Concrete Calculator
Enter your dimensions for an instant answer — the exact quantity to buy (waste included), a material cost, and an estimated installed cost. Change any figure to update the result live.
| ITEM | QTY | PRICE RANGE |
|---|---|---|
| cubic yards (ready-mix) | 1.9 | $237.5–$313.5 |
| Estimated material total | $237.5–$313.5 |
- 86 × 80 lb bags — $473–$688
- 115 × 60 lb bags — $517.5–$805
How much concrete do I need?
Enter your slab's length, width and thickness. The calculator returns cubic yards for a ready-mix order and the equivalent bag counts in both common sizes — an 80 lb bag yields about 0.6 cubic feet, a 60 lb bag about 0.45 — with a 7% waste allowance built in.
Ready-mix or bags? Below roughly one cubic yard, bags are usually cheaper and simpler; above it, ready-mix delivery wins on price and your back. It takes 45 eighty-pound bags to make a single cubic yard, so a 12 × 12 ft slab at 4 inches (1.9 yards with waste) would mean 86 bags — order the truck.
Round forms use cylinder math instead: volume = π × radius² × depth. Per foot of depth, an 8-inch sonotube takes about 0.35 cubic feet (0.6 bags), a 10-inch about 0.55 (0.9 bags), and a 12-inch about 0.79 (1.3 bags). For standard fence posts, plan two 50 lb bags of fast-setting mix per post. Self-leveling underlayment is its own product — a 50 lb bag typically covers 40–50 sq ft at 1/8 inch; check your bag's rating.
Formula & methodology
cubic yards = (length ft × width ft × thickness in ÷ 12) ÷ 27, + 7% waste
Slab volume converted to cubic yards with a 7% allowance for spillage, uneven subgrade and over-excavation. Bag counts divide the same volume by each bag's yield (0.6 ft³ for 80 lb, 0.45 ft³ for 60 lb) and round up — you cannot buy a fraction of a bag.
All constants are published on the material reference; the full model is on the methodology page.
12 ft × 12 ft, 4 in thick: you need 1.78 cubic yards (ready-mix) (buy 1.9 with waste), about $237.5–$313.5 in materials. See how we calculate this →
Popular sizes
Concrete cost
Ready-mix runs $125–$165 per cubic yard delivered; installed slab pricing adds forming, pouring and finishing labor by state.
concrete cost by state →Frequently asked questions
How much concrete do I need?
Multiply length × width × thickness (in feet) for cubic feet, divide by 27 for cubic yards, and add 7% waste. A 12 × 12 ft slab at 4 inches needs 1.78 cubic yards exactly — order 1.9. The calculator above does all of it, including bag counts.
How many bags of concrete do I need?
Divide your volume by the bag yield: 0.6 cubic feet per 80 lb bag or 0.45 per 60 lb bag, then round up. A 12 × 12 ft, 4-inch slab takes 86 eighty-pound bags (or 115 sixty-pound) — at that size, ready-mix is the better buy.
How many bags of concrete in a yard?
It takes 45 bags of 80 lb concrete mix (or 60 bags of 60 lb mix) to make one cubic yard, since each 80 lb bag yields about 0.6 cubic feet.
How many bags of concrete for a 10x10 slab?
A 10 × 10 ft slab at 4 inches is 1.23 cubic yards exactly — about 60 eighty-pound bags or 80 sixty-pound bags with waste included. That is right at the point where ordering ready-mix starts to beat bagging.
How much concrete per fence post?
Two 50 lb bags of fast-setting concrete per post is standard for a residential wood fence. The fence calculator counts posts and bags for your whole run automatically.
Embed this calculator
Bloggers and contractors: embed the free concrete calculator on your own site — copy the snippet below. Please keep the attribution link visible.