Use this concrete slab calculator to turn measured slab or pad dimensions into a concrete order. Enter the plan dimensions and actual thickness; the result separates the geometric volume from the optional material allowance so you can discuss a clear quantity with a supplier.
Measure the slab before entering dimensions
Measure the length and width of the area that will actually be filled, then record the specified or already chosen slab thickness. Length and width use feet in the default path; thickness uses inches. A value such as 5 ft 6 in can be entered as an explicit feet-and-inches pair, while 5.6 ft is a decimal length. Those values are different, so use the format shown beside each field rather than converting by eye.
For a simple rectangle, use the inside edges of the forms that contain the pour. If an existing pad has a notch, a separate landing, or a section with a different thickness, add it as a separate non-overlapping area or use the known-area option for each thickness. Do not count an overlapping corner twice. The calculator begins with blank inputs; its result stays empty until you calculate with valid measurements or intentionally load the example.
What the volume formula means
The rectangular volume is length × width × thickness. With feet for length and width and inches for thickness, the direct cubic-yard form is yd³ = length(ft) × width(ft) × thickness(in) ÷ 324. The divisor accounts for twelve inches per foot and twenty-seven cubic feet per cubic yard. The underlying calculation uses the full value; displayed rounding should not affect a bag count or the order quantity.
Net volume describes the geometric space inside the forms. Order volume applies the selected waste allowance to that net amount: order volume = net volume × (1 + allowance ÷ 100). The default 10% allowance is a changeable planning input, not a code rule or a guarantee that every site needs exactly that amount. Uneven subgrade, form movement, spillage, placement method, and supplier ordering increments can change what is practical.
Worked example: a 10-by-10-foot pad
For a 10 ft × 10 ft slab that is 4 in thick, the area is 100 ft². Multiplying by one-third of a foot gives 33.333 ft³, or about 1.23457 yd³ net. Applying a 10% allowance gives about 1.35802 yd³ to plan for. The calculator can show both values so the allowance is not mistaken for the slab’s actual geometric volume.
If you choose QUIKRETE Concrete Mix No. 1101, its data sheet lists approximate yields of 0.60 ft³ for an 80 lb bag and 0.45 ft³ for a 60 lb bag. Using the unrounded order volume gives 62 bags at 80 lb or 82 bags at 60 lb. Those are rounded-up material counts for this named product, not a claim that every concrete mix bag yields the same amount. Check the selected product label before buying.
Choose an ordering path
A ready-mix order is usually discussed in cubic yards, while bagged mix is counted by package and product yield. A slab’s calculated volume does not include delivery charges, a pump, labor, base preparation, reinforcement, or finishing. If you want a broad project budget, open the slab cost page and keep its installed-price mode separate from this material quantity. When you contact a supplier, give the dimensions, net volume, allowance, and final requested order amount rather than only saying the area in square feet.
The page can also add distinct slab sections before applying the allowance once to their combined net volume. This is useful for a porch pad adjoining a larger rectangle or for a slab with a separately measured extension. The section boundary should follow the actual pour geometry. If the extension has a different thickness, calculate it separately and add its volume; multiplying the entire combined area by one thickness would misstate the material requirement.
Thickness and project limits
This tool answers a quantity question for dimensions you provide. It does not choose the thickness for a garage, patio, shed, driveway, or structural floor. Loads, soil, frost depth, reinforcement, drainage, joints, local requirements, and the slab’s support conditions can change the design. Use approved plans or a qualified local professional to determine those details before construction; a mathematically correct volume does not show that a slab design is adequate.
The calculation treats the slab as a uniform rectangular prism unless you add separate areas. It does not infer thickened edges, beams, steps, drain depressions, slopes, or subgrade voids from a photograph. Add known, non-overlapping shapes as separate quantities when the tool supports them, or calculate unusual portions separately. For a sloped surface, confirm whether the drawing gives horizontal plan dimensions or the actual slope length before choosing which measurement belongs in the formula.
From estimate to order
Before ordering, recheck every dimension at the forms and note whether the entered thickness is inches or a decimal foot value. Confirm whether the displayed quantity includes the allowance and whether the product is ready-mix or a specific bagged mix. Suppliers may quote minimum loads or delivery increments that the volume calculation cannot know. Ask the supplier how it handles partial loads, timing, access, and any project-specific charges, then compare that confirmed order with the calculator’s unrounded result.
If the slab dimensions are still being planned, the calculator can compare candidate thicknesses as quantities only. For example, increasing thickness from 4 in to 5 in increases rectangular volume by 25% when length and width remain fixed, because thickness changes directly in proportion to volume. That observation helps compare material amounts; it is not a recommendation to use either thickness. Use the slab cost calculator for a sourced planning range, and use the project’s plans for construction decisions.
Frequently asked questions
Does the calculator add 10% automatically?
The default order allowance is 10%, shown separately from net volume and adjustable by the user. It is a planning setting, not a mandatory construction value. Confirm a suitable quantity and any supplier increment for the actual pour.
How many 80 lb bags cover this slab?
The answer depends on the slab dimensions and the selected product yield. For the 10 × 10 × 4 in example, QUIKRETE Concrete Mix No. 1101 at about 0.60 ft³ per 80 lb bag gives 62 bags with a 10% allowance.
Can I enter a slab by square footage?
Yes. Use the known-area path when the area is measured and thickness is known. Area alone cannot produce volume; the thickness is still needed, and separate thickness zones should be entered as separate quantities.
Is a concrete pad different from a slab here?
The volume math is the same for a flat rectangular pad when dimensions and thickness are known. The tool does not decide whether a particular use needs a pad, slab, reinforcement, or a different foundation system.
Should I enter my slab thickness as feet or inches?
Use the unit attached to the thickness field, which defaults to inches. If switching to a feet-based dimension, convert explicitly: 4 in is one-third of a foot, not 4 ft.
Does the result include labor, delivery, or a pump?
No. This page estimates material quantity. Labor and delivery are not included; use a separate cost estimate and confirm site charges with the supplier or contractor.