Slab Foundation Calculator
— How Much Do You Need?

A slab foundation is a flat, horizontal layer of poured concrete that sits directly on the ground and serves as both the foundation and the floor of a structure. Instead of digging down for a basement or framing up a crawl space, builders pour a single concrete slab that carries the entire load of the building above it. What most people do not realize is that the concrete itself is only part of the story. A proper slab foundation relies on several layers of preparation before a single bucket of concrete is poured. First, the site is graded and leveled. If the ground is low or uneven, fill dirt is brought in to raise the grade and encourage drainage away from the building. Once the soil is at the right elevation, a compacted aggregate base layer, typically 4 to 6 inches of crushed stone or gravel, is packed down over the prepared soil. This base layer is what keeps the slab stable over the long run. It distributes weight evenly, drains water away from beneath the concrete, and prevents the ground from shifting with seasonal changes. A vapor barrier usually goes on top of the gravel just before the concrete is poured. Hello Gravel supplies the fill dirt and compacted gravel base materials that make this critical prep work possible. If you want your slab to last, the material underneath it matters just as much as the concrete on top.

For most residential and commercial slab foundations, you want a dense, angular crushed stone or compactable base material. Crusher run, also known as road base or dense-grade aggregate, is one of the most widely used options. It combines crushed stone with fine stone dust, and when compacted, the particles lock together to create a firm, load-bearing surface that resists movement over time. Clean crushed stone, such as number 57 gravel, is another solid choice. It does not compact as tightly as crusher run, but it drains exceptionally well and reduces the risk of water pooling beneath your slab. Your concrete contractor will often have a preference based on your local soil conditions, building codes, and the drainage characteristics of your site. The key quality you are looking for in any base material is that it compacts firmly and stays put under load. Rounded river gravel, for example, tends to shift rather than lock together, which makes it a poor choice for slab prep. Angular crushed stone performs much better in this application. If you are unsure which material is the right fit for your project, the Hello Gravel team is glad to help you think through the options. Reach out before you order and they will point you toward the right product for your job.

The amount of aggregate you need depends on the square footage of your slab and the target depth of your base layer. For most residential slabs, contractors call for 4 to 6 inches of compacted gravel base beneath the concrete. To give you a real-world reference point, a 20x20 slab covering 400 square feet would require roughly 7 to 8 tons of crushed stone at 4 inches of compacted base depth. If your contractor calls for 6 inches of compacted base, that same 400-square-foot footprint would need closer to 11 to 12 tons. These figures are based on standard material weights for compacted crushed stone, but your actual quantity will vary depending on the specific material you choose and your site conditions. For larger footprints, the numbers scale up quickly. A 40x60 slab at 4 inches of base depth, for example, would put you in a much higher tonnage range. Rather than trying to estimate by hand, use the Hello Gravel online calculator. Enter your length, width, and depth, and the calculator will give you a precise tonnage figure for your project. Hello Gravel delivers bulk aggregate by the truckload, so you can order the exact amount you need in a single delivery.

In many cases, yes. If your build site is low-lying, uneven, or needs to be raised to a specific grade for proper drainage and code compliance, fill dirt is the first material you bring in. Fill dirt builds up the soil profile to the right elevation so that water flows away from the structure rather than pooling beneath it. Think of the two materials as doing different jobs at different stages. Fill dirt shapes the land and establishes your grade. The gravel base goes on top of the compacted fill and creates the stable, drainage-friendly platform the concrete slab needs. Skipping the fill step when your site calls for it can leave you with a slab that sits too low, drains poorly, or settles unevenly as the soil beneath it compresses over time. Your contractor or a site engineer can assess your lot and tell you how much fill, if any, your project requires before gravel base goes in. Once you have those numbers, Hello Gravel can deliver both fill dirt and aggregate base materials. Use the calculator to estimate your quantities for each material separately before you place your order, so you have a clear picture of everything your project needs.

Generally, yes. A slab foundation is the most cost-effective foundation option for most residential and light commercial construction. Because you are not excavating for a basement or building up block walls for a crawl space, the labor, material, and time costs are significantly lower. Slab foundations are also faster to build, which keeps your overall construction schedule tight. The aggregate materials that Hello Gravel supplies, fill dirt and the gravel base layer, are a big part of what keeps slab prep costs reasonable. Getting the materials you need in a single bulk delivery at upfront pricing means you are not making multiple trips to a retail supplier or paying a markup for small quantities. That said, your total slab project cost will vary based on your slab thickness, the amount of site prep required, local labor rates, and how much fill dirt your site needs before the gravel base goes in. For current pricing on the materials Hello Gravel supplies, check the product pages or use the calculator to build a materials estimate. The Hello Gravel team is also available if you want to talk through your project before ordering.