Concrete Slab Calculator

This calculator estimates the concrete material quantity — volume, and optionally bags, truckloads, and material cost — for simple rectangular and circular slabs. It does not design the slab: thickness, reinforcement, and subgrade preparation are project-specific decisions that come from your plans, a qualified professional, or local requirements.

For a rectangular or square slab, measure length and width along the finished slab edges. For a circular slab, measure the diameter — the distance straight across through the center — since it's usually easier to measure accurately than a radius. Measure thickness as the depth of concrete you plan to place, not an existing subgrade or base layer.

“Calculated volume” is the geometric volume of your slab exactly as measured — length × width × thickness, or the circular equivalent. “Concrete to order” adds your order allowance on top. These are kept separate deliberately: the calculated figure reflects your plan dimensions exactly, while the order figure is what you'd actually ask a supplier for, or mix from bags, once you plan for real-world variation.

Field dimensions rarely match plan dimensions exactly — subgrade over-excavation, form deflection, spillage, and minor thickness variation all use extra concrete. NRMCA's own technical guidance (CIP 8, “Discrepancies in Yield”) recommends ordering roughly 4-10% over the calculated volume for these contingencies, explicitly as a range rather than one fixed number — a controlled, repetitive placement needs less; an irregular or sporadic one needs more. This calculator's 5% default sits in that range but is fully editable; there is no universally correct figure for every project.

Ready-mix concrete is delivered by truck in bulk (cubic yards or cubic meters) and is typically used for larger pours where the volume justifies a delivery. Bagged concrete mix is measured out and mixed on site bag by bag, and is typically used for smaller pours, repairs, or posts where a full truck delivery isn't practical. Neither is universally “better” — the right choice depends on your project's size, access, and your supplier's minimum order.

Two bags of the same weight from different manufacturers do not necessarily produce the same mixed volume — yield depends on the product's formulation, not just how much it weighs. This calculator's bag count is always based on a MIXED YIELD you enter (with QUIKRETE Concrete Mix's own published figures offered as a quick-fill shortcut), never inferred from bag weight alone.

An irregularly shaped pour can usually be broken into simpler rectangular or circular sections and added together. Add a slab for each section — the calculator sums every section's volume into one combined total before applying the order allowance, rather than rounding or estimating each section separately.

Volume scales directly with thickness: doubling the thickness doubles the concrete needed for the same area. A seemingly small difference matters more than it looks — NRMCA's own example notes that just 1/8 in (3 mm) of extra thickness on a 4-inch (100 mm) design slab is about a 3% volume difference. This calculator does not tell you what thickness your project requires — that depends on project loads, soil/subgrade conditions, reinforcement, local requirements, and your project's design.

• Confirm actual field dimensions once forms are set.
• Confirm thickness and any other requirements against your project design.
• Confirm your bag product's stated mixed yield, if using bagged concrete.
• Confirm your supplier's minimum order and delivery policy.
• Confirm ordering increments — suppliers don't always sell arbitrary fractional quantities.
• Confirm any local or project-specific requirements that apply.

This is a material quantity planning estimator, not structural design or engineering software. It does not calculate or recommend slab thickness, reinforcement (bar size, spacing, or mesh), subgrade design, control-joint layout, concrete strength/mix design, or code compliance. Reinforcement and structural requirements depend on your project's design, loads, and local requirements — they should come from your plans, specifications, or a qualified professional, not from this calculator.