Concrete Slab Calculator

How to Use the Concrete Slab Calculator

Using this concrete slab calculator takes three measurements. First, measure the length and width of the slab forms in feet — measure the forms themselves, not the drawing, because as-built dimensions often differ from the plan. Second, enter the slab thickness in inches; the calculator converts it automatically. Third, set a waste factor, typically 5–10%, to cover uneven subgrade, spillage, and finishing losses. The slab concrete calculator instantly returns the volume in cubic yards, cubic feet, and cubic meters, plus the number of 40 lb, 60 lb, or 80 lb bags. You can switch any dimension between feet, inches, yards, and meters using the unit toggle next to each input, so metric projects work just as well as imperial ones.

What Can You Calculate With a Slab Calculator?

Use this calculator for rectangular patios, sidewalks, garage floors, shed bases, driveways, and workshop slabs. Measure the finished form inside the edges, then use the planned thickness rather than the depth of the excavation. For a slab with more than one rectangle, calculate each section separately and add the results; for a round or equipment pad, the concrete pad calculator uses the appropriate shape.

Concrete Slab Formula Explained

To calculate concrete slab volume by hand, use the formula: length × width × (depth ÷ 12) = cubic feet, then divide by 27 to get cubic yards. Depth is divided by 12 because it is usually measured in inches while the other dimensions are in feet, and the division by 27 converts cubic feet to cubic yards — there are 3 × 3 × 3 = 27 cubic feet in one cubic yard. Take a 20 ft × 15 ft garage slab poured at 5 inches: 20 × 15 × (5 ÷ 12) = 125 cubic feet, and 125 ÷ 27 = 4.63 cubic yards. With a 10% waste factor the order becomes about 5.1 cubic yards. If you would rather see the answer purely in yards, our concrete yard calculator runs the same math and supports a second area. For bag-based projects, the concrete bag calculator divides the volume by the yield of each bag size.

Concrete Thickness Calculator for Slabs

Slab thickness drives both the concrete slab calculation and the durability of the finished pour. Walkways and patios that only carry foot traffic are commonly poured at 4 inches. Garage floors and residential driveways that carry cars are typically 4 to 6 inches, with 6 inches preferred where pickups or trailers park. Slabs for workshops, RV pads, or light equipment often run 6 inches or more, sometimes with thickened edges. Shed bases are usually 4 inches. These are planning conventions, not engineering advice: soil conditions, climate, reinforcement, and local building codes can all change the required thickness, so confirm structural slabs with a professional before you pour.

10 × 10 ft Slab Thickness Comparison

The footprint stays at 100 square feet, so every change comes from thickness. A 6-inch slab uses 50% more concrete than a 4-inch slab with the same length and width. Bag counts include the table's 10% waste factor and round up using a typical 0.60 cubic foot yield for an 80 lb bag. Use the calculator above for the actual dimensions, selected bag yield, and waste allowance; confirm the required thickness independently.

Calculating a Slab With a Footing

Garages, sheds, and workshops are often poured as a single piece with a deepened band around the outside edge, so the slab and its footing arrive in the same truck. There is no separate shape to enter for this: calculate the flat slab at its normal thickness, calculate the deepened band as its own volume, then add the two. Treating the whole footprint as one deep pour badly overestimates the order, because only the perimeter band is deep.

The band volume is easier than it looks. Subtract the inner rectangle from the outer one to get the band area, then multiply by how much deeper the band goes below the slab — not by its full depth, or you will count the slab thickness twice. For a 20 × 20 ft garage at 4 inches with a band 12 inches wide and 12 inches deep overall: the slab is 20 × 20 × 0.333 = 133 cubic feet, the band area is 400 − 18² = 76 square feet, and the band runs 8 inches deeper than the slab, so 76 × 0.667 = 51 cubic feet. Together that is 184 cubic feet, or 6.82 cubic yards before waste.

20 × 20 ft Slab With Perimeter Edge

The edge detail can add more than a third to the order on a footprint this size, which is why it is worth measuring before ordering rather than after. For footings poured separately from the slab — strip footings under walls, pier holes, or sonotubes — the concrete footing calculator handles those shapes directly, and steel for the slab portion can be planned with the slab rebar calculator. Edge width, depth, and steel are structural choices set by the design and local requirements, so take them from the project documents rather than from this example.

Tips for Ordering Concrete

Once the concrete slab estimator gives you a volume, round up rather than down — running short mid-pour creates a cold joint that permanently weakens the slab. Concrete suppliers usually sell in quarter-yard increments with a minimum load, and short-load fees can make very small deliveries expensive; below roughly one cubic yard, bagged concrete is often cheaper and simpler. Tell the dispatcher the slab thickness and finish you plan, confirm the truck can reach the forms, and have help on site, because a yard of concrete weighs about two tons and sets on its own schedule. The Quick Cost Estimate below the calculator multiplies your yardage by a price per cubic yard for a fast material-only budget check.