Concrete Slab Calculator
Enter the length, width and thickness of your slab and get the cubic yards to order, bag counts if you're mixing on site, gravel base, rebar and an estimated concrete cost. Built by the crew behind PourPad, who pour this stuff for a living.
Slab dimensions
Concrete to order (with waste)
5.43 yd³
Order 5.50 yards — most plants sell in quarter-yard increments.
Estimated concrete cost
$896
Ready-mix only. Short-load fees, forming, base and labor are extra.
Slab
- 400 sq ft
- 133.33 cubic feet
- 4.94 yd³ before waste
- ≈ 20,000 lb of concrete
Bags (mixing on site)
- 245 × 80 lb bags
- 326 × 60 lb bags
- 392 × 50 lb bags
Base & rebar
- 5.93 yd³ crushed stone
- ≈ 8.30 tons of base
- 560 lin ft rebar at 18" o.c.
- ≈ 28 × 20 ft sticks
Need more than yardage?
This page handles rectangles. If your pour is L-shaped, curved, a duplex approach or anything the tape measure argues with, use the free PourPad estimator: sketch the real shape on a grid, drag the corners where they belong, and get square footage, yardage and a full price — then request an on-site field verification.
How to calculate concrete for a slab
The formula is length × width × thickness. The only trick is that thickness is in inches while everything else is in feet, so divide the thickness by 12 first, then divide the total by 27 to convert cubic feet into cubic yards.
Worked example: a 20 ft × 20 ft patio at 4 inches thick is 400 sq ft × 0.333 ft = 133.3 cubic feet, or 4.94 cubic yards. Add 10% for waste and uneven subgrade and you order 5.5 yards.
Always round up. Being a quarter yard short on a pour means a cold joint, a second truck charge, or a crew standing around — all of which cost more than the extra concrete.
Typical slab thickness by job
| Job | Thickness | Base | Reinforcement |
|---|---|---|---|
| Sidewalk / walkway | 4" | 4" crushed stone | Fiber or none |
| Patio | 4" | 4" crushed stone | Wire mesh |
| Residential driveway | 4"–5" | 4"–6" | #4 rebar 18" o.c. |
| Truck / RV parking | 6" | 6" | #4 rebar 12"–16" o.c. |
| Garage slab | 4"–6" | 4"–6" | Mesh or rebar grid |
| Foundation / footing | Per drawings | Per drawings | Per engineer |
General guidance only — local code and engineered drawings always win.
Bags vs. ready-mix
An 80 lb bag yields about 0.60 cubic feet, so it takes roughly 45 bags to make one cubic yard. A 60 lb bag yields about 0.45 cubic feet — around 60 bags per yard. Below about half a yard (post footings, a small landing), bags make sense. Above it, order ready-mix: it's cheaper per yard, arrives at a consistent mix design, and you get the whole slab placed before anything starts setting up.
Common questions
How much does a concrete slab cost?
Ready-mix generally runs $140–$200 per cubic yard delivered, plus a short-load fee under about 4–5 yards. A finished slab — forming, base, reinforcement, placing, finishing and cleanup — usually lands between $6 and $12 per square foot depending on region, thickness and finish. Prices move, so confirm your yard rate before quoting.
How much gravel goes under a slab?
Four to six inches of compacted crushed stone is typical. Calculate it like concrete — square feet × depth in feet ÷ 27 — and add about 20% because the base compacts.
How much does a concrete slab weigh?
Concrete weighs roughly 150 lb per cubic foot, so a 4-inch slab is about 50 lb per square foot. A 20 × 20 patio comes in near 20,000 lb.
How much waste should I add?
Five to ten percent covers spillage, over-excavation and a subgrade that isn't as flat as it looked. On rough ground or a hand-dug base, use ten.