Free concrete calculator. Enter slab dimensions to get concrete volume in cubic yards and the 80-lb bags needed, incl. 10% waste.
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Volume is length × width × depth converted to cubic yards (/27). One cubic yard of concrete needs about 45 eighty-pound bags. A 10% waste allowance is added for spillage and uneven subgrade.
Concrete is ordered by volume in cubic yards. For any rectangular slab, the formula is:
Cubic yards = length (ft) × width (ft) × thickness (ft) / 27
Since thickness is usually given in inches, divide it by 12 first. Bag counts come from the bag yield: an 80-lb bag of mix yields about 0.60 cubic feet, so 27 / 0.60 = 45 bags per cubic yard.
Worked example — a 10 × 10 ft patio slab, 4 in thick, with 10% waste:
1. Convert thickness: 4 / 12 = 0.333 ft 2. Volume: 10 × 10 × 0.333 = 33.33 cubic feet 3. Cubic yards: 33.33 / 27 = 1.23 yd3 4. Add 10% waste: 1.23 × 1.10 = 1.36 yd3 to order 5. 80-lb bags: ceil(1.23 × 45 × 1.10) = 62 bags
Columns and round footings use the same idea with cylinder geometry: pi × radius² × height.
Every concrete estimator follows this same volume-first workflow: measure, convert units, divide by 27, then add waste.
1. Enter the slab length and width in feet. 2. Enter the thickness (depth) in inches — 4 in is the standard residential default. 3. Your results appear instantly: cubic yards to order (with 10% waste built in), cubic feet, and the number of 80-lb bags.
Measure to the nearest inch for the best accuracy, and always double-check an oddly shaped area by breaking it into rectangles first.
A standard 80-lb bag of premix yields about 0.60 cubic feet of concrete, so the bags needed for one full cubic yard (27 cubic feet) are:
| Bag size | Yield per bag | Bags per cubic yard | |---|---|---| | 80 lb | 0.60 cu ft | 45 | | 60 lb | 0.45 cu ft | 60 | | 40 lb | 0.30 cu ft | 90 |
Always round up to whole bags — you cannot buy part of a bag — and check the yield printed on your specific mix, since brands vary slightly.
For small jobs, bagged concrete is cheaper and far more convenient — no truck minimum, no coordination. Once a job crosses about half a cubic yard, ready-mix delivery usually wins: it costs less per yard, arrives pre-mixed, and sets more consistently.
The numbers: 45 bags of 80-lb mix make one yard, so bagged material alone runs $270-$360 per equivalent yard plus a full day of mixing. A ready-mix truck costs roughly $130-$190 per delivered yard but charges a $50-$150 short-load fee on small orders. Under ~0.5 yd3 (a 4x4 or 5x5 slab, or a handful of footings), bags make sense; above it, price both.
The right thickness depends on the load the slab will carry:
- Patios, walkways and shed floors: 4 inches - Light-duty paths: 3-4 inches - Driveways and garage floors carrying vehicles: 5-6 inches - Structural slabs and heavy loads: 6 inches or more, with rebar
Going from 4 to 6 inches raises material cost by about 50%, so run both thicknesses through the calculator before you commit. Always check local building code minimums for your project.
Every experienced contractor orders about 10% more than the raw volume, and the calculator builds this in automatically. The buffer covers three real costs:
Cold joints. Concrete starts setting in about 90 minutes. Running short mid-pour creates a seam between the first and second batches — a weak point that is likely to crack.
Short-load fees. If you underestimate and need a top-up, the plant charges a minimum-order or short-load fee that can cost more than the extra concrete itself.
Overpour and losses. The subgrade is never perfectly level, forms flex, and concrete is lost to spillage and wheelbarrow residue. A half-inch of extra depth across a 10x10 slab adds 0.15 yd3 on its own.
Concrete estimators and contractors treat the 10 percent waste factor as standard practice — ordering exactly the calculated volume is the most common estimating mistake on DIY pours.
Cubic yards for common slab sizes at 4 inches thick (before waste — the calculator adds 10% on top):
| Slab size | Cubic yards | 80-lb bags | |---|---|---| | 8 × 8 ft | 0.79 | 36 | | 10 × 10 ft | 1.23 | 56 | | 12 × 12 ft | 1.78 | 80 | | 16 × 20 ft | 3.95 | 178 | | 20 × 20 ft | 4.94 | 223 | | 24 × 24 ft | 7.11 | 320 |
Thicker slabs scale linearly: multiply the figures above by 1.25 for 5 inches and 1.5 for 6 inches.
About 45 eighty-pound bags (or 60 sixty-pound bags) yield one cubic yard of concrete. An 80-lb bag yields roughly 0.60 cubic feet, and 27 cubic feet / 0.60 = 45. Always round up and add a waste allowance.
A 10x10 ft slab at 4 inches thick is 33.33 cubic feet, or 1.23 cubic yards. Add the 10% waste factor and order about 1.36 yards — roughly 62 bags of 80-lb mix.
To avoid cold joints and short-load fees. The subgrade is never perfectly level, forms flex, and concrete is lost to spillage and screeding. Ordering 10% extra costs a few dollars per yard and keeps you from running short mid-pour.
Under about half a cubic yard (a 4x4 or 5x5 slab, or a handful of footings), bags are cheaper because the truck's short-load fee dominates. Above that, ready-mix is usually cheaper per yard and saves a full day of hand mixing. Price both at your volume.
Residential patios and sidewalks are typically poured at 4 inches. Driveways and garage floors that carry vehicles need 5-6 inches, and structural slabs 6 inches or more with rebar. Always check local code minimums.
A cubic yard is 27 cubic feet — a cube measuring 3 ft on each side. Ready-mix concrete is priced and ordered in cubic yards; at 4 inches thick, one yard covers about 81 square feet.
Cement is an ingredient in concrete — the fine grey powder (Portland cement) that acts as a binder. Concrete is the finished material made by mixing cement with water, sand and gravel. You pour concrete; you don't pour cement.
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