Calculate cubic yards, bags, and costs for slabs, footings, post holes, stairs — with a live 3D pour preview.
Calculating concrete volume is one of the most foundational skills in construction estimating, yet it's one of the most commonly done incorrectly. Over-ordering ready-mix concrete costs real money — a cubic yard of 3000 PSI concrete runs $130–$200 depending on your region. Under-ordering means sending the truck away and scheduling a second pour with a cold joint seam that compromises structural integrity. Getting the number right matters.
All concrete volume calculation reduces to a single principle: multiply the area of the pour by its depth. For a rectangular slab, that is length × width × depth. The result comes out in cubic feet (assuming measurements in feet and inches), which you then divide by 27 to convert to cubic yards — since one cubic yard equals 27 cubic feet. Always add a waste factor of 5–10% for slabs and 8–15% for footings and irregular forms, because fresh concrete spreads into irregularities in the sub-base, forms are never perfectly sized, and some spill and waste is inevitable during the pour.
A 10 × 10 foot patio at 4 inches depth: 10 × 10 × (4/12) = 33.33 cubic feet ÷ 27 = 1.23 cubic yards. With 10% waste: 1.36 cubic yards. Round up to the nearest quarter yard when ordering ready-mix. For bags, 1.36 cubic yards = 36.7 cubic feet. At 0.45 cubic feet per 60-lb bag, that is 82 bags. At 0.60 cubic feet per 80-lb bag, that is 62 bags.
| Application | Thickness | Min PSI | Rebar? | Notes |
|---|---|---|---|---|
| Sidewalk / walkway | 4" | 3000 | Optional | #3 bar at 18" OC if desired |
| Residential patio | 4" | 3000 | Optional | Fiber mesh is a good alternative |
| Residential driveway | 6" | 3500 | Recommended | #4 bar at 12" OC both ways |
| Garage floor | 6" | 4000 | Yes | #4 at 12" OC; vapor barrier required |
| Structural slab | 8–12" | 4000 | Required | Engineer design required |
| Strip footing | 8–12" deep | 3000 | 2 bars cont. | Width = 2× wall thickness |
| Column / pier | 12–24" dia | 3000 | Optional | Below frost line minimum |
The decision between mixing bags and ordering ready-mix concrete comes down to one number: volume. For pours under approximately 0.5 cubic yards (about 13.5 cubic feet), bags are almost always more economical when you factor in the ready-mix truck's minimum load fee and short-load surcharge. Most ready-mix suppliers charge a short-load fee of $50–$150 for orders under 3 cubic yards because it is not economical for them to dispatch a full truck for a small pour. For projects between 0.5 and 1 cubic yard, the economics are close enough that local pricing, rental costs for a mixer, and labor time become the deciding factors.
For pours over 1 cubic yard, ready-mix is almost universally more economical and produces a more consistent mix than hand-mixing bags. Ready-mix also arrives in a single batch with a consistent water-to-cement ratio, which is critical for large structural pours where cold joints (the seam between a hardened and a fresh pour) must be avoided. A standard ready-mix truck carries 8–10 cubic yards and can pour at approximately 1 yard per minute through a chute, enabling rapid placement before initial set time (typically 30–60 minutes depending on temperature and admixtures).
Reinforcing bar (rebar) is placed in a grid pattern at a consistent spacing measured on-center (OC). For a residential driveway at 12 inches OC with #4 rebar, you run bars the full length of the slab every 12 inches, then run bars the full width every 12 inches, creating a grid. The number of bars in each direction equals the dimension divided by the spacing plus one. Rebar comes in 20-foot sticks, so you divide total linear footage by 20 to get the number of sticks, adding a 10% overlap factor because each splice requires an 18–24 inch overlap. Rebar chairs (plastic spacers) hold the grid at the correct height — typically at 1/3 of the slab depth from the bottom for slabs, or centered for footings.
