How to use the Concrete Calculator
- Add your slabs, footings, columns, steps, hollow tubes, curb & gutter or block walls in Measurements. Select a component to edit its dimensions and see the measured view.
- Check the extra allowance and use the depth-change comparison for slab measurement uncertainty.
- Open Supplier comparison and enter delivery, rounding, minimum billing and small-load rules for each quote.
- Open Pour brief, add delivery details and confirmations, then save the project or export the brief.
One estimate for every pour
Add each slab, footing, column, set of steps, form tube and curb run as a named component. The calculator adds measured volumes before applying your extra allowance, then rounds premix bags up. Block walls are counted separately by face area; their blocks are never added to concrete volume. Ready-mix quotes include only the delivery amount and supplier fees you enter. Saved projects remain in this browser on this device.
How to calculate concrete
Concrete is ordered by volume. Multiply length × width × thickness to get cubic feet, then divide by 27 to get cubic yards (one cubic yard is 3 ft × 3 ft × 3 ft). For a 10 ft × 10 ft slab that is 4 inches thick: 10 × 10 × (4 ÷ 12) = 33.3 cu ft ÷ 27 = 1.23 cubic yards. For round columns use π × radius² × height.
How many bags of concrete do I need?
A bag’s yield is printed on the label. Typical yields are:
- 80 lb bag ≈ 0.60 cu ft (45 bags per cubic yard)
- 60 lb bag ≈ 0.45 cu ft (60 bags per cubic yard)
- 40 lb bag ≈ 0.30 cu ft (90 bags per cubic yard)
- 25 kg bag ≈ 0.012 m³ (about 83 bags per m³)
Bags make sense for small jobs such as post holes, steps and small pads. Compare your local ready-mix and bag quotes, including delivery and minimum orders; volume alone does not determine the cheaper option. Confirm the mix specifications for your project.
How a depth change affects concrete volume
Use the thickness specified for your project. The depth-change check compares quantities if all slab components are deeper than the entered measurement; it does not recommend a structural thickness. A 10 × 10 ft slab needs about 0.154 extra cubic yards for an additional half-inch of depth, before allowance. Footings, columns and block walls are unchanged by this check.
Compare ready-mix delivery and small-load fees
A lower unit price can produce a higher delivered bill. Enter each supplier’s price, delivery fee, order increment, minimum billed volume and any small-load fee. The planned order rounds the takeoff up to the stated increment. The minimum billed volume affects the charge, while a small-load fee applies when the planned order is below the threshold you enter. Confirm these rules and all other charges with the supplier. Both quotes use the same selected currency; amounts are not converted.
Prepare a concrete delivery brief
Save the measured components, quotes, requested date, specified mix and access notes together. Download or copy the brief for a supplier conversation. Keep unresolved items visible in the checklist. The NRMCA ordering guide explains why project requirements and delivery arrangements matter. The calculator does not choose a mix or certify site readiness.
Steps, form tubes and curbs
Steps are treated as solid down to the ground: each step adds one run of depth and one more rise of height, and an optional top landing sits at full height. Three 7 in × 11 in steps, 4 ft wide, need about 0.48 cubic yards. Hollow round tubes subtract the core: an 8 ft tube with a 12 in outside and 8 in inside diameter holds about 0.13 cubic yards. Curb & gutter multiplies the cross-section by the length: the curb stands on the gutter flag, so its height is measured above the flag.
Why add a waste allowance?
Sub-grades are never perfectly flat, forms bow slightly and some concrete is always left in the mixer or wheelbarrow. Adding 5–10% avoids the expensive problem of running short halfway through a pour. Running out leaves a cold joint, which is weaker and visible.