CONCRETE

Measure the length and width of the area you're pouring, in feet. For an irregular shape, switch the Shape toggle to "L-shape" and enter the cutout dimensions — the calculator subtracts the missing rectangle for you. For shapes that aren't rectangles or L-shapes, break the area into rectangles, run each one through the calculator, and add the results.

Pick the slab thickness in inches: 4 inches is standard for patios, walkways, and shed pads; 6 inches for driveways and garages; 8–12 inches for footings (check your local code). The result returns cubic yards (already padded with 10% waste) plus the bag count for both 60lb and 80lb sizes. If you're ordering ready-mix, give the plant the cubic-yard number; if you're mixing yourself, the bag count is what to load into your cart.

Say you're pouring a 12 ft × 12 ft patio at 4 inches thick.

The volume is 12 × 12 × (4 ÷ 12) = 48 cubic feet, which is 48 ÷ 27 = 1.78 cubic yards. With the built-in 10% waste factor, the calculator shows 1.96 cubic yards.

In bags: 48 ÷ 0.45 = 107 bags of 60lb, or 48 ÷ 0.60 = 80 bags of 80lb. At roughly $4 per 60lb bag from Home Depot, that's about $428 in materials — versus a typical short-load ready-mix delivery at around $200–300 per yard plus a $100–150 short-load fee, putting a 2-yard delivery in the $500–700 range.

For this 1.78-yard pour, bags are slightly cheaper, but ready-mix saves several hours of mixing and gives a more uniform finish. Once you cross 2 yards, ready-mix wins on both price and time.

Forgetting the waste factor. Concrete shrinks slightly as it cures and a surprising amount sticks to the mixer, wheelbarrow, and chute. Order at least 10% extra — this calculator already adds that, but if you're double-checking with napkin math, don't forget to pad.

Confusing slab thickness units. Length and width are in feet; thickness is in inches. Entering "0.33" for a 4-inch slab (instead of 4) is a common slip that throws results off by a factor of 12.

Under-ordering for footings. Trench footings rarely match nominal dimensions — the soil collapses inward, the trench widens, the bottom is uneven. Order 15–20% extra for any below-grade pour, not the standard 10%.

Mixing partial bags. A 60lb bag is calibrated for a specific water ratio. Splitting one bag in half almost always gives you a weaker mix than full-bag pours. Round bag count up, don't fudge it down.

0.5 cubic yards is the bags/ready-mix breakeven. Below that, bags are more convenient and roughly the same cost. Above that, ready-mix is cheaper, faster, and more uniform.

4 inches = standard residential slab. Patios, walkways, shed pads, AC condenser pads — 4 inches with wire mesh or fiber reinforcement is the default.

6 inches + rebar = driveways and garages. Anywhere a vehicle parks needs 6 inches of concrete and a #3 or #4 rebar grid on 12–18 inch centers.

One cubic yard ≈ 45 bags of 80lb or 60 bags of 60lb. Useful for sanity-checking large pours against bag counts.

One cubic yard covers 81 ft² at 4 inches deep, or 54 ft² at 6 inches. Quick mental math: divide your slab area in ft² by 81 for a 4" pour, or by 54 for a 6" pour, to get cubic yards.