10×10×4 in → 33.3 ft³ → 1.23 yd³
• 12×16×4 in patio → 64 ft³ → 2.37 yd³
• 20×40×6 in driveway → 400 ft³ → 14.8 yd³

Add extra, then decide bags vs truck

Waste covers spillage, chute residue, and uneven bases. Under ~0.75 yd³, bags usually win. Past a yard or two, price the truck including short-load fees. Put both prices on the same pour in the cost calculator.

Never round every small pier to a quarter yard on its own and then add. Sum neat volumes, add waste once, round once. Read ordering extra concrete for where the “missing” concrete goes on pour day.

Coverage shortcuts when you know area

One yard covers about 81 ft² at 4 inches and 54 ft² at 6 inches. Area ÷ coverage ≈ yards before waste. Confirm with full dimensions when the shape is not a simple rectangle.

Do it in Concrete Calculator

Open the homepage calculator, pick the shape, leave units on each field, set waste, and read yards and bags together. Dedicated pages for slabs, bags, and yards match how Google sent you — same math, clearer intent.

Checklist before you order

Forms measured · thickness from the plan · waste percent chosen · all pours added · truck rounded once · bags compared if under ~2 yd³ · short-load fee asked · washout and access confirmed. That checklist beats a second calculator with the same thin paragraph about “length times width.”