Concrete for a 20 x 24 ft Slab
Everything for a 20 x 24 ft slab (480 sq ft) — two-car garage floor: cubic yards at 4, 5 and 6 inches thick, bag counts, base gravel, rebar and cost.
Quick answer
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Volume at each thickness
| Thickness | Cubic feet | Cubic yards | Order (ready-mix) | 80 lb bags | 60 lb bags | Weight |
|---|---|---|---|---|---|---|
| 4 in | 160 | 5.93 | 6.75 yd³ | 294 | 392 | 13.2 tons |
| 5 in | 200 | 7.41 | 8.25 yd³ | 367 | 489 | 16.5 tons |
| 6 in | 240 | 8.89 | 10 yd³ | 440 | 587 | 19.8 tons |
volume = 20 ft x 24 ft x thickness/12 | yd³ = ft³ / 27
At 5 in thick
Concrete to order
8.25yd³
7.41 yd³ before waste
80 lb bags
367
Base gravel (4 in)
11.88tons
Compacted crushed stone
#4 rebar at 16 in
37sticks
487 lb, or 11 mesh sheets
Concrete cost
$1,361
At $165/yd³, before delivery
Pouring a 20 x 24 ft slab
At 480 square feet this pour needs 8.25 yards, which is comfortably worth a ready-mix truck; mixing this by hand would be well over a hundred bags. Excavate for 4 inches of compacted base plus the slab thickness, and set forms to a slope of about 1/8 inch per foot away from any structure.
Control joints go in at one quarter of the slab depth, spaced no more than two to three times the thickness in inches converted to feet — for a 5 inch slab that is roughly every 12.5 feet. Keep the surface moist for seven days.
Source & method: Volumes are geometry (20 x 24 x thickness), bag counts use published nominal yields, weight uses 150 lb/ft³, and the gravel figure uses 135 lb/ft³ for compacted road base. Cost uses a $165/yd³ default — adjust it on the slab cost calculator.
Supplies for this slab
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- Form stakes (24 in steel)5 in slabHold form boards to line and grade against the pressure of wet mix.Check price
- Expansion joint stripIsolate the slab from footings, walls and adjoining concrete.Check price
- Vapor barrier (6 mil poly)Required under interior slabs and worth it under any garage floor.Check price
- Concrete cure and sealHolds moisture in for the first week so the slab reaches strength.Check price
Frequently asked questions
- How much does a 20 x 24 ft concrete slab cost?
- Concrete alone is about $1,361 at $165 per cubic yard, plus roughly $120 delivery. Add base gravel and reinforcement for materials near $2,341, or $4,261 installed with labor at $4 per square foot.
- How thick should a 20 x 24 ft slab be?
- 4 inches for foot traffic — patios, shed floors and walkways. Step up to 5 or 6 inches where a vehicle will park or where you will store heavy equipment. Two-car garage floor is the typical use for this size.
- How much rebar does a 20 x 24 ft slab need?
- About 37 twenty-foot sticks of #4 bar for a 16 inch grid both ways — 729 linear feet weighing 487 lb. Welded wire mesh is the cheaper alternative at about 11 sheets.
Related calculators & charts
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- Concrete Bag CalculatorHow many 40, 50, 60 or 80 lb bags a pour needs, by published bag yield.
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- Cubic Yard CalculatorConvert any length, width and depth into cubic yards, feet and tons.
- Gravel CalculatorTons and cubic yards of gravel, crushed stone or road base for any area.