Direct concrete answer
How much concrete for a 20x24 slab (4 inches thick)?
A 20 ft by 24 ft by 4 inch concrete slab requires 5.93 cubic yards of concrete, or 267 bags of 80-lb premix before field waste. With 10% overage, order about 6.6 cubic yards.
Quick answer
| Project dimensions | 20 ft x 24 ft x 4 in |
|---|---|
| Area or count | 480 sq ft |
| Volume | 160.00 cubic feet |
| Volume | 5.93 cubic yards |
| Metric volume | 4.53 cubic meters |
| 80-lb bags needed | 267 bags |
| 60-lb bags needed | 356 bags |
| 50-lb bags needed | 427 bags |
| Ready-mix order with 10% overage | 6.6 cubic yards |
| Ready-mix planning cost at $150/yd3 | $978 |
| Bagged planning cost at $5/80-lb bag | $1,335 |
Calculation steps
- Convert depth to feet: 4 inches / 12 = 0.333 ft.
- Calculate cubic feet: 20 ft x 24 ft x 0.333 ft = 160.00 ft3.
- Convert to cubic yards: 160.00 ft3 / 27 = 5.93 yd3.
- Add waste for ordering: 5.93 yd3 x 1.10 = 6.6 yd3. Round according to the supplier's ordering increment.
This is the exact math behind the answer, but the field order should reflect the actual formed dimensions, the base condition, and the supplier's minimum order policy. Measure the finished inside form dimensions. For this 20x24 slab, 4 inches thick, even a half-inch depth change across 480 sq ft changes the order.
Number of bags by size
Bag counts use common premix yields and round up to whole bags. Always check the yield printed on the exact product before buying because specialty mixes can differ.
| Bag size | Common yield | Calculation | Bags to buy |
|---|---|---|---|
| 80-lb bag | 0.6 ft3 | 160.00 / 0.6 | 267 |
| 60-lb bag | 0.45 ft3 | 160.00 / 0.45 | 356 |
| 50-lb bag | 0.375 ft3 | 160.00 / 0.375 | 427 |
Cost snapshot
At a planning price of $150 per cubic yard, the waste-adjusted ready-mix concrete is about $978 before short-load, delivery, pump, labor, forms, base stone, reinforcement, tax, or finish upgrades. The 80-lb bag material estimate is about $1,335 before mixer rental or labor.
Cost is where small concrete jobs surprise people. A short-load fee can make the delivered price look high, while bagged concrete can look cheap until the crew has to mix 267 heavy bags fast enough to place and finish one continuous surface. Use the material number as a quote starting point, not as the final installed price.
Ready-mix vs bagged concrete
Order ready-mix if schedule, access, and short-load fees make sense; use bags only when a truck cannot reach the pour or the crew can mix fast enough.
As a rule of thumb, ready-mix becomes easier once a project is near or above one cubic yard. This page's base volume is 5.93 cubic yards, so the practical choice depends on access, crew size, weather, finish timing, and whether the supplier charges a short-load fee. If the pour must be continuous, the truck often reduces risk even when the invoice includes a delivery charge.
Recommended PSI and reinforcement
| Use | Typical planning PSI | Notes |
|---|---|---|
| Patio or walkway | 3,000 PSI | Light foot traffic on compacted base. |
| Garage floor | 3,500 PSI | Verify vapor barrier, joints, and reinforcement. |
| Driveway | 3,500 to 4,000 PSI | Use higher strength for trucks, RVs, or weak soil. |
| Footings | 3,000 to 4,000 PSI | Plan and local code control final mix. |
Use 3,500 to 4,000 PSI for typical residential vehicle traffic, higher if the design or supplier recommends it.
A 4 inch slab for light residential use may use welded wire mesh or fiber, but vehicle loads and poor soil can require stronger reinforcement.
Project-specific notes
This answer is for a garage-size slab, work pad, or broad patio. The geometry is straightforward, but the site details decide whether the estimate is clean in the field. Before ordering, compare the calculated dimensions against the actual form layout, the base depth, and the planned finish elevation.
- The 20 by 24 footprint is close to a common two-car garage size, so reinforcement and vapor barrier decisions may apply.
- If this is a garage floor, check whether the design calls for thickened edges or a turndown perimeter.
- If the project includes thickened edges, a landing, a flare, a step, or a separate footing, calculate that concrete separately and add it to the base result.
Common mistakes
- Using drawing dimensions after forms have moved. Recheck inside length and width before ordering.
- Forgetting that thickness is converted to feet before multiplying. Four inches is 0.333 ft and six inches is 0.5 ft.
- Ordering the exact mathematical volume with no allowance for low spots, spillage, or surface finishing.
The expensive mistake is running short during placement. The second most expensive mistake is ordering more concrete than the site can place before it starts setting. The best order is not the smallest number; it is the number that fits the measured work, the crew, the truck access, and the supplier's rounding policy.
What to say when ordering
I am estimating a 20 ft by 24 ft by 4 inch concrete slab. My calculated volume is 5.93 cubic yards, and I want to plan around 6.6 cubic yards with 10% overage. Can you confirm the order size, mix strength, short-load fee, delivery charge, chute reach, and whether this job needs a pump or different placement method?
Related calculations
Sources and methodology
BuilderCalc uses standard geometric volume formulas, the 27 cubic feet per cubic yard conversion, common premix bag yields, and planning cost ranges that should be verified with local suppliers.