Concrete is sold by the cubic yard, but you measure your project in feet and inches. This calculator bridges that gap — enter your slab dimensions and get cubic yards, cubic feet, bag equivalents, and a delivered cost estimate with a waste allowance built in.
Pouring a footing, curb, or narrow strip instead? See our Linear Feet for Concrete calculator for length-based pours.
Concrete Slab Calculator
Enter your slab dimensions. Results show volume in cubic yards and cubic feet, bag counts, and estimated ready-mix cost.
The Concrete Volume Formula
Every concrete quantity calculation reduces to one formula. The only trick is that thickness is given in inches while length and width are in feet, and the answer is sold in cubic yards.
Cubic Yards = (Length ft × Width ft × Thickness in ÷ 12) ÷ 27
The division by 12 converts inches of thickness into feet. The division by 27 converts cubic feet into cubic yards, because a cubic yard is 3 ft × 3 ft × 3 ft = 27 cubic feet. Ready-mix trucks, dispatchers, and quotes all speak in cubic yards, so that final conversion is the one that matters when you place an order.
Worked example: A 24 × 24 ft garage floor at 4 inches. 24 × 24 = 576 sq ft. 4 in ÷ 12 = 0.333 ft. 576 × 0.333 = 192 cubic feet. 192 ÷ 27 = 7.11 cubic yards. With a 10% waste allowance, order 7.8 cubic yards — which in practice means asking for 8 yards.
Concrete Volume by Common Slab Size
These figures are exact volume with no waste allowance. Add 10% before ordering. Bag counts assume 80 lb bags at 0.60 cubic feet each.
| Slab Size | Sq Ft | 4 in (cu yd) | 6 in (cu yd) | 80 lb Bags (4 in) | Typical Use |
|---|---|---|---|---|---|
| 4 × 4 ft | 16 | 0.20 | 0.30 | 9 | AC pad, landing |
| 8 × 10 ft | 80 | 0.99 | 1.48 | 45 | Small shed floor |
| 10 × 10 ft | 100 | 1.23 | 1.85 | 56 | Patio, hot tub pad |
| 12 × 12 ft | 144 | 1.78 | 2.67 | 80 | Patio, single-car pad |
| 12 × 20 ft | 240 | 2.96 | 4.44 | 133 | Single-car driveway |
| 16 × 16 ft | 256 | 3.16 | 4.74 | 142 | Large patio |
| 20 × 20 ft | 400 | 4.94 | 7.41 | 222 | Two-car garage floor |
| 24 × 24 ft | 576 | 7.11 | 10.67 | 320 | Standard 2-car garage |
| 20 × 40 ft | 800 | 9.88 | 14.81 | 444 | RV pad, long driveway |
| 30 × 40 ft | 1,200 | 14.81 | 22.22 | 667 | Shop / barn floor |
Above roughly 1 cubic yard, ready-mix delivery is cheaper and far less labor than mixing bags. Below 1 cubic yard, bags usually win because short-load fees dominate the price of a small ready-mix order.
Bags of Concrete Per Cubic Yard
A cubic yard is 27 cubic feet. Divide 27 by the yield of your bag size to get bags per yard. Yield varies slightly by manufacturer, so treat these as planning numbers and buy a couple of spare bags.
| Bag Size | Yield (cu ft) | Bags per Cu Yd | Water Needed | Cost per Cu Yd |
|---|---|---|---|---|
| 80 lb | 0.60 | 45 | ~6 pt/bag | $290–$400 |
| 60 lb | 0.45 | 60 | ~4.5 pt/bag | $300–$420 |
| 50 lb | 0.375 | 72 | ~3.5 pt/bag | $320–$450 |
| Ready-Mix Truck | — | — | Batched at plant | $160–$195 |
Ready-mix is 2–3x cheaper per cubic yard than bagged concrete, but carries a short-load fee of $60–$120 on orders under 4–5 yards. The crossover point where ready-mix becomes cheaper is typically around 1–1.5 cubic yards.
