How this instrument works
A concrete slab — a patio, a shed or garage floor, a sidewalk section, a small equipment pad — is a rectangular volume of concrete: length times width times thickness. Because length and width are conventionally measured in feet while slab thickness is specified in inches, this instrument converts thickness to feet before multiplying, then divides the resulting cubic-foot volume by 27 to return the figure in cubic yards, the unit ready-mix concrete is ordered and priced by.
Slab thickness is the dimension most likely to get entered wrong, since it's almost always specified in inches on plans and quotes while the other two dimensions are in feet. A 4 inch slab is 4/12, or about 0.333 ft — a small number that's easy to mistype as a whole '4' if the unit conversion is done by hand rather than left to the instrument.
Required thickness depends entirely on what the slab supports: a garden shed floor with light foot traffic can get by with a thinner slab than a garage floor expected to support vehicle weight, and load-bearing or structural slabs may call for reinforcement (rebar or wire mesh) that this volume-only calculation doesn't account for. Always check the thickness and reinforcement appropriate for the specific slab's use against local code or an engineer's recommendation.
This instrument returns pure concrete volume with no price attached. For a driveway specifically — where vehicle loads, thickness code minimums, and an installed cost estimate are usually all part of the same question — this site's concrete driveway cost instrument runs the identical volume formula and adds a price-per-cubic-yard total on top.
- Enter the slab's length into Slab length (ft) — the longer footprint dimension.
- Enter the slab's width into Slab width (ft) — the shorter footprint dimension.
- Enter the planned thickness into Slab thickness (in) — check local code or a structural recommendation for the appropriate thickness for the slab's intended use.
- Read Concrete needed (cubic yards) beneath the inputs — the volume figure to order from a ready-mix supplier.
Worked example — a 20 ft × 20 ft slab, 4 in thick
Enter 20 into Slab length (ft), 20 into Slab width (ft), and 4 into Slab thickness (in). The instrument converts 4 in to 0.3333 ft, computes 20 x 20 x 0.3333 = 133.33 ft³, and divides by 27: 133.33 / 27 ≈ 4.9383. Concrete needed (cubic yards) reads 4.9383 yd³.
That 4.9383 yd³ figure is the raw material volume for a 20 ft square patio or pad at a common 4 inch thickness — enough to plan a ready-mix order around, though most contractors add a small overage beyond this exact figure to cover sub-base irregularities and minor spillage during the pour.
Questions
How thick should a concrete slab be?
It depends on what the slab needs to support — a light-duty patio or shed floor commonly uses a thinner slab than a garage floor or anything expected to bear vehicle weight, and load-bearing applications may need reinforcement this volume calculation doesn't account for. Check local building code or a structural recommendation for the specific slab's intended use rather than assuming one thickness fits every application.
Why convert thickness to feet before dividing by 27?
Because 27 cubic feet make one cubic yard, and length and width are already in feet, so thickness needs to match that same unit before the three dimensions can be multiplied together meaningfully. Leaving thickness in inches and multiplying it directly against feet-based length and width would produce a volume figure off by a factor of 12.
Does this include a cost estimate?
No — this instrument returns concrete volume only, in cubic yards, with no price attached. For a project where cost matters as much as volume — particularly a driveway, where thickness and vehicle-load considerations usually come bundled with a budget question — this site's concrete driveway cost instrument runs the same volume formula and adds a price-per-cubic-yard total.
Should I add extra concrete beyond the calculated volume?
Yes — a modest overage beyond the exact calculated figure is standard practice, since sub-base irregularities, minor form movement, and spillage during the pour typically use somewhat more concrete than the theoretical volume alone. Many contractors add roughly 5 to 10 percent as a buffer, and ready-mix trucks deliver in fixed load increments rather than arbitrary fractions of a yard.
Does the slab need to be a perfect rectangle for this?
Yes, this calculation assumes a simple rectangular footprint. An L-shaped, curved, or otherwise irregular slab needs to be broken into separate rectangular sections, each calculated individually and then summed, or handled with a more general area calculation before multiplying by thickness.
Is a thicker slab always better?
Not necessarily — thickness should match the load the slab actually needs to support, since concrete volume, and therefore cost, scales directly with thickness. Over-specifying thickness for a light-duty application like a garden path adds material cost without a meaningful structural benefit, while under-specifying it for a load-bearing application like a garage floor risks cracking or failure under weight it wasn't designed for.