Concrete Calculator
Estimate the concrete needed for slabs, footings, walls, columns, tubes, curbs and stairs in cubic yards, cubic feet or cubic meters, with waste allowance, pre-mix bag counts, weight and cost.
Change the values and press Calculate to work out your own figures.
Slab, square footing or wall (rectangular block)
Length × width × thickness, for patios, driveways, floors and square footings. Tick the thickened edge to add a deeper footing around the outside.
Show the working
- Measurements in feet: length 10 ft, width 10 ft, thickness 0.333333 ft
- Volume = length × width × thickness = 33.33 cu ft
- 33.33 cu ft ÷ 27 = 1.23 cu yd = 0.94 m³
- Bags: 33.33 cu ft ÷ 0.60 cu ft per 80 lb bag = 55.56, rounded up to 56
- Cost = 1.23 cu yd × $150 = $185.19
Round slab
Enter the diameter (the distance across) and thickness. Tick the thickened edge to add a deeper ring around the outside.
Show the working
- Measurements in feet: diameter 10 ft, thickness 0.333333 ft
- Volume = π/4 × diameter² × thickness = 26.18 cu ft
- 26.18 cu ft ÷ 27 = 0.97 cu yd = 0.74 m³
- Bags: 26.18 cu ft ÷ 0.60 cu ft per 80 lb bag = 43.63, rounded up to 44
Round slab with hole or tube
For round forms with a hole, such as tube forms around a post. Use an inner diameter of 0 for a solid cylinder.
Show the working
- Measurements in feet: outer diameter 5 ft, inner diameter 4 ft, height 0.5 ft
- Volume = π/4 × (outer² − inner²) × height = 3.53 cu ft
- 3.53 cu ft ÷ 27 = 0.13 cu yd = 0.10 m³
- Bags: 3.53 cu ft ÷ 0.60 cu ft per 80 lb bag = 5.89, rounded up to 6
Continuous footing
A long strip footing under a wall. Tick stem wall to add the short wall poured on top.
Show the working
- Measurements in feet: length 20 ft, footing width 1.33333 ft, footing depth 0.666667 ft
- Volume = length × footing width × footing depth = 17.78 cu ft
- 17.78 cu ft ÷ 27 = 0.66 cu yd = 0.50 m³
- Bags: 17.78 cu ft ÷ 0.60 cu ft per 80 lb bag = 29.63, rounded up to 30
Wall
Length × height × thickness of a poured wall.
Show the working
- Measurements in feet: length 10 ft, height 4 ft, thickness 0.666667 ft
- Volume = length × height × thickness = 26.67 cu ft
- 26.67 cu ft ÷ 27 = 0.99 cu yd = 0.76 m³
- Bags: 26.67 cu ft ÷ 0.60 cu ft per 80 lb bag = 44.44, rounded up to 45
Hole, column, pier or round footing
For post holes, piers and round columns. Tick bell footing if the bottom of the hole is flared wider.
Show the working
- Measurements in feet: diameter 2.5 ft, height 6 ft
- Volume = π/4 × diameter² × height = 29.45 cu ft
- 29.45 cu ft ÷ 27 = 1.09 cu yd = 0.83 m³
- Bags: 29.45 cu ft ÷ 0.60 cu ft per 80 lb bag = 49.09, rounded up to 50
Curb and gutter
A curb with an attached gutter flag, measured along its length.
Show the working
- Measurements in feet: length 10 ft, curb depth 0.333333 ft, curb height 0.333333 ft, gutter width 0.833333 ft, flag thickness 0.416667 ft
- Volume = curb L × depth × (height + flag) + gutter L × width × flag = 5.97 cu ft
- 5.97 cu ft ÷ 27 = 0.22 cu yd = 0.17 m³
- Bags: 5.97 cu ft ÷ 0.60 cu ft per 80 lb bag = 9.95, rounded up to 10
Stairs
Solid steps poured on the ground. The top step uses the platform depth when it is deeper than the others.
Show the working
- Measurements in feet: width 3 ft, rise 0.583333 ft, run 0.916667 ft, platform depth 3 ft, steps 3
- Volume = width × rise × (run × n(n − 1)/2 + platform × n) = 20.56 cu ft
- 20.56 cu ft ÷ 27 = 0.76 cu yd = 0.58 m³
- Bags: 20.56 cu ft ÷ 0.60 cu ft per 80 lb bag = 34.27, rounded up to 35
How to estimate concrete
- Pick the shape that matches your pour: a slab, a round slab, a tube form, a strip footing, a wall, a post hole or column, a curb and gutter, or steps.
