How Much Concrete Do I Need for a 24×24 Slab?
A 24’x24’ slab at 4 inches thick needs 7.11 cubic yards of concrete. At 6 inches thick it needs 10.67 cubic yards. Always add 10% for waste, so order 7.8 yards for a 4-inch pour and 11.7 yards for a 6-inch pour. One cubic yard of ready-mix covers about 81 square feet at 4 inches thick.
The question sounds simple, but the answer has a few moving parts. The volume of concrete you need depends on three things: the length, the width, and the thickness of the slab. Change any one of those and the order changes. And ordering the wrong amount is one of the most expensive mistakes you can make on a concrete job, because coming up short mid-pour means scrambling for an emergency delivery at premium rates, and having too much means you are paying for mud that goes back on the truck.
This guide walks through the math step by step, shows you the cubic yards for every common slab size, explains when to use bags instead of a ready-mix truck, and makes sure you never over-order or under-order again.
Enter your dimensions and thickness. The calculator handles the math, adds the waste factor, and shows you the bag count or ready-mix order in seconds.
Open the Concrete Slab Calculator →The Concrete Volume Formula: How It Works
Concrete volume is calculated in cubic feet first, then converted to cubic yards because that is how ready-mix suppliers sell it. Here is the formula:
Cubic feet = Length (ft) x Width (ft) x Thickness (ft)
Cubic yards = Cubic feet / 27
The key step most people miss is converting thickness from inches to feet. A 4-inch slab is 0.333 feet thick. A 5-inch slab is 0.417 feet. A 6-inch slab is 0.5 feet. Forget this conversion and your order will be significantly off.
One cubic yard equals 27 cubic feet, which is why you divide by 27. That is the standard unit that concrete plants, suppliers, and ready-mix trucks across the United States price by. When a driver hands you a ticket, the volume on it is in cubic yards.
What one cubic yard actually covers
At 4 inches thick, one cubic yard of concrete covers approximately 81 square feet. That is a patch roughly 9 feet by 9 feet. At 6 inches thick, the same yard covers only about 54 square feet. Understanding this relationship is what lets you sanity-check any estimate before you commit to an order.
The 24×24 Slab: Full Math, Both Thicknesses
A 24-foot by 24-foot slab is one of the most common pours in the country. It fits a standard two-car garage and is a frequent backyard workshop size. Here is the complete calculation at both standard thicknesses.
At 4 inches thick (patios, basic shed floors)
24 x 24 = 576 square feet
576 x 0.333 (4 inches in feet) = 191.9 cubic feet
191.9 / 27 = 7.11 cubic yards
Add 10% waste: order 7.8 cubic yards (round up to 8 yards)
At 6 inches thick (garage floors, driveways, foundations)
24 x 24 = 576 square feet
576 x 0.5 (6 inches in feet) = 288 cubic feet
288 / 27 = 10.67 cubic yards
Add 10% waste: order 11.7 cubic yards (round up to 12 yards)
Uneven subgrade, spillage at the chute, and overfilling forms all eat into your order. The industry standard is to add 5% to 10% on top of your calculated volume. For large pours over 10 yards, 5% is usually enough. For smaller pours with complex forms, use 10%. Running short mid-pour is the scenario you are paying the waste factor to avoid.
The calculator works for any slab size and any thickness. It also shows the bag count if you want to compare DIY bagged concrete against a ready-mix order.
Calculate My Slab →Cubic Yards by Common Slab Size
Here are the pre-calculated cubic yard figures for the most searched residential slab sizes in 2026. Both 4-inch and 6-inch thicknesses are shown, with the waste-adjusted order quantity in parentheses.
| Slab size | Sq ft | 4-inch (order qty) | 6-inch (order qty) |
|---|---|---|---|
| 10’ x 10’ | 100 | 1.23 yds (1.4) | 1.85 yds (2.1) |
| 12’ x 12’ | 144 | 1.78 yds (2.0) | 2.67 yds (3.0) |
| 12’ x 20’ | 240 | 2.96 yds (3.3) | 4.44 yds (4.9) |
| 16’ x 16’ | 256 | 3.16 yds (3.5) | 4.74 yds (5.2) |
| 20’ x 20’ | 400 | 4.94 yds (5.5) | 7.41 yds (8.2) |
| 24’ x 24’ | 576 | 7.11 yds (7.8) | 10.67 yds (11.7) |
| 24’ x 30’ | 720 | 8.89 yds (9.8) | 13.33 yds (14.7) |
| 30’ x 30’ | 900 | 11.11 yds (12.3) | 16.67 yds (18.4) |
| 40’ x 40’ | 1,600 | 19.75 yds (21.8) | 29.63 yds (32.6) |
Bags vs Ready-Mix: Which One Do You Need?
