Concrete Slab Calculator

Article — Concrete Slab Calculator

Concrete Slab Calculator: Cubic Yards, Bags, and Cost

A 20 ft × 10 ft × 6 in concrete slab needs 3.70 cubic yards of concrete (4.07 yd³ with 10% waste). That translates to roughly 167 80-lb bags or one short-load ready-mix delivery at $140-$200 per cubic yard. Pour thickness should follow the American Concrete Institute (ACI) 332 residential standard: 4 inches for patios and walkways, 5-6 inches for driveways carrying cars and light trucks.

This calculator handles the volume math, the bag count, and the cost estimate. Length and width go in any unit (feet, inches, meters, centimeters); thickness gets its own unit selector because slabs are almost always specified in inches even when the slab area is in feet.

Concrete slab volume math

The formula is straightforward: length × width × thickness, all converted to consistent units. The catch is that thickness is almost always in inches while length and width are in feet. Divide inches by 12 to convert to feet, then multiply through to get cubic feet. Divide cubic feet by 27 to get cubic yards — the unit ready-mix concrete is sold in.

A 20 ft by 10 ft patio at 4 inches thick: 20 × 10 × (4/12) = 66.7 ft³ ÷ 27 = 2.47 yd³. The same patio at 6 inches thick: 100 ft³ ÷ 27 = 3.70 yd³. Adding 50% to slab thickness adds 50% to volume — bumping a 4-inch patio to 6 inches for a heavier load costs half again as much in materials.

Concrete slab thickness guide

The American Concrete Institute publishes thickness guidance in ACI 332 (Residential Concrete) and ACI 360 (Slabs-on-Ground). For most home projects: 4 inches handles foot traffic and passenger vehicles; 5 inches adds margin for trucks and SUVs; 6 inches covers heavy trucks, RVs, and dumpsters; 8 inches enters commercial territory.

The decision depends on three factors: expected load, soil conditions, and reinforcement. A 4-inch slab on a properly compacted 4-inch gravel base supports passenger vehicles indefinitely. The same slab on uncompacted soil cracks within a year. Adding rebar lets you thin the slab for the same load — a 5-inch reinforced slab matches an 8-inch unreinforced one for crack resistance.

Ready-mix vs. bagged concrete

The crossover point is roughly 1 cubic yard. Below that, bagged mix from a home-improvement store is cheaper and easier — no truck access, no minimum order, no time pressure. Above 1 cubic yard (about 45 80-lb bags), ready-mix delivery from a concrete supplier is faster, cheaper per yard, and produces better consistency because the mix is batched mechanically.

Ready-mix has a short-load fee under 3 cubic yards, typically $50-$100 per yard short. A 2.5-yard order on a 5-yard minimum costs an extra $125-$250 on top of the per-yard price. Either pour multiple smaller projects on the same delivery or use bagged mix when project size is between 1 and 3 yards. Concrete must be placed within 90 minutes of mixing — there is no extending that window with admixtures.

Concrete slab bag counts

Each bag of concrete mix yields a specific volume when mixed with water at the recommended ratio. A 40-lb bag yields about 0.30 ft³ (about 90 bags per cubic yard). A 60-lb bag yields about 0.45 ft³ (about 60 bags per cubic yard). An 80-lb bag yields about 0.60 ft³ (about 45 bags per cubic yard).

Most DIY pours use the 60-lb size — lighter to lift, easier to mix in a wheelbarrow or small mixer. The 80-lb bag is more economical per cubic foot but harder to handle. For pours over half a cubic yard, rent a 9 cu ft electric mixer; mixing 30+ bags by hand or in a wheelbarrow is brutal.

• 1 cubic yard = 27 cubic feet = 0.7646 cubic meters
• 40-lb bag yields 0.30 ft³ (90 bags per yard)
• 60-lb bag yields 0.45 ft³ (60 bags per yard)
• 80-lb bag yields 0.60 ft³ (45 bags per yard)
• Ready-mix minimum typically 1 yard; short-load fee under 3 yards
• ACI 332 minimum = 4 inches for residential driveways
• Cure time = 24-48 h for foot traffic, 7 d for vehicles, 28 d for full strength
• Crack spacing = control joints every 8-10 feet, depth = 1/4 of slab thickness

Concrete slab cost estimate

Ready-mix concrete runs $140-$200 per cubic yard delivered in most US markets as of 2024-2026, with regional variation of about ±25%. Bagged 80-lb mix runs $5-$8 per bag at home centers, which works out to $220-$360 per cubic yard equivalent. Bagged mix is more expensive per yard but avoids delivery and short-load fees.

The slab itself is only part of total project cost. Forms (lumber and stakes) add $1-$3 per linear foot. Rebar or mesh adds $50-$150 per cubic yard. Labor, if you hire it, adds $4-$8 per square foot for a finished slab including form work, pour, and finishing. A DIY 20 ft × 10 ft 4-inch patio runs roughly $500-$700 in materials alone; hiring it out lands at $1,500-$2,500.

Waste factor and extra yardage

Concrete losses happen at every stage. Spillage during pour, base absorption (especially over dry compacted gravel), form deflection, and last-minute adjustments to grade all consume more material than the rectangular volume calculation suggests. Industry standard adds a 10% waste factor to every order.

For tight DIY pours into preformed boxes, 5% is often enough. For irregular shapes, multiple sections poured at once, or first-time work, push to 15%. The cost of over-ordering by 0.5 yd³ ($70-$100) is much lower than the cost of running short during a pour — partial second deliveries cost short-load fees, and a cold-joint pour creates a visible seam in the finished slab.

Rebar, mesh, and curing

Reinforcement keeps cracks from spreading. For driveways and garage floors, #4 rebar (1/2 inch diameter) on 12 to 18 inch centers in both directions handles residential loads. Welded wire mesh (6 inch by 6 inch grid, W1.4 wire) suffices for patios and sidewalks under light foot traffic. Either must be elevated to mid-slab depth using rebar chairs or doblies — rebar lying on the gravel base does almost nothing.

Curing matters as much as reinforcement. Fresh concrete needs to stay damp for 7 days minimum, ideally 14, to develop full strength. Cover with plastic sheeting, wet burlap, or apply a liquid curing compound right after finishing. Skipping cure increases shrinkage cracking by 30-50% and reduces the slab's ultimate compressive strength from a target of 3,000-4,000 psi down to 2,000-2,500 psi.

Common concrete slab mistakes

The first mistake is under-ordering. Adding a 10% waste factor is non-optional. The second is mixing inches with feet without converting — a 6-inch thickness entered as "6" instead of "0.5" gives a volume 12 times too large. The third is choosing thickness by feel instead of by code: ACI 332 specifies minimums for a reason, and undersizing leads to cracking within the first year.

The fourth mistake is skipping the gravel base. A 4-inch compacted gravel base under a 4-inch slab makes the slab perform like a 6-inch slab on bare soil. The fifth is pouring in extreme temperatures: concrete cures poorly below 40°F (use heated blankets) and above 90°F (use evaporation retardants and pour at dawn). The sixth is finishing too early, before bleed water has evaporated, which traps water under the surface and produces dusting and flaking later.