- Measure each dimension and enter it in the unit you used. Thickness boxes default to inches and lengths to feet; tape-measure readings such as 10' 6" work in any box set to feet.
- Use quantity for identical pours, such as eight post holes of the same size.
- Add extra for waste if you want a margin for an uneven base, spillage and form bulge. Many contractors add 5–10%.
- Read the volume to order, then the number of bags for each bag size, or enter a price to see the cost.
Ready-mix concrete is sold by the cubic yard (or cubic metre). For small jobs, bagged pre-mix is easier: each bag makes a set volume once water is added, so the calculator divides the volume by that yield and rounds up to whole bags.
Concrete volume formulas
Slab, footing or wall: length × width × thickness Round slab or post hole: π × diameter² ÷ 4 × depth Tube: π ÷ 4 × (outer diameter² − inner diameter²) × height Curb and gutter: length × (curb depth × (curb height + flag thickness) + gutter width × flag thickness) Stairs: width × rise × (run × n(n − 1) ÷ 2 + platform depth × n) Cubic yards = cubic feet ÷ 27
For example, a 10 × 10 ft patio 4 inches thick is 10 × 10 × 4/12 = 33.33 cubic feet, or 1.23 cubic yards. At 0.60 cubic feet per 80 lb bag that is 55.56 bags, so you would buy 56. At $150 per cubic yard, ready-mix for it would cost about $185.19 before delivery charges.
Bag sizes and yields
The bag counts use published yields for a common general-purpose concrete mix: 40 lb makes about 0.30 cubic feet, 50 lb about 0.375, 60 lb about 0.45, 80 lb about 0.60 and 90 lb about 0.675. Other products, especially fast-setting or high-strength mixes, can differ, so check the yield printed on your bag and enter it as a custom yield if it is different.
The dry mix weight is the weight of bagged pre-mix needed, based on 80 lb per 0.60 cubic feet (about 133 lb per cubic foot of finished concrete). It is not the weight of cured concrete, which is usually heavier.
Thickened edges, stem walls and bells
A thickened (monolithic) edge is a deeper strip poured at the same time as the slab, around its outside. The edge width is measured in from the outside, and the edge depth is how far it goes below the underside of the slab. A stem wall is the short wall poured on top of a strip footing. A bell footing is the flared base at the bottom of a post hole; the calculator treats it as a cone with its top cut off, inside the overall hole depth.
Questions
How many bags of concrete do I need for a 10 × 10 slab?
At 4 inches thick, a 10 × 10 ft slab needs 33.33 cubic feet of concrete: 56 bags of 80 lb, 75 bags of 60 lb or 112 bags of 40 lb, before any extra for waste.
Should I use bags or ready-mix?
Bags suit small pours such as post holes, steps and small pads. Above about one cubic yard (45 bags of 80 lb), ready-mix delivery is usually faster and more consistent, although suppliers may charge a short-load fee for small orders.
How much extra concrete should I order?
This calculator gives the exact geometric volume, with no extra unless you add a waste percentage. Many people add 5–10% to cover an uneven base, spillage and form bulge.
Sources
- OpenStax Prealgebra 2e, 9.6 Solve Geometry Applications: Volume and Surface Area
- OpenStax Calculus Volume 1, 6.2 Determining Volumes by Slicing
- OpenStax Prealgebra 2e, 9.4 Use Properties of Rectangles, Triangles, and Trapezoids
- NIST SP 811 Appendix B.9, conversion factors by kind (volume, mass, density)
- QUIKRETE Concrete Mix No. 1101 product data sheet
Limitations
- Bag yields are the manufacturer-published approximations for one brand's general-purpose concrete mix (40 lb 0.30, 50 lb 0.375, 60 lb 0.45, 80 lb 0.60, 90 lb 0.675 cu ft); actual yield varies by product, water content and compaction. The tool always shows this assumption next to bag counts and allows a custom yield.
- Dry-mix weight defaults to bag weight ÷ bag yield (400/3 lb per cu ft of finished concrete); it is not the density of hardened or ready-mix concrete.
- Volumes are geometric; no allowance for sub-base irregularity, form deflection or rebar displacement other than the waste percentage.
- Stairs are modelled as solid blocks on grade (no underside void or sloped soffit); the bell footing is modelled as a frustum inside the overall column height.
- Metric pre-mix bag sizes (e.g. 20 kg, 25 kg) are not listed; use the custom yield.
- Ready-mix minimum orders, short-load fees and delivery charges are not modelled.