Slabs are the most common residential pour, but footings, post holes, and stairs each have their own calculation method — and their own failure modes when under-engineered. The most frequent residential concrete failure is not a slab crack but a footing that is too shallow or too narrow, allowing frost heave to shift the structure above it.
Strip footings (also called continuous footings) run the full length of walls — both bearing walls and foundation walls. The footing is always wider than the wall it supports, typically twice the wall thickness. A standard 8-inch concrete block wall sits on a footing 16 inches wide and 8–12 inches deep. The footing depth must always extend below the frost line for your climate zone. In Minneapolis, that means 42 inches; in Atlanta, 12 inches is sufficient. The footing must be at least as deep as it is wide — so a 16-inch-wide footing must be at least 16 inches deep. Typical residential footings contain two continuous #4 rebar bars running the full length, lapped 24 inches at corners and splices.
Post holes are cylindrical, so their volume is calculated using the cylinder formula: π × radius² × depth. A standard 4×4 fence post (3.5 inches actual) requires a hole 8–10 inches in diameter and at least one third of the total post length deep. For a 6-foot fence (posts at 8 feet above grade, 2.5 feet below), that means a 30-inch deep hole. The post itself displaces some concrete volume — deduct π × (post radius)² × depth to get the net concrete needed. For 10 post holes at 8 inches diameter and 30 inches deep, the math is: π × 4² × 2.5 = 125.7 cubic inches per hole × 10 holes = 0.73 cubic feet per hole × 10 = 7.3 cubic feet = 10 bags of 80-lb Quikrete (0.60 cu ft each).
Concrete stair volume is calculated per step as a triangular prism. Each step has a rise (vertical height) and a run (horizontal depth). The cross-sectional area of each stair, treated as a right triangle, is (rise × run) / 2. The total triangular cross-section for all steps is: for step 1, triangle = rise×run/2; for step 2, triangle = (2×rise)×run/2; and so on. The total volume is the sum of all triangular areas times the stair width. In practice, most estimators calculate the full rectangular block (total height × total run × width) and divide by 2, since a staircase is roughly half the rectangle it occupies. A 3-step stair at 7-inch rise, 11-inch run, 48-inch width: total height = 21 inches, total run = 33 inches. Block volume = 1.75 × 2.75 × 4 feet = 19.25 cu ft ÷ 2 = 9.6 cu ft × a landing factor ≈ 0.36 cubic yards.
| Structure | Formula | Key Variable | Common Mistake |
|---|---|---|---|
| Slab | L × W × D ÷ 27 | Depth in feet | Forgetting to convert inches to feet |
| Strip Footing | L × W × D ÷ 27 | Width must ≥ 2× wall | Footing above frost line |
| Column / Pier | π × r² × D ÷ 27 | Radius in feet | Too small diameter |
| Post Hole | π × r² × D − post volume | Net volume per hole | Forgetting to deduct post |
| Stairs | (R × Run × W × n²) ÷ 2 | n = number of steps | Using rectangular not triangular calc |
Concrete compressive strength (PSI) is the force per square inch required to crush a 4-inch × 8-inch cylinder of the cured concrete at 28 days. The water-to-cement ratio is the single most important factor in achieving target strength — lower ratio means stronger concrete but less workable mix. For 3000 PSI concrete, the water-to-cement ratio is approximately 0.50 by weight. For 4000 PSI, it is approximately 0.44. The aggregate size also matters: 3/4-inch maximum aggregate is standard for most residential pours. Smaller aggregate (3/8-inch pea gravel) is used for tight forms and thin sections but costs more per yard.
Air-entrained concrete is required in any climate that experiences freeze-thaw cycles. Air-entraining admixtures create microscopic air bubbles that give water room to expand when it freezes, preventing spalling and scaling of the surface. The specified air content for freeze-thaw exposure is typically 5–7% total air. When you order ready-mix in a cold climate, always specify air-entrained and confirm the percent target with your supplier. Non-air-entrained concrete will scale and spall within a few freeze-thaw cycles in climates with road salt or repeated temperature cycling.