How to measure for your concrete project
- Measure length and width. Measure the slab in feet. For an L-shape or other irregular area, split it into rectangles, figure each one, and add them together.
- Choose a thickness. Enter the thickness in inches. Four inches is standard for patios, walkways, and shed floors; use 5–6 inches for driveways, garage floors, or anything that carries a vehicle.
- Read the result and add waste. You’ll get cubic yards plus bag counts for 40, 60, and 80 lb pre-mix. Round up and add about 10% for spillage and an uneven subgrade.
How the concrete calculator works
Concrete is sold by volume, so the math is length × width × depth converted to cubic yards. Thickness is in inches, so divide it by 12 to get feet, multiply by the length and width for cubic feet, then divide by 27 (a cubic yard is 27 cubic feet). For a 10 × 10 slab at 4 inches: 10 × 10 × (4 ÷ 12) = 33.3 cubic feet, and 33.3 ÷ 27 = 1.23 cubic yards. To convert to bags, divide the cubic feet by each bag’s yield — an 80 lb bag makes about 0.6 cu ft, a 60 lb bag about 0.45, and a 40 lb bag about 0.30 — so 33.3 ÷ 0.6 ≈ 56 eighty-pound bags.
Which type are you estimating?
Slab, patio, or shed floor
A flat rectangular pour — the calculator’s default. Use your length and width with a 4-inch thickness (5–6 inches for a driveway).
Enter: length × width, 4 in thick
Footing or strip foundation
A long, narrow trench pour under a wall, deck, or block. Enter the trench length as the length, the footing width as the width, and the depth as the thickness.
Enter: trench length × footing width × depth
Round column or sonotube pier
A round pour fills less than the square estimate, so trim the result by about a fifth — or measure it as π × radius² × height. A 12-inch tube 4 ft deep holds roughly 3.1 cu ft.
Enter: estimate, then reduce ~20%
Fence-post footings
Setting posts in concrete subtracts the post from the hole. Use the dedicated Fence Post Concrete calculator for an accurate per-post bag count.
Enter: use the fence-post calculator
Steps or stairs
Treat each step as its own box (rise × run × width), calculate each, and add them up for the total volume.
Enter: sum each step box
Tips & ways to save
- Order about 10% extra — spillage, over-excavation, and an uneven subgrade always eat into a pour.
- Above ~1 cubic yard (about 56 eighty-pound bags), ready-mix delivery is usually cheaper per yard and far less labor than mixing by hand.
- Bag yields differ — check the cubic-foot yield printed on the bag (≈0.6 for 80 lb, 0.45 for 60 lb, 0.30 for 40 lb) before you divide.
- For driveways or anything holding a vehicle, pour at least 5–6 inches thick and add rebar or wire mesh.
- Concrete is heavy: one cubic yard weighs about 4,000 lb. Plan delivery access, or several trips if you’re hauling bags.
Concrete bags by slab size (4 inches thick)
| Slab size | Concrete | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 10 × 10 ft | 1.23 cu yd | 56 | 75 |
| 10 × 12 ft | 1.48 cu yd | 67 | 89 |
| 12 × 12 ft | 1.78 cu yd | 80 | 107 |
| 12 × 16 ft | 2.37 cu yd | 107 | 143 |
| 20 × 20 ft | 4.94 cu yd | 223 | 297 |
At 4 inches thick; a 6-inch slab needs 1.5× as much. Order about 10% extra for spillage and uneven subgrade.
Frequently asked questions
How many bags of concrete are in a cubic yard?
How many bags of concrete do I need for a 10×10 slab?
Is it cheaper to use bagged or ready-mix concrete?
How thick should a concrete slab be?
How much does a cubic yard of concrete cover?
How long does concrete take to cure?
Sources
Related calculators
Reviewed by the BackyardCalc editorial team. Figures are computed from the formula above and checked against manufacturer yields.