Concrete Slab Calculator
Enter your measurements
Extra to cover cuts, breakage, and mistakes.
Results
- Concrete needed1.36 cubic yards
- Volume36.67 cubic feet
- 60 lb bags82if mixing by bag
- 80 lb bags62if mixing by bag
Estimated cost
per cubic yard of ready-mix, delivered
Material cost is per cubic yard of ready-mix concrete delivered to the site; short loads under the full-truck minimum typically add $50 to $150 or more per yard under the minimum. Installed cost is the per-square-foot labor to place, screed, float, and broom-finish a standard 4-inch residential slab, on top of the material; decorative finishes, saw-cut control joints, and reinforcing steel are additional.
Estimate only — prices vary by region, supplier, and season. Get a local quote before buying.
This concrete calculator for slabs takes your slab's length, width, and thickness and converts those dimensions into cubic yards of ready-mix, cubic feet of total volume, and the number of 60 lb or 80 lb bags you'd need if mixing yourself. It also applies a configurable waste factor — typically 10% for slabs — so you don't fall short on pour day. Concrete is sold by the cubic yard from ready-mix plants, and a full truck holds about 8 to 10 yards, so smaller orders trigger short-load fees. Bagged concrete in 60 lb or 80 lb sacks is sold individually at home improvement stores and only pencils out for small pours. Most residential patio and walkway slabs are 4 inches thick, garage slabs carrying vehicles run 5 to 6 inches, and engineer-specified structural slabs go deeper. Knowing your cubic yardage before you call a supplier or load a cart matters because under-ordering risks a cold joint where fresh concrete meets a set edge, while over-ordering means paying for material you'll dump and wheelbarrow off.
How it’s calculated
Volume = Length × Width × Thickness, converted to cubic yards (÷27) and to 60 / 80 lb bag counts.
Worked example
For a 20 ft by 12 ft slab poured 4 inches thick with a standard 10% waste allowance, the calculator returns 3.26 cubic yards (88 cubic feet) of concrete. That same volume works out to 196 sixty-pound bags or 147 eighty-pound bags if you mix by hand instead of ordering ready-mix. At a little over 3 cubic yards, this job sits right in the short-load zone where a ready-mix supplier tacks on a partial-truck fee, so the bag-count output is genuinely useful for weighing whether to order a truck or buy sacks.
Inputs
- Length
- 20 ft
- Width
- 12 ft
- Thickness
- 4 in
- Waste / overage
- 10 %
Result
- Concrete needed
- 3.26 cubic yards
- Volume
- 88 cubic feet
- 60 lb bags
- 196
- 80 lb bags
- 147
- Estimated material cost
- $456 – $603
Materials & pricing near you
Ready-mix concrete is priced per cubic yard and varies meaningfully by region. Dense metro markets with several competing batch plants tend to run lower than rural areas where a truck drives farther, often adding $5 to $10 per mile beyond a base radius. Short loads — typically any order under the 8-to-10-yard full-truck minimum — carry a premium of $50 to $150 or more per yard under the minimum because the plant still ties up a truck and driver. Bagged 80 lb concrete at big-box stores runs roughly $6 to $10 per bag; fast-setting and fiber-reinforced mixes cost more. Many jurisdictions require a permit once a slab is attached to a structure or exceeds a set square footage (often around 120 sq ft), so check your local building department before you pour — unpermitted flatwork can surface during a home inspection or sale.
Frequently asked questions
How thick should a concrete slab be?
A standard residential patio or walkway slab is 4 inches thick. Garage slabs that support vehicles should be 5 to 6 inches, and driveways typically call for at least 4 inches with a 3,500 to 4,000 PSI mix. Structural slabs carrying engineer-specified loads may be 6 to 8 inches or more. When in doubt, your local building code or a structural engineer's spec sheet is the authoritative source, since thickness drives both your yardage and the slab's capacity.
When does it make sense to use bagged concrete instead of ordering ready-mix?
Bagged concrete is cost-competitive only for jobs under about 1 cubic yard — a small pad, a few fence-post footings, or a repair. Once you cross 1 cubic yard, a ready-mix truck is usually cheaper per yard and far less labor. Mixing 147 eighty-pound bags by hand for this 3-yard example, for instance, is roughly 12,000 pounds of material and risks inconsistent water ratios between batches that weaken the finished slab.
Why does the calculator add a 10% waste factor?
Volume math assumes perfectly level, true-to-dimension forms, but real subgrade is rarely that even. A 10% overage covers low spots in the gravel, form flex, spillage during placement, and the concrete left clinging to the truck chute. Some contractors drop to 5% on tightly formed work and go up to 15% on irregular shapes or rough subgrade. You can adjust the waste percentage in the calculator to match how confident you are in your base prep.
How do I convert cubic feet to cubic yards to check the math myself?
There are exactly 27 cubic feet in a cubic yard, so divide your cubic-foot total by 27. For the 20 x 12 x 4-inch example, 88 cubic feet divided by 27 equals about 3.26 cubic yards. If you measured the slab in inches instead of feet, multiply length x width x thickness in inches, then divide by 46,656 (the number of cubic inches in a cubic yard) to go straight to yards.