Concrete intimidates first-timers, but the estimate itself is simple: it's just length × width × thickness, with a couple of unit conversions and a safety margin.
Step 1: Measure your pour
Get the length and width in feet, and decide the thickness. For a slab that's usually 4 inches; for footings it's deeper. Convert the thickness to feet by dividing by 12 (4 inches = 0.33 ft).
Step 2: Calculate volume
Multiply the three numbers to get cubic feet, then divide by 27 to get cubic yards. A 10 × 10 ft slab at 4 inches is 100 × 0.33 = 33 ft³ ≈ 1.23 cubic yards. Add 10% waste and you're at about 1.36 yards. The concrete slab calculator does this instantly.
Step 3: Bags or ready-mix?
Bagged mix is convenient for small jobs but adds up fast:
- 40 lb bag ≈ 0.30 ft³
- 60 lb bag ≈ 0.45 ft³
- 80 lb bag ≈ 0.60 ft³
A cubic yard takes about 45 eighty-pound bags — that's a lot of mixing. As a rule, once you pass roughly half a cubic yard, ordering ready-mix from a truck is cheaper and far easier.
Step 4: Don't forget the base
Good concrete sits on a compacted gravel base — usually 4 inches. Estimate that separately with the gravel calculator or paver base calculator.
How thick should it be?
4 inches for patios, walkways and shed floors; 5–6 inches for driveways or anything carrying vehicle weight. When in doubt, go thicker — concrete is cheap compared to redoing a cracked slab.
The bottom line
Measure, multiply, divide by 27, add 10%. Or skip the arithmetic and use the concrete calculator — it gives you cubic yards and the bag count side by side.