This is the decision point for most homeowners, and the answer is simpler than it looks. The break-even is roughly one cubic yard.
When to use bagged concrete
For pours under about one cubic yard, bagged concrete wins on convenience and cost. You avoid the short-load delivery fee that ready-mix suppliers charge on small orders, which can add $75 to $200 to a small job. A 10’x10’ shed pad at 4 inches is 1.23 yards, right at the line. Many homeowners pour that over a weekend with an 80-lb bag mixer rental.
An 80-pound bag of concrete mix yields about 0.60 cubic feet. That means you need 45 bags to make one cubic yard. At roughly $7 to $9 per bag in 2026, one yard of bagged concrete costs $315 to $405 in material alone, before the mixer rental. That is about twice the cost of ready-mix, which is why bags only win when the convenience and small-load savings outweigh the premium.
When to use ready-mix
Above one cubic yard, a ready-mix truck is almost always cheaper, faster, and produces a more consistent result. In 2026, ready-mix concrete runs $125 to $175 per cubic yard delivered nationally. For a 24’x24’ 4-inch slab at 7.8 yards ordered, that is a material cost of roughly $975 to $1,365 just for the mud, compared to around $2,500 if you tried to do the same pour with bags.
Ready-mix suppliers typically require a minimum delivery of 3 to 4 cubic yards. Orders below that threshold trigger a short-load surcharge that can run $75 to $200 or more, pushing the effective per-yard price up by 20% to 50%. If your pour is under 3 yards, either use bags or schedule your pour alongside a neighbor’s project to share a truck.
| Pour size | Cubic yards | Bags needed | Bag cost (approx) | Ready-mix cost (approx) |
|---|---|---|---|---|
| 10’ x 10’ at 4” | 1.4 | 63 bags | $440 to $570 | $175 to $245 + short-load |
| 12’ x 12’ at 4” | 2.0 | 90 bags | $630 to $810 | $250 to $350 + short-load |
| 20’ x 20’ at 4” | 5.5 | 248 bags | $1,735 to $2,230 | $688 to $963 |
| 24’ x 24’ at 4” | 7.8 | 351 bags | $2,460 to $3,160 | $975 to $1,365 |
| 24’ x 24’ at 6” | 11.7 | 527 bags | $3,690 to $4,740 | $1,463 to $2,048 |
Why Thickness Changes Everything
Of all the inputs in the concrete calculation, thickness has the biggest impact on your order, your cost, and how long your slab lasts. Going from 4 inches to 6 inches adds 50% more material on the exact same footprint. Going from 4 inches to 5 inches adds 25%. The table below shows why getting thickness right before you call the plant matters more than almost anything else.
| Thickness | Common use | Cu yds per 100 sq ft | 24×24 order (with waste) |
|---|---|---|---|
| 3.5 inches | Light walkways only | 1.08 yds | 6.9 yds |
| 4 inches | Patios, shed floors | 1.23 yds | 7.8 yds |
| 4.5 inches | Light residential drives | 1.39 yds | 8.8 yds |
| 5 inches | Standard driveways | 1.54 yds | 9.8 yds |
| 5.5 inches | Heavy-use drives | 1.70 yds | 10.8 yds |
| 6 inches | Garages, foundations | 1.85 yds | 11.7 yds |
| 8 inches | Commercial floors | 2.47 yds | 15.6 yds |
The Portland Cement Association recommends a minimum 4-inch thickness for residential flatwork, with 5 to 6 inches for any slab that will carry vehicle traffic. Going thinner to save money on material is one of the most common reasons residential slabs crack prematurely.
Three Real Concrete Orders From American Projects
Here is how the math plays out on three real jobs, the kind contractors price every week across the country.
Example 1: Backyard patio in Phoenix, Arizona
A homeowner wants a 14’x20’ patio slab (280 square feet) at 4 inches thick. The Phoenix climate is dry with no freeze-thaw concern, so a basic 3,000 PSI mix works fine. The calculation: 280 sq ft x 0.333 ft = 93.2 cubic feet / 27 = 3.45 cubic yards. With 10% waste, the order is 3.8 yards, rounded up to 4. This clears the short-load minimum so the ready-mix truck comes without a surcharge. Material cost at $130 per yard in the Phoenix market: approximately $520.
Example 2: Two-car garage floor in Minneapolis, Minnesota
A detached garage slab at 24’x24’, poured at 6 inches with a 4,000 PSI air-entrained mix for freeze-thaw protection. The order: 10.67 yards base volume plus 10% = 11.7 yards, ordered as 12. Minneapolis ready-mix runs about $155 per yard in 2026. Material cost for the slab alone: approximately $1,860. The contractor adds rebar at 6-inch spacing on a 6-inch grid, which is the standard for a Midwest garage floor that will see road-salt vehicles year-round.
