Concrete Calculator
This concrete calculator is the shape tool next to the slab, footing, and post-hole pages. Slab or footing is a rectangle: length times width times depth. Column or tube is a cylinder: pi times the radius squared times the height. The post is not subtracted. Solid steps are count times tread depth times width times rise. Ready-mix is the wasted volume converted to cubic yards and rounded up to the next 0.25 yard. That is an ordering convention, not a price.
Last reviewed: 2026-10-28 · How we calculate
How we calculate
Rectangle: length × width × depth. Column: π × (diameter ÷ 2)² × height. The diameter uses the unit shown beside the field. Steps: number × tread depth × width × rise, all in feet, or metres on the metric toggle, as solid rectangles with no gap. Volume with waste = volume × (1 + waste %). Cubic feet = wasted cubic metres ÷ 0.3048³. Ready-mix yards = wasted cubic metres ÷ (0.3048³ × 27), rounded up to the next 0.25. Bags = wasted volume ÷ litres per bag, and only when that litre field is above zero.
Worked example
A 10 × 10 ft slab at 4 in is 1.23 yd³ before waste and 1.25 yd³ of ready-mix after quarter-yard rounding.
A slab, a post hole, and solid steps
A patio slab, a fence-post hole, and a set of solid steps are three different shapes, and this page keeps them on one form so you do not add a slab calculator to a post-hole calculator by hand. Choose the shape, fill only the dimensions that shape uses, and read yards and 80 lb bags. The 80 lb yield is the checked QUIKRETE 0.60 cubic foot figure. A sonotube and a square column are both the cylinder path if you enter the outside diameter.
Measuring a concrete shape
A general concrete calculator is useful when the job is not only a patio slab. The shape control picks one formula. A slab or footing is the rectangle you already know from the slab and footing pages: length times width times depth, with depth in millimetres so 100 mm is a tenth of a metre. A 2 m square at 100 mm is 0.40 cubic metres. A column or a tube is a cylinder. Diameter is millimetres, so 200 mm is a radius of 0.1 m, and pi times 0.01 times a 1 m height is about 0.0314 cubic metres, shown as 0.03 after the usual two-decimal rounding. The post that will stand in that hole is not removed from the volume. The post-hole page works the same way. If you truly need only the concrete around a post, this is the wrong subtraction and you should measure the ring yourself. Solid steps are blocks. Three steps, each 0.25 m deep, 1 m wide, and 0.18 m high, are 0.135 cubic metres, shown as 0.14. There is no nosing and no open riser in that product.
How a slab, column, or steps become yards
Waste is a percentage on the shape volume. It starts at 0 on this page so the net shape is visible. Ready-mix rounding is separate and always applied to the wasted volume. Cubic yards equal cubic metres divided by 0.3048 cubed, then divided into yards by the 27 cubic feet in a yard. The order line then jumps to the next 0.25 yard. 0.40 cubic metres is about 0.523 yards, and the next quarter yard is 0.75. That quarter yard is a common way to talk to a plant. It is not their price list and it is not a promise they sell in quarter yards. Ask them. Cubic feet are the same wasted volume divided by 0.3048 cubed, so 0.40 cubic metres is 14.13 cubic feet.
Ordering ready-mix and 80 lb bags
The 16.99 litre default matches about 0.60 cubic feet, the yield on the QUIKRETE 80 lb Concrete Mix No. 110180 Home Depot listing checked on 24 October 2026 (https://www.homedepot.com/p/QUIKRETE-80-lb-Concrete-Mix-110180/100318511). Edit it if your bag prints a different yield. This is not a price. On a net 0.40 cubic metre pour, 16.99 litres per bag is 0.01699 cubic metres per bag. 0.40 divided by 0.01699 is about 23.5, so the bag line rounds up to 24. A separate check with a 20 litre bag is an even 20 bags, which is how the unit test audits the division. If your bag prints a different yield, type that yield and ignore 24. Sixty-pound and forty-pound yields are not cited. Both fields start at 0 and those bag lines stay 0. Type a yield only from the bag you are holding.
