About the Concrete Calculator
A concrete calculator computes the cubic yards (or cubic meters) of concrete needed for slabs, footings, columns, walls, and similar pours. Concrete is sold by volume, so accurate measurement is essential — over-ordering wastes money and the leftover concrete is hard to use; under-ordering halts the pour and forces a much-more-expensive short delivery. Add a 5–10% waste factor on top of the calculated volume.
How concrete is sold and the standard waste factor
Ready-mix concrete is sold by the cubic yard in the U.S. (cubic meter elsewhere). One cubic yard equals 27 cubic feet — a 9-foot × 9-foot × 4-inch slab is exactly 1 cubic yard. Most ready-mix companies have minimum-load fees and per-yard pricing; a 'short load' under 3 cubic yards typically incurs a delivery surcharge.
Always order 5–10% more than calculated. Forms can shift slightly during the pour, ground can absorb water or have small variations in depth, and stopping mid-pour to order more concrete is far more expensive than ordering a slight surplus. A residential 4-inch slab calculation showing 5.5 cubic yards should be ordered at 6 cubic yards.
Common project volumes
Slab on grade: length × width × depth. A 20-ft × 20-ft × 4-inch slab: 400 sq ft × 0.333 ft = 133.3 cubic feet ÷ 27 = 4.94 cubic yards (order 5.5).
Footings: typically 12–24 inches wide and 12–24 inches deep, at lengths matching the foundation perimeter. A 100-foot perimeter at 16 inches wide × 12 inches deep: 100 × 1.33 × 1.0 = 133 cu ft ÷ 27 = 4.93 cubic yards.
Sonotube columns: π × (radius)² × height. A 12-inch-diameter, 4-foot-deep column: π × 0.5² × 4 = 3.14 cubic feet, or 0.116 cubic yards. Multiple columns add up — 6 columns at this size = 0.7 cubic yards (round up to 1 for delivery).
Walls and steps require more careful measurement, especially when stepped or sloped. Break complex shapes into rectangles, multiply each, and sum.
Mix specs and what they mean
Concrete mix is specified by compressive strength in pounds per square inch (psi) at 28 days of cure. Common residential mixes: 3,000 psi (general purpose), 3,500 psi (typical residential foundations), 4,000 psi (heavy-duty slabs, exposed concrete), 5,000 psi (commercial). Higher strength means higher cost per yard.
Slump — measured in inches — describes how 'wet' the mix is. Lower slump (3–4 inches) is stiffer, used for slabs and footings where workability matters less; higher slump (5–6 inches) is wetter, easier to pump and finish but slightly weaker. The supplier matches slump to your project; communicating what you're pouring helps them deliver the right mix.
Air-entrained concrete (with tiny dispersed air bubbles) better resists freeze-thaw damage in cold climates. Required for exterior concrete in most northern states. Specify when ordering for any outdoor pour that will see freezing temperatures.
DIY vs. ready-mix: when bag concrete makes sense
Bagged concrete (60-lb or 80-lb bags from home stores) is much more expensive per cubic yard than ready-mix but suits small projects (fence posts, single sonotubes, repair patches) where the minimum ready-mix load wouldn't make sense. An 80-lb bag yields about 0.6 cubic feet — 45 bags equal one cubic yard.
Cost crossover: ready-mix typically costs $130–$200 per cubic yard delivered (regionally variable, plus delivery and minimum fees). Bagged concrete equivalent for a cubic yard: 45 × $5–$7/bag = $225–$315 plus the labor to mix it. For projects above ~1.5 cubic yards, ready-mix is usually cheaper and easier; below that, bagged often makes sense.
Formula
- 27 = Cubic feet per cubic yard (3 ft × 3 ft × 3 ft)
- depth (ft) = Depth in feet — 4 inches = 0.333 ft, 6 inches = 0.5 ft
- Cubic meters = Volume in m³ = length (m) × width (m) × depth (m); 1 cubic yard ≈ 0.7646 cubic meter
Worked examples
Driveway slab
20-ft × 30-ft driveway, 4-inch slab. Volume: 20 × 30 × 0.333 = 200 cubic feet. Cubic yards: 200 / 27 = 7.41. Order with 10% waste: ~8.2 cubic yards (round to 8.5 for delivery).
Strip footing for a garage
Perimeter 96 ft (24×24 garage), footing 16" wide × 12" deep. Volume: 96 × 1.333 × 1.0 = 128 cubic feet ÷ 27 = 4.74 cubic yards. With 10% waste: 5.2 cubic yards.
Sonotube columns for a deck
Six 10-inch-diameter sonotubes, 48 inches deep. Each: π × (5/12)² × 4 = π × 0.174 × 4 = 2.18 cubic feet. Six columns: 13.1 cubic feet ÷ 27 = 0.49 cubic yards. Likely below ready-mix minimum — bagged concrete probably more economical.
Frequently asked questions
How much waste should I add to my concrete order?
Typically 5–10% extra. Forms can shift, ground depth can vary slightly, and the cost of running short mid-pour (forcing a much-more-expensive partial delivery) far exceeds the cost of slight over-ordering. For complex pours or on-grade slabs, lean toward 10%.
Why is concrete sold by the cubic yard?
It's the traditional volume unit for ready-mix in the U.S. Trucks are sized in cubic yards (typical mixer carries 8–10 yards). Cubic meters are used in metric countries; the conversion is 1 cubic yard = 0.7646 cubic meter.
Should I use 3,000 psi or 4,000 psi concrete?
Depends on the application. 3,000 psi works for general slabs, sidewalks, and basic footings. 4,000 psi is recommended for foundations supporting structures, heavy-duty slabs (driveways used by large trucks), and exposed concrete that will see freeze-thaw cycles. Higher psi costs more per yard but provides longer service life and durability.
What's the difference between concrete and cement?
Cement is the binder (Portland cement is the most common type). Concrete is the finished material — cement plus sand, gravel (aggregate), and water. Concrete contains cement; the terms aren't interchangeable. 'Cement truck' is a common misnomer; the trucks deliver concrete.
Can I mix my own concrete instead of ordering ready-mix?
Yes, for small projects. 80-lb bags from home stores are convenient for fence posts, small slabs, repair patches. Mixing larger volumes by hand or with a small mixer is labor-intensive and quality is less consistent than ready-mix. Above ~1.5 cubic yards, ready-mix is almost always faster, cheaper, and higher quality.
How long does concrete take to cure?
Concrete reaches roughly 70% of its 28-day strength within 7 days and most of its full strength by 28 days. You can typically walk on a slab after 24 hours, drive light vehicles on it after 7 days, and load it fully after 28 days. Curing isn't drying — the concrete needs water for the chemical reactions, so keeping the surface damp during the first week (especially in hot weather) improves final strength.