Example 3: RV pad in Nashville, Tennessee
A 12’x40’ RV parking pad (480 square feet) at 6 inches to handle the 15,000-pound vehicle. The math: 480 x 0.5 = 240 cubic feet / 27 = 8.89 yards. With 10% waste: 9.8 yards, ordered as 10. At $140 per yard in Nashville: approximately $1,400 in ready-mix. The homeowner specifies wire mesh plus rebar at the edges, adding structural rigidity where the tires track most often.
How to Place the Concrete Order the Right Way
Once you have your cubic yards calculated, calling the plant correctly saves money and avoids a half-loaded truck showing up on pour day.
What to tell the dispatcher
Every ready-mix order requires at least four pieces of information: the volume in cubic yards, the mix strength in PSI, the slump (how workable the mix is, measured in inches), and the delivery date and time window. For standard residential flatwork, a 4-inch slump is typical. In hot weather, ask for 4 to 5 inches of slump so the mix stays workable during placement. In cold weather, ask about heated water or accelerators.
Timing the delivery
Order the truck to arrive at least 30 minutes after your crew is set up and forms are checked. Once the drum starts turning on-site, you have roughly 90 minutes before the mix starts to lose workability. For pours over 10 yards, consider ordering two trucks with staggered arrivals 20 minutes apart so you are never rushing to beat a hardening drum.
Every ready-mix delivery comes with a batch ticket that shows the actual volume delivered, mix design, water added, and time of batching. Keep this ticket. It is your proof of what was poured if cracking disputes come up later, and most concrete warranties require it.
Frequently Asked Questions
How many yards of concrete for a 24×24 slab at 4 inches?
A 24×24 slab at 4 inches thick requires 7.11 cubic yards of concrete. With a 10% waste factor added, you should order 7.8 yards and round up to 8 when calling your supplier. Use our free slab calculator to get the exact figure for any thickness.
How many 80-pound bags do I need for a 24×24 slab?
At 4 inches thick you need about 7.11 cubic yards. One 80-pound bag yields roughly 0.60 cubic feet, and 27 cubic feet equals one yard, so you need about 45 bags per yard. For 7.11 yards that is approximately 320 bags without waste, or 351 bags with 10% added. For a pour this large, ready-mix concrete is far more practical and cost-effective.
What is the formula for calculating concrete volume?
Multiply the length in feet by the width in feet by the thickness in feet to get cubic feet. Then divide by 27 to convert to cubic yards. The key step is converting thickness from inches to feet: divide the inch measurement by 12. So a 4-inch slab is 0.333 feet thick.
How thick should a 24×24 garage slab be?
A garage floor that will park passenger vehicles should be a minimum of 4 inches thick, but most contractors recommend 5 to 6 inches with a rebar grid for a detached two-car garage. In freeze-thaw climates like the Midwest and Northeast, 6 inches with a 4,000 PSI air-entrained mix is the right spec. The extra thickness and concrete quality pay for themselves in a slab that lasts 30 or more years without major cracking.
What is the minimum order for a ready-mix concrete truck?
Most ready-mix suppliers in the US have a minimum order of 3 to 4 cubic yards. Below that minimum you pay a short-load fee that typically runs $75 to $200. For very small pours under 2 yards, bagged concrete from a hardware store is usually the more economical option, even accounting for the labor difference.
Does adding rebar change how much concrete I need?
Technically rebar displaces a tiny amount of concrete volume, but in practice the displacement is negligible and no one adjusts their order for it. Calculate your cubic yards based on full slab dimensions and add the standard 10% waste factor. The rebar will not meaningfully affect your order quantity.
How much does one yard of concrete weigh?
A cubic yard of ready-mix concrete weighs approximately 3,900 to 4,050 pounds, or roughly two tons. This is useful to know when calculating truck axle weights for tight access situations, and when estimating how much a slab itself weighs if you ever need to demo it.
Can I mix a 24×24 slab by hand?
Not practically. A 24×24 slab at 4 inches requires 7.11 cubic yards, which is about 320 bags of 80-pound concrete. Mixing that by hand or with a small portable mixer would take multiple days and produce an inconsistent result, since different batches cure at different rates and create visible joints. A 24×24 slab should always be poured with a ready-mix truck in a single continuous pour.
Related Calculators and Guides
For the official standards on residential concrete mix design, the Portland Cement Association’s mix design resources are the authoritative US reference.