20x24 Concrete Slab: Cost, Yards & Bags of Concrete
Quick answer: a 20 × 24 ft slab (480 sq ft) 5 inches thick needs 7.41 yd³ of concrete, 8.15 yd³ with 10% waste (order 8.25 yd³ of ready-mix) or 367 bags of 80-lb mix. Installed, it costs about $3,580 – $6,330 ($7.00 – $13.50 per sq ft); DIY materials run $1,540 – $2,300.
A 20 × 24 ft slab (480 sq ft) is a compact two-car garage: two cars fit, with little room for a workbench. Garage floors are usually poured 5 in thick with #4 rebar, which takes 8.15 yd³ with waste.
Concrete for a 20x24 Slab at 4, 5 and 6 Inches
| Thickness | Net volume | With 10% waste | Ready-mix order | 80-lb bags | 60-lb bags | 40-lb bags | Installed |
|---|---|---|---|---|---|---|---|
| 4 in | 5.93 yd³ | 6.52 yd³ | 6.75 yd³ | 294 | 392 | 587 | $3,330–$6,020 |
| 5 in (typical) | 7.41 yd³ | 8.15 yd³ | 8.25 yd³ | 367 | 489 | 734 | $3,580–$6,330 |
| 6 in | 8.89 yd³ | 9.78 yd³ | 10 yd³ | 440 | 587 | 880 | $3,820–$6,640 |
How Much Extra to Order
The table above adds 10%, the usual allowance for a formed slab on a graded base. Pick less only for rigid forms on a laser-flat base, more for soft or uneven ground:
| Waste allowance (5 in slab) | Concrete | 80-lb bags |
|---|---|---|
| 0% (exact volume) | 7.41 yd³ | 334 |
| 5% | 7.78 yd³ | 350 |
| 10% | 8.15 yd³ | 367 |
| 15% | 8.52 yd³ | 384 |
Bags or a Ready-Mix Truck?
| Option | What you buy | Concrete bill |
|---|---|---|
| 80-lb bags | 367 bags at $6.50 + mixer rental $75 | $2,461 |
| Ready-mix | 8.25 yd³ at $150 + $120 delivery | $1,358 |
Ready-mix, without question. 367 bags would cost $2,461 and take about 15 hours of mixing, far longer than concrete stays workable, so the slab would end up with cold joints. Ready-mix trucks bring it as fast as the crew can place it (2 trucks of about 8–10 yd³ each). Try your own prices in the bags vs. ready-mix calculator.
What a 20x24 Slab Costs
| Line item (5 in slab) | Basis | Cost |
|---|---|---|
| Ready-mix concrete + delivery | 8.15 yd³ at $150 + $120 | $1,342 |
| Forms | 88 lin ft at $2.20 | $194 |
| Vapor barrier | 480 sq ft at $0.35 | $168 |
| #4 rebar at 18 in | 480 sq ft at $1.25 | $600 |
| Broom finish | 480 sq ft at $0.75 | $360 |
| Labor (place & finish) | 480 sq ft at $5.00 | $2,400 |
| Direct cost | Before overhead and profit | $5,064 |
- Installed price: $3,580 – $6,330. The low end is a plain slab with lower-cost labor; the high end is the full scope above plus 25% contractor overhead and profit.
- Site prep and gravel base (excavation and 4 in of compacted stone): about $670 – $1,580 more.
- Thickened edge (12 in wide, 12–18 in deep) if the building sits on the slab: 2.0–3.7 yd³ more concrete, about $300 – $560, plus trenching and two #4 bars around the edge.
- Permit, where one is needed: about $50 – $350.
Have a contractor's price? Put it into the concrete quote checker, or compare two or three bids side by side there.
