Concrete Slab Calculator — Yards, Bags & Rebar
Get cubic yards, bag counts, rebar and base gravel for any slab, plus a delivered concrete budget range.
Your numbers
Results
- Slab area
- 240 sq ft
- Concrete to orderraw volume + 10% waste
- 3.26 cu yd
- Ready-mix costplus short-load and pump fees
- $538 – $672
- Bagged alternativeonly practical under ~1 cu yd
- 134 bags (80 lb)
- Rebar grid @ 18"9 + 15 bars
- 360 lin ft
- Gravel base4" compacted
- 2.96 cu yd
- Control joints2.5 × slab thickness in feet
- every 10 ft
Quantities are estimates for planning. Verify spans, spacing and local amendments before ordering.
How the math works
- Volume (cu yd) = length × width × (thickness ÷ 12) ÷ 27. Always order about 10% extra for spillage and subgrade variation.
- One 80 lb bag yields about 0.60 cu ft, so bagged mix is only realistic for small pours.
- Rebar grid length = bars in each direction × their run length; add 6" lap at splices.
Code & best practice
- IRC R506 requires a minimum 3.5" slab on a 4" compacted gravel base with a vapor retarder under habitable space.
- Driveways typically use 4" thickness for cars and 5–6" where trucks park.
- Cut control joints within 6–12 hours of the pour, at a depth of one quarter of the slab thickness.
Frequently asked questions
How many yards of concrete for a 20x12 slab?
A 4" thick 20x12 slab is about 2.96 cu yd raw, so order roughly 3.25 cu yd.
How many bags of concrete in a cubic yard?
About 45 eighty-pound bags per cubic yard.
How thick should a concrete driveway be?
4 inches for passenger vehicles, 5–6 inches with rebar where heavy trucks will park.