Ordering Concrete for a Slab: Volume & Waste
Turn slab measurements into an order estimate without confusing inches, feet and cubic yards. This guide separates geometric volume, an allowance for uncertainty and the quantity your supplier will actually sell.
Figur Editorial · Practical calculations, examples and planning assumptions · 2026-10-26
Measure the pour, not just the floor plan
Write down the slab's length and width in feet and its planned thickness in inches. Measure inside the forms where possible. Split an irregular footprint into rectangles, calculate each separately and avoid counting overlapping areas twice.
Thickness is a frequent source of error: four inches is one-third of a foot, not four feet or 0.4 feet. Thickened edges, footings and steps need separate volume calculations. A rectangular slab estimate does not automatically include them or determine a safe structural thickness.
Worked example: a 20 × 30 ft slab
For a 20 ft by 30 ft slab at 4 inches thick, volume = 20 × 30 × (4 ÷ 12) = 200 cubic feet. Divide by 27 cubic feet per cubic yard: 200 ÷ 27 = 7.4074 cubic yards before any allowance.
A chosen 10% allowance gives 7.4074 × 1.10 = 8.1481 cubic yards, approximately 8.15. That is an estimate, not an instruction to order exactly 8.15 yards: ask the supplier about order increments, delivery minimums and how quantities are rounded.
Choose an allowance deliberately
No single waste percentage fits every pour. Uneven excavation, variation in actual depth, form movement and placement losses affect the amount needed. Ask the contractor and supplier to review the allowance rather than treating 10% as a universal rule.
Depth matters substantially. At 4.5 inches instead of 4, the same slab needs 8.3333 cubic yards before waste, 12.5% more than the original geometric volume. Re-measuring the base can be more useful than adding an arbitrary safety margin to an uncertain measurement.
Confirm the purchase before the truck arrives
Give the supplier both the measured volume and the allowance used. Ask about the specified mix, truck access, unloading time and charges for a short load or an additional delivery. Include separate sections of the pour in the discussion.
Bagged concrete is a different purchasing problem: use the yield printed on the exact product, not a generic bag-to-yard conversion. Reinforcement, base preparation, drainage and strength requirements need qualified project guidance; this calculator estimates quantity, not structural suitability.
- Keep measurements and units beside the order.
- Separate slab, footings and thickened edges.
- Confirm yield or delivery increments with the seller.