Concrete quantity calculator

What a concrete calculator does

A concrete calculator works out the volume of concrete a pour needs, in cubic yards or cubic metres, from the length, width and depth of the form. This one handles slabs, walls, footings and round columns, adds a waste allowance, and converts the order volume into premix bags at the yield printed on the bag you plan to buy.

The basic formula is length × width × thickness. In imperial units, multiply the length and width in feet by the thickness in inches, divide by 12 to get cubic feet, then divide by 27 to get cubic yards. A 10 × 10 ft slab 4 in thick is 33.3 ft³, or 1.23 yd³.

Use the result to order ready-mix by the yard, to decide whether a small job is better done with bags, and to price the pour. It is a quantity tool only: it counts bars or mesh at the spacing you enter, but it does not size footings, choose a mix strength or design reinforcement.

How to use the concrete quantity calculator

1. Choose Imperial or Metric. In imperial mode, lengths are in feet, thickness and diameter in inches, and volumes in cubic yards. Your choice is remembered across the free tools.
2. Pick the Form: Slab, Wall, Footing, Circular column, Stairs or Curb / gutter. The fields change to match. A slab asks for length, width and thickness; a wall for length, thickness and height; a footing for length, width and depth; a column for diameter and height; stairs for the number of risers, riser height, tread depth and stair width (a solid block on a level base); a curb for run length and cross-section area.
3. Enter the Number of identical forms, for example 6 for six identical piers, a Waste allowance (10% by default) and the Mixer truck capacity (10 yd³ by default) to see how many loads the pour needs.
4. Pick a Premix bag preset (80, 60 or 40 lb; 25 or 20 kg, each with a typical label yield) or choose Custom yield and enter the figure printed on the bag, such as 0.60 ft³ for a typical 80 lb bag. Add an optional price per cubic yard (or m³) to get a cost for the order volume.
5. Set Reinforcement to Rebar grid to pick a bar size (US #3–#18, AS/NZS N bars or EU/UK B500), a spacing each way, an edge cover and the number of mats, or to Mesh sheets for a sheet area and lap allowance. Bars are laid out on the slab plan, wall face or footing plan.
6. Read the net concrete volume, the ready-mix volume with waste, premix bags, truck loads and the rebar length and mass or mesh sheets. Download a PDF or CSV, copy the result, or use Save inputs to reopen the calculation later.

Worked example

Imperial: A 20 × 12 ft slab, 4 in thick, needs 2.96 yd³ net or 3.26 yd³ with 10% waste.

Metric: A 6 × 4 m slab, 0.1 m thick, needs 2.4 m³ net or 2.64 m³ with 10% waste.

How much concrete do I need for a 10x10 slab?

A 10 × 10 ft slab needs 1.23 yd³ at 4 in thick, 1.54 yd³ at 5 in and 1.85 yd³ at 6 in, before waste. Add 10% and the order becomes 1.36, 1.70 and 2.04 yd³. The chart below uses the same formula as the calculator for other common slab sizes.

A useful shortcut: one cubic yard covers 81 ft² at 4 in thick, 64.8 ft² at 5 in and 54 ft² at 6 in. Divide your slab area by the figure for your thickness to check any quote in your head.

How many bags of concrete make a yard?

A cubic yard is 27 ft³. Standard concrete mix in 80, 60 and 40 lb bags is commonly labelled to yield about 0.60, 0.45 and 0.30 ft³ per bag, which works out at 45, 60 and 90 bags per cubic yard. Yields vary by product: high-strength, fast-setting and fiber-reinforced mixes can yield differently, so enter the figure printed on the bag you are buying.

Bags make sense for piers, post holes, steps and small pads. At 45 bags per yard, a 1 yd³ pour means mixing and lifting 3,600 lb of dry material, so larger slabs are usually cheaper and more consistent from a ready-mix truck. Many suppliers set a minimum order or charge a short-load fee for small loads, so ask before you choose.

How thick should a concrete slab be?

For slab-on-ground floors in houses, the 2021 International Residential Code (R506.1) sets a minimum thickness of 3-1/2 in. In practice, 4 in is the usual thickness for patios, walkways and shed bases, and 5 to 6 in is common where vehicles or heavier loads sit on the slab. Your local code, soil conditions and any engineer's design take priority over rules of thumb.

Thickness has a large effect on volume. Every extra half inch on a 4 in slab adds 12.5% more concrete, and an uneven subgrade can do that without anyone noticing. Check the depth at several points before the pour and raise the waste allowance if the base is rough.

• Patios, walks, shed bases: 4 in is typical.
• Driveways and garage floors: 5 to 6 in is common, often with thickened edges.
• Thickened edges and grade beams: enter them as a separate Footing and add the volumes.
• Walls and stem walls: use the Wall form with the wall thickness in inches.

Worked example: a shed slab and six deck piers

A 12 × 16 ft shed slab, 4 in thick, is 2.37 yd³ net and 2.61 yd³ with 10% waste. In 80 lb bags at 0.60 ft³ each that is 118 bags, which is a strong case for a ready-mix delivery.

The same project has six round piers, 10 in in diameter and 4 ft deep. Choose Circular column, enter a 10 in diameter, 4 ft height and 6 forms. The net volume is 0.49 yd³ and 0.53 yd³ with 10% waste, or 24 bags of 80 lb mix. The table shows the volume per foot of depth for common round form sizes so you can check pier and post-hole counts quickly.

For footings and walls the method is the same. A 100 ft run of footing 24 in wide and 12 in deep is 7.41 yd³ net. A 40 ft wall, 8 in thick and 4 ft high, is 3.95 yd³.

Who it is for and when to use it

Method, formulas & assumptions

Rectangular volume = length × width/thickness × depth/height. Circular column = π × (diameter/2)² × height. Solid stairs = width × riser × tread × n(n + 1) ÷ 2. Curb = length × section area. Order volume includes waste; bags, mixer loads, rebar (bars at the spacing inside the cover × nominal mass) and mesh sheets are derived from it.

Imperial inputs are converted with exact factors (1 ft = 0.3048 m, 1 lb = 0.45359237 kg, 1 US gal = 3.785411784 L) before the calculation runs in SI, then results are converted back.

Common mistakes to avoid

• Entering slab thickness in feet instead of inches. A 4 in slab entered as 4 ft is twelve times too large; in imperial mode the thickness field is already in inches.
• Ordering the exact net volume. Forms bow, subgrades dip and some concrete stays in the truck or pump. Running short mid-pour leaves a cold joint.
• Ignoring subgrade variation. Half an inch of extra depth on a 4 in slab adds 12.5% to the volume.
• Using the 80 lb bag yield for a different product. Fast-setting and high-strength mixes can yield less per bag than standard mix.
• Forgetting thickened edges, footings under the slab or turned-down edges on a monolithic pour. Add them as separate footings.
• Treating the result as a structural design. Thickness, reinforcement, joints and strength need to come from the code, the drawings or an engineer.

Scope & limits

Quantity only. Bag yields are typical label values and rebar masses are standard nominal values; laps, chairs and dowels are excluded. This does not size foundations, reinforcement or concrete strength.

Frequently asked questions

Further reading

• Foundation types template
• Deck and pergola template
• All materials & quantities tools
• All free architecture & construction tools
• Harth resource library

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