Materials List for a 20 × 24 ft Slab
| Concrete | 8.25 yd³ ready-mix, or 367 × 80-lb / 734 × 40-lb bags |
|---|---|
| Gravel base (4 in compacted) | 6.81 yd³, about 9.5 tons |
| Forms | 8 × 2x6 boards 12 ft long, 34 stakes, 68 screws (88 lin ft) |
| Rebar (#4 at 18 in) | 705 lin ft = 36 × 20-ft sticks, 63 chairs |
| Or wire mesh (6x6) | 15 sheets of 5 × 10 ft, about $180 – $300 |
| Vapor barrier | 480 sq ft plus 6 in laps |
| Control joints | 4 panels of about 12 × 10 ft: 44 lin ft of saw cut, 1.25 in deep |
Recommended Specs for a Garage Floor
| Thickness | 4 in minimum, 5 in typical; 6 in for heavy trucks or a lift |
|---|---|
| Strength | 3,500 PSI air-entrained (IRC minimum for garage floors in severe weathering areas) |
| Base | 4 to 6 in of compacted crushed stone, plus a 10-mil vapor barrier under heated or finished garages |
| Reinforcement | #4 rebar at 18 in on chairs |
| Edge | Monolithic thickened edge (12 in wide, 12-18 in deep) or a stem wall on footings, per your permit |
- Pour 3,500 PSI air-entrained concrete; it is the code minimum for garage floors in freeze-thaw areas.
- Slope the floor toward the overhead door, about 1/8 in per foot, so melting snow drains out.
- If the garage will be built on the slab, it needs a thickened edge or stem wall; the cost section shows that add-on.
Pour Day for a 20x24 Slab
- Crew time: about 17 crew-hours to set forms, place, finish and cut joints (0.035 h per sq ft for a broom finish).
- Trucks: 8.25 yd³ is 2 trucks, about 41–66 minutes of unloading in total. Concrete must be placed within 90 minutes of batching.
- Reach: a truck chute reaches about 15–20 ft, enough for this slab if the truck can park beside it. If it can't, budget wheelbarrows or a line pump.
- Joints: saw-cut 4–12 hours after finishing with a wet saw, or 1–4 hours with an early-entry saw.
- Curing: foot traffic after 24–48 hours, cars after 7 days, full strength at 28 days.
Price Your Own 20x24 Slab
The calculator below starts at 20 × 24 ft, 5 inches. Change the thickness, finish or reinforcement, pick your state for local prices, and the Live Estimate updates with a blueprint of the joint and rebar layout.
Concrete Slab Cost & Volume Estimator
Precision multi-layer project budgeting: Materials, Sub-Base, Reinforcement, Labor & Equipment
Picking a location sets the ready-mix, excavation and labor rates below to that area's typical level. You can still type your own local prices afterwards. Slab prices by state.
Closer spacing uses more steel and raises the rebar cost.
Not charged when bagged concrete is selected.
A scaled plan of your slab: rebar grid at the spacing you chose, support chairs about every 3 to 4 ft, and the recommended control-joint saw cuts (2.5 times the thickness in inches, in feet, capped at 15 ft). It redraws as you change the inputs.
| Scope Item / Material Description | Amount |
|---|
Enter the total your contractor quoted. We compare it with a benchmark for the same slab and scope you set in the calculator: materials at your entered prices plus labor at the regional standard rate, before any overhead or profit. Make sure the calculator scope (base, rebar, finish, pump, demolition) matches what the bid includes.
Optional: enter the bid's line items to see where the markup sits
Enter the forecast for pour day. The check follows ACI 306R (cold weather) and ACI 305R (hot weather) and estimates the surface evaporation rate that drives plastic shrinkage cracking.
20x24 Slab Questions
About $3,580 to $6,330 installed for a 5-inch slab with forms, a broom finish and labor ($7.00 to $13.50 per sq ft), at U.S. average prices. DIY materials run about $1,540 to $2,300. Site prep and a gravel base add about $670 to $1,580.
At 4 inches thick with 10% waste: 294 bags of 80-lb mix, 392 bags of 60-lb mix or 587 bags of 40-lb mix. At 5 inches: 367 bags of 80-lb mix.
At 4 inches, 5.93 cubic yards (order 6.75 with waste); at 5 inches, 7.41 (order 8.25); at 6 inches, 8.89 (order 10.00). Ready-mix is sold in quarter yards, so round up.
Ready-mix. 8.25 yards delivered costs about $1,358, while 367 bags would cost about $2,461 and take roughly 15 hours of mixing. Bags only win below about 1.3 cubic yards.
A #4 grid at 18 inches each way needs about 705 linear feet of bar including laps: 36 twenty-foot sticks and about 63 chairs. Welded wire mesh instead takes 15 sheets of 5 x 10 ft.
It fits two cars, but tightly. 24 × 24 ft is the more common two-car size because it leaves room to open doors and walk around; see our 24x24 garage slab guide.