Concrete Square Foot Calculator
Order 1.28-1.36 yd³
Plan volume 1.23 yd³ (33.33 ft³); range = NRMCA CIP-8 4-10% ordering allowance.
- Volume is plan geometry; NRMCA CIP 8 says order 4-10% over plan (subgrade, forms, spillage).
- A 1/8" thickness variation on a 4" slab is a ±3% volume swing; measure thickness honestly.
- Ready-mix increments and short-load fees are vendor practice; confirm with your plant.
Why This One Is Different
You've already got a square footage off the permit or the plan; what you don't have yet is how many yards of concrete that turns into once you add a thickness. This calculator takes the footprint you already measured (the number a permit clerk would recognize) and turns it into cubic yards using the same closed-form geometry a ready-mix dispatcher would run: length times width times depth, divided by 27 for cubic feet to cubic yards. Square footage alone tells you nothing about how much concrete to order until you multiply it by a thickness nobody wrote on the plan. A 400 sq ft slab poured 4 inches deep needs about 4.9 cubic yards of plan volume before the standard ordering allowance is added.
Where the Numbers Come From
- Turning a footprint into an order starts with definitional geometry, not an estimate: slab volume is length × width × (thickness in inches ÷ 12), converted to cubic yards by dividing cubic feet by 27. Source: worked example (closed-form geometry)
- Order 4-10% more concrete than plan volume calls for: form deflection, subgrade irregularity, spillage, and air/settlement losses mean placed volume never quite equals plan volume, which is what NRMCA guidance accounts for. Source: NRMCA, CIP 8: Discrepancies in Yield (2021 reprint)
- A 1/8" thickness variation on a 4" slab alone shifts total volume by roughly 3%: thickness, not square footage, is usually the shakier input. Source: NRMCA CIP 8, applied via construction-estimating engine convention
More Questions
Do I need to account for waste when ordering by square footage?
Yes: order 4-10% over your calculated plan volume per NRMCA's CIP 8 guidance, since spillage, subgrade irregularity, and forming losses mean the truck's actual delivered yardage should exceed the raw geometric number.
Related Calculators
Formulas, waste factors, and cost models on this page follow the sourced conventions on our methodology page. Estimates only; verify with your supplier and local code.