Concrete Slab Cost — Material vs Installed
Material cost is only part of a slab. Forms, base prep, reinforcement, finishing, and control joints usually cost more than the concrete itself on residential jobs.
| Cost Component | Cost per Sq Ft | Cost on 20×20 Slab | Notes |
|---|---|---|---|
| Concrete material (4 in) | $1.98–$2.41 | $790–$963 | Ready-mix at $160–$195/cu yd |
| Gravel base (4 in) | $0.60–$1.20 | $240–$480 | Compacted crushed stone |
| Forms & stakes | $0.50–$1.10 | $200–$440 | Lumber, often reusable |
| Rebar or wire mesh | $0.15–$0.60 | $60–$240 | #3/#4 grid or 6×6 WWM |
| Labor (place & finish) | $3.00–$7.00 | $1,200–$2,800 | Crew, screed, float, trowel |
| Short-load fee | — | $60–$120 | Under 4–5 cu yd orders |
| Total installed (4 in) | $6.50–$10.50 | $2,600–$4,200 | Plain broom finish |
| Total installed (stamped) | $10–$20 | $4,000–$8,000 | Decorative pattern & color |
Sources: ready-mix and installed slab pricing from ConcreteNetwork.com (updated July 2026), which compiles NRMCA national data and supplier quotes. Component line items (base, forms, rebar, labor) are planning estimates, not sourced figures. Metro markets on the coasts run 20–40% above these ranges; rural inland markets often fall 10–20% below. Always get three local quotes before budgeting.
How Thick Should the Slab Be?
Thickness drives both volume and cost. Going from 4 to 6 inches adds 50% to your concrete order, so match thickness to the actual load rather than over-building by default.
| Application | Thickness | PSI Mix | Reinforcement | Cu Yd per 100 Sq Ft |
|---|---|---|---|---|
| Sidewalk / garden path | 3–4 in | 3,000 | Fiber mesh or none | 0.93–1.23 |
| Patio | 4 in | 3,000–3,500 | 6×6 wire mesh | 1.23 |
| Shed / garage floor | 4 in | 3,500 | Wire mesh or #3 rebar | 1.23 |
| Driveway (cars) | 4–5 in | 3,500–4,000 | #3 rebar 18 in grid | 1.23–1.54 |
| Driveway (trucks / RV) | 6 in | 4,000 | #4 rebar 12–18 in grid | 1.85 |
| Shop / heavy equipment | 6–8 in | 4,000–4,500 | #4/#5 rebar, both mats | 1.85–2.47 |
Going from 4 to 5 or 6 inches increases a slab’s load-carrying capacity by nearly 50% according to ConcreteNetwork, so the extra depth pays for itself anywhere vehicles will park or turn. Note that thickened edges add volume beyond the flat slab calculation: a 12-inch-deep, 12-inch-wide perimeter footing on a 20 × 20 slab adds roughly 0.9 cubic yards on top of the 4.94 yard slab figure.
Common Concrete Estimating Mistakes
Forgetting to Divide Thickness by 12
Plugging 4 into the formula instead of 0.333 gives you a number 12 times too large. A 20 × 30 slab at 4 inches is 7.4 cubic yards, not 88.9. If your answer looks absurdly large, this is almost always the reason. Thickness must be converted from inches to feet before multiplying.
Ordering Exact Volume With No Waste Allowance
Subgrade is never perfectly flat, forms bow slightly under load, and some concrete is lost in the chute and wheelbarrow. Ordering exactly 7.11 yards for a 7.11 yard slab means running short. A second truck triggers a fresh short-load fee of $60–$120 and creates a cold joint where the two pours meet — a permanent weak line in the slab.
Ignoring Thickened Edges and Footings
Many slabs are not uniform thickness. Garage slabs and shed pads often have a 12-inch-deep perimeter footing, and driveways frequently have thickened edges where vehicles turn. These add real volume that the flat length × width × thickness formula misses entirely. Calculate the perimeter footing separately as its own linear-feet-times-cross-section volume and add it to the slab total.
Mixing Bags for Pours Over One Cubic Yard
One cubic yard is 45 eighty-pound bags — roughly 3,600 lbs of material to haul, open, and mix. Beyond about one yard the labor becomes punishing and the concrete quality suffers because you cannot place and finish it as a single monolithic pour. Batches mixed minutes apart begin setting at different times, producing cold joints and inconsistent surface finish across the slab.
About these price ranges
Pricing varies widely by region, season, project size, and site conditions. Treat every figure on this page as a planning estimate for budgeting only — not as a quote. Coastal metro markets commonly run 20–40% above national ranges while rural inland markets can fall 10–20% below. Always collect at least three written quotes from local contractors before committing to a budget.
Sourced figures: ConcreteNetwork.com concrete prices (compiles NRMCA national data, updated July 2026).
Unsourced estimates: Base gravel, forms, rebar, and labor line items. These are informed planning figures rather than data drawn from a published price index, and they carry a correspondingly wider margin of error.