Concrete Calculator
Estimating concrete accurately before ordering saves money and avoids the cost of short-load surcharges or extra deliveries. This concrete calculator handles four common pour shapes: slabs (the most common), round columns, cardboard tube forms, and circular slabs such as those used for round patios or stepping stones. Enter your dimensions in feet and inches and the tool converts everything to cubic feet, then divides by 27 to give cubic yards. A 10% waste factor is applied automatically to account for spillage, sub-base irregularities, and residual concrete in the mixer. The results section also shows how many 60 lb and 80 lb bags of premix concrete you would need if you are not ordering ready-mix. A 60 lb bag yields approximately 0.45 cubic feet of cured concrete; an 80 lb bag yields approximately 0.60 cubic feet. For pours under about 0.5 cubic yards, bagged mix is usually more economical. For anything larger, get quotes from local ready-mix suppliers and compare the cost per cubic yard against the convenience of bagged mix. Always round up your order to the nearest quarter yard to avoid running short during a pour.
Concrete volume formulas
Slab: Volume (cu ft) = Length (ft) x Width (ft) x Thickness (ft)
Column: Volume (cu ft) = Length (ft) x Width (ft) x Height (ft)
Tube/Round column: Volume (cu ft) = pi x (Radius in ft)^2 x Height (ft)
Circular slab: Volume (cu ft) = pi x (Radius in ft)^2 x Thickness (ft)
Cubic yards = Cubic feet / 27
With 10% waste = Cubic yards x 1.10
Worked example: 10 ft x 12 ft driveway apron, 4 inches thick
- Thickness in feet: 4 in / 12 = 0.333 ft
- Volume: 10 x 12 x 0.333 = 40.0 cubic feet
- Cubic yards: 40.0 / 27 = 1.48 cu yd
- With 10% waste: 1.48 x 1.10 = 1.63 cu yd (order 1.75 cu yd)
- 80 lb bags (0.60 cu ft each): 1.63 x 27 / 0.60 = approx 74 bags
Concrete calculator: frequently asked questions
How many cubic yards do I need for a 10x10 slab at 4 inches thick?
Multiply length by width by depth, all in feet, then divide by 27. A 10x10 slab at 4 inches (0.333 ft) deep = 10 x 10 x 0.333 = 33.3 cubic feet / 27 = 1.23 cubic yards. Add a 10% waste factor and you need about 1.36 cubic yards. This calculator does all of that for you automatically.
How many 80 lb bags of concrete make a cubic yard?
One 80 lb bag of premix concrete yields approximately 0.60 cubic feet of cured concrete. There are 27 cubic feet in a cubic yard, so you need roughly 45 bags of 80 lb concrete per cubic yard. For 60 lb bags, which yield about 0.45 cubic feet each, you need approximately 60 bags per cubic yard. This calculator shows both estimates.
Should I add a waste factor when ordering concrete?
Yes. It is standard practice to add at least 10% to your calculated volume to account for spillage, uneven sub-base, over-excavation, and residual concrete left in the mixer drum. For odd-shaped areas or rough sub-bases, some contractors add 15 to 20%. This calculator includes a 10% waste factor in all results by default.
What is the difference between cubic feet and cubic yards for concrete?
One cubic yard equals exactly 27 cubic feet (3 ft x 3 ft x 3 ft). Concrete ready-mix trucks are priced and ordered in cubic yards. Premix bags are sized in cubic feet of yield. This calculator shows both units so you can order ready-mix by the yard or count bags for smaller pours.
When should I use ready-mix versus bagged concrete?
Ready-mix (delivered by truck) is cost-effective for pours of 1 cubic yard or more. Below that threshold, bagged premix is usually more economical because minimum ready-mix orders carry a short-load surcharge. For very large projects (driveways, foundations), always use ready-mix. For small footings, fence posts, or repairs, 60 lb or 80 lb bags are more practical.
Sources
- Portland Cement Association: Concrete Construction.
- US DOT Federal Highway Administration: concrete design references at fhwa.dot.gov.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology. For structural pours consult a licensed engineer.