Checking a concrete order
Screed is not this concrete. A floor screed has its own page and its own bag yield, which also starts at 0. A room addition reports slab volume with no bags. Use this page when you are ready to turn that volume into bags or a ready-mix order. Footings with a thickened edge are not a single rectangle. Split the deep strip and the thin slab into two shapes and add the volumes. Steps that are hollow in the middle are not the solid model. The stair calculator checks rise and going. It does not pour the steps. None of these lines is a price per yard or a price per bag.
Concrete fields and starting values
Shape starts at 0. Choose which dimensions the volume uses. The other fields stay in the form but only the selected shape is multiplied. Length starts at 10 ft. Slab length in feet. Switch to m / cm for metres. Width starts at 10 ft. Slab width in feet. Switch to m / cm for metres. Depth starts at 4 in. Slab depth in inches, or millimetres on the metric toggle. Diameter starts at 8 in. Column or hole diameter in inches, or millimetres on the metric toggle. Height starts at 3 ft. Column height in feet. Switch to m / cm for metres. Number of steps starts at 3. How many solid rectangular steps. Open risers are not this model. Tread depth starts at 10 in. Step tread depth in inches. Step width starts at 3 ft. Step width in feet. Switch to m / cm for metres. Rise per step starts at 7 in. Step rise in inches. Waste allowance starts at 0 %. Added after the shape volume. Leave 0 to see the net shape. 80 lb bag yield starts at 0.6 cu ft. 0.60 cu ft matches the QUIKRETE 80 lb Concrete Mix listing checked on 24 October 2026. Edit it if your bag differs. 60 lb bag yield starts at 0.45 cu ft. 0.45 cu ft (12.7 L) is the yield QUIKRETE prints for a 60 lb bag of Concrete Mix, product No. 1101, data sheet revised October 2022. 40 lb bag yield starts at 0.3 cu ft. 0.30 cu ft (8.5 L) is the yield QUIKRETE prints for a 40 lb bag of Concrete Mix, product No. 1101, data sheet revised October 2022.
Concrete yards and 80 lb bags for a slab
Waste is 0. The 80 lb count uses 16.99 L per bag, about 0.60 cu ft, from the QUIKRETE Concrete Mix No. 110180 listing checked 24 October 2026.
| Slab | Depth | Ready-mix | 80 lb bags |
|---|---|---|---|
| 10×10 ft | 4 in | 1.25 yd³ | 56 |
| 12×12 ft | 4 in | 2.00 yd³ | 81 |
| 20×10 ft | 6 in | 3.75 yd³ | 167 |
Ready-mix rounds up to the next quarter yard. Sixty-pound and forty-pound bags stay off because those yields were not checked.
Common mistakes
- Reading a 60 lb or 40 lb bag count while the litre field is still 0. Those counts are 0 until you type a yield.
- Subtracting the post from a column. This model, like the post-hole page, fills the whole cylinder.
- Using the step model for hollow or open-riser stairs. It is solid rectangles only.
- Treating the quarter-yard ready-mix line as a price or as a minimum order the plant must accept.
- Leaving the 80 lb yield at 16.99 when your bag prints a different number.
- Mixing up depth in millimetres on a slab with height in feet, or metres on the metric toggle on a column.
Frequently asked questions
How many bags of concrete do I need?
Divide the volume with waste by the litres one bag yields. The 80 lb field starts from the checked QUIKRETE listing. The 60 lb and 40 lb fields start at 0.
How do I round ready-mix?
Convert cubic metres to cubic yards and round up to the next 0.25 yard. A net 0.40 m³ becomes 0.75 yd³ on that convention. Confirm the plant's actual increment.
Does a column subtract the post?
No. The volume is the full cylinder. That matches the post-hole calculator.
Can I calculate a strip footing?
Yes. Use the slab or footing shape and enter the trench length, width, and depth. The separate footing page does the same rectangle.
What about steps?
Count times tread depth times width times rise, as if each step were a solid block. It is not a stair-code check.
Is 16.99 litres exact?
It is 0.60 cubic feet converted with 0.3048 metres per foot, rounded to two decimals for the field. The listing was checked on 24 October 2026. Edit the field to the bag in your hand.