Concrete Slab Calculator
Watch your slab update as you type, dial in the waste factor, and walk away with a material list that's ready to print.
Most pours need 5–10% extra to cover spillage, uneven subgrade, and formwork. Skipping this often means a costly second trip.
Enter dimensions to calculate.
| Slab size | Volume (cu yd) | 80-lb bags | 60-lb bags | Weight (lb) |
|---|---|---|---|---|
| 8′ × 8′ × 4″ | 0.87 | 40 | 53 | 3,520 |
| 10′ × 10′ × 4″ | 1.36 | 62 | 82 | 5,490 |
| 10′ × 10′ × 6″ | 2.04 | 92 | 123 | 8,240 |
| 12′ × 12′ × 4″ | 1.96 | 88 | 118 | 7,910 |
| 12′ × 12′ × 6″ | 2.93 | 132 | 177 | 11,870 |
| 16′ × 16′ × 4″ | 3.48 | 157 | 209 | 14,060 |
| 20′ × 20′ × 4″ | 5.43 | 245 | 326 | 21,970 |
| 20′ × 20′ × 6″ | 8.15 | 367 | 489 | 32,960 |
| 24′ × 24′ × 6″ | 11.73 | 528 | 705 | 47,470 |
| Driveway: 12′ × 20′ × 4″ | 3.26 | 147 | 196 | 13,180 |
| Driveway: 20′ × 24′ × 6″ | 9.78 | 440 | 587 | 39,550 |
Add or subtract bags proportionally for other sizes — or use the calculator above for exact numbers.
How to measure
Measure the longest length and width of the area at ground level, in feet or meters. For thickness, measure the depth of the form or excavation. Take measurements from at least two spots and use the larger value to be safe.
The math
Volume is length × width × thickness. Convert each measurement into the same units before multiplying, then convert the result into cubic yards or cubic meters.
Why add a waste percentage?
Forms flex, the subgrade settles, and a small amount of mix is always lost in the pour. A 5–10% waste allowance prevents you from coming up short before the truck pulls away.
Bagged mix vs. ready-mix
Bagged concrete is convenient for small pours under roughly half a cubic yard. Above that, ready-mix delivered by truck is almost always cheaper per cubic yard and avoids cold-joint problems from mixing one bag at a time.
Frequently asked questions
- How many bags do I need for a 10 × 10 × 4-inch slab?
- A 10 × 10 × 4-inch slab works out to 33.3 cubic feet of concrete. At 0.6 cubic feet per 80-lb bag that's about 56 bags net, and once you add a 10% waste allowance and round up you'll want to buy 62 80-lb bags. If you're using 60-lb bags (yield is about 0.45 ft³), the same slab takes roughly 82 bags with waste factored in.
- Is 4 inches thick enough?
- It depends on what's going on top. Four inches is the standard for sidewalks, patios, and foot-traffic flatwork. For driveways and garage floors that will see passenger vehicles, 4 inches is the practical minimum and 5–6 inches is more common; pickups, trailers, or anything heavier generally call for 6 inches or more. Always check with your local building department, since some jurisdictions require thicker.
- Do I need rebar?
- For pretty much any slab larger than a few feet across, yes — either #3 or #4 rebar in an 18–24 inch grid, or 6×6 welded wire fabric. Reinforcement doesn't prevent cracks, but it holds them tight and keeps the slab acting as one piece if it does crack. The IRC addresses concrete floors on ground in Section R506; check your local jurisdiction's adopted code for specific requirements, which vary by use and soil conditions.
- Can I pour in cold weather?
- You can, but it takes precautions once temperatures get near freezing. ACI 306 — the industry guide for cold-weather concreting — generally treats conditions as cold-weather once the average daily air temperature drops below 40°F for several consecutive days. In that range you need to protect fresh concrete from freezing for at least the first 24–48 hours using insulated blankets, heated enclosures, or an accelerating admixture, and you should never pour onto frozen ground.
- How long until I can walk on it or drive on it?
- For a standard mix, you can walk on the slab after roughly 24–48 hours, drive a passenger car on it after 7 days, and treat it as fully cured at 28 days. Trucks and heavy equipment should wait the full 28 days. Cold or wet weather slows the cure noticeably; what matters most isn't calendar time but keeping the surface moist for the first 3–7 days while it hydrates.
- Bagged mix or ready-mix — when does it pay to switch?
- The rule of thumb is one cubic yard (27 ft³). Below that, bagged mix from the home center is usually cheaper once you account for ready-mix truck minimums and short-load fees. Above one yard, a ready-mix delivery is almost always cheaper, faster, and gives you a more consistent pour with fewer cold joints. A 10 × 10 × 4 slab is about 1.2 yards — right on the line either way.
- How much waste should I add?
- Ten percent is a safe default for most DIY slabs and is what this calculator uses out of the box. For very simple, small pours with experienced finishers you can drop to about 5%; for irregular shapes, sloped sites, or first-time pours, 10–15% is more realistic. Buying a couple of extra bags or scheduling a half-yard buffer is always cheaper than running short mid-pour.
- What does a concrete slab cost?
- For a basic 4-inch slab, expect roughly $6–$12 per square foot installed by a contractor in most U.S. markets, including labor, forms, mix, and basic finishing. DIY material-only cost is closer to $2–$4 per square foot. Stamped, colored, or thicker slabs run higher, and remote sites, heavy reinforcement, or significant grading push the number up further. Local market and access drive most of the variation, so get two or three quotes before you commit.