From measured concrete volume to a complete bid
For contractors planning the quantities and costs of a rectangular slab or uniform perimeter footing. Use measured dimensions and supplier quotes; the visual is a quantity sketch rather than a structural specification.
Formula and worked example
Solid slab: ft³ = length(ft) × width(ft) × depth(in) ÷ 12. yd³ = ft³ ÷ 27. Perimeter footing: area = outside rectangle − inside rectangle. Order = net volume × (1 + allowance/100). Bid = total entered cost ÷ (1 − margin/100).
A 24 × 24 ft slab, 6 in deep, contains 288 ft³ or 10.6667 yd³. With 10% order allowance, purchase volume is 11.7333 yd³. At 0.60 ft³ per 80 lb bag, that is 528 whole bags. Without gravel or rebar costs, $165/yd³ concrete plus $1,000 prep/labor gives $2,936 cost and a $3,670 bid at 20% margin. These are hypothetical quantities and prices.
Measure the pour before ordering
Measure the actual planned concrete dimensions, not just a building's nominal size. Enter length and width in feet, then depth in inches. The tool converts the depth to feet before calculating volume. Confirm whether forms establish outside or inside dimensions and whether the bottom is level enough for a uniform-depth assumption. Small depth errors across a large slab can change the order materially.
The slab sketch changes as the dimensions change, but its vertical depth is exaggerated so a thin layer remains visible on a screen. Read the numeric depth label instead of measuring the illustration. Irregular edges, steps, thickened sections and different elevations require separate quantity calculations. Do not force a complicated pour into one rectangle simply because its overall outside dimensions are easy to measure.
Understand the perimeter-footing model
A solid slab fills its whole rectangular footprint. The footing mode instead removes an inside rectangle from the outside rectangle, using the same entered width around all four sides. That subtraction avoids counting the corners twice. The outside length and width remain feet, while the footing width and depth are entered in inches. The inside opening must remain positive for this model.
For example, a 24 ft square outside footprint with a 1.5 ft wide perimeter has a 21 ft square inside opening. Its plan area is 576 minus 441, or 135 square feet. Multiplying by the footing depth gives its concrete volume. Crossings, pads, isolated piers and different footing widths are excluded. Calculate those separately rather than treating the ring result as a foundation design.
Separate measured volume from order allowance
The net volume describes the entered geometry. The order quantity multiplies that volume by your allowance percentage. An allowance can reflect expected placement variation or purchasing practice, but the calculator does not prescribe a universal amount. Use the same measurement basis as the supplier and discuss the order, site access, discharge method and scheduling before committing to a delivery.
The quantity is not silently rounded to a truck size. A ready-mix supplier may use delivery increments, minimum-load charges or other terms that require a different final order. Include those costs in the prep, labor and other-cost input. Pinning down which amount is measured and which amount is purchased also prevents a waste allowance from being added twice in another estimating worksheet.
Compare bagged mix with ready-mix
The bag alternative divides the order volume in cubic feet by the selected product yield, then rounds up to whole bags. The starting yields are specific to QUIKRETE Concrete Mix: an 80 lb bag is approximately 0.60 cubic feet and a 60 lb bag approximately 0.45 cubic feet. Bag weight alone is not enough to establish the yield of a different product.
Changing the supply selection also changes the concrete cost unit: ready-mix uses dollars per cubic yard, while bagged mix uses dollars per bag. Enter the actual quoted price for that selection. The pallet or truck illustration is symbolic; it does not assign a delivery capacity or guarantee that hand mixing is practical for the size of your pour. Verify mixing, handling and placement requirements separately.
Use gravel and rebar as quantity assumptions
The gravel toggle adds a uniform layer beneath the same modeled concrete footprint. Its order volume uses the gravel depth and the order allowance, with a separate gravel unit price. This is a purchase estimate, not a compaction specification. Confirm whether the quoted gravel quantity is loose or placed and whether delivery, compaction labor or extra excavation belongs in the other-cost allowance.
For a slab, the grid estimates bars in two directions using the entered spacing and edge clearance. It includes both ends and distributes the bars so the modeled gap does not exceed the chosen spacing. The #4 or #5 selector changes the visual line weight; update the price for your chosen steel yourself. Counts and linear feet exclude laps and bends, and must not be interpreted as an engineered reinforcement schedule.
Turn the takeoff into a reviewable quote
Material cost includes concrete and the enabled gravel and grid-steel allowances. Add preparation, labor, equipment, overhead and other costs that you expect to bear. The bid divides that combined cost by one minus the margin as a decimal. A 20% margin uses a denominator of 0.80. Adding 20% as a markup would produce a different, lower margin on the same costs.
Copy the summary to keep the quantity assumptions with the price. Prepare an estimate line to pass the bid to the document studio, then choose Add takeoff line there. That second step preserves an existing document until you decide to add the new work. Before sharing a quote, review scope, exclusions, delivery assumptions, tax treatment and payment timing against the actual job.
Frequently asked questions
How many cubic yards are in a 24 × 24 × 6 in slab?
The net volume is 10.6667 cubic yards: 24 × 24 × 0.5 feet divided by 27. With a hypothetical 10% order allowance it becomes 11.7333 cubic yards. Supplier increments and site conditions may change the final order.
Does the rebar overlay tell me what reinforcement to install?
No. It visualizes entered quantity assumptions. It does not select a code-compliant bar size, spacing, cover, lap length or support arrangement. Use the project's approved specifications and calculate additional fabrication needs separately.
Are gravel and concrete both included in the bid?
Concrete is included using the selected supply unit. Gravel and slab-grid steel are included only when enabled, using their separate prices. Enter every other cost you need to recover in the prep, labor and other-cost allowance; avoid counting the same cost twice.