Concrete Volume & Bag Estimator

Estimate the cubic footage, cubic yards, and number of pre-mix bags needed for a rectangular concrete slab or footing — including an optional waste allowance.

How it works

cubicFeet = length x width x (thickness / 12) x (1 + waste% / 100). cubicYards = cubicFeet / 27. Bag yields: 40 lb = 0.30 ft3, 60 lb = 0.45 ft3, 80 lb = 0.60 ft3. bags = ceiling(cubicFeet / yieldPerBag).

Worked example

A 10 ft x 10 ft slab at 4-inch thickness with 80 lb bags and no waste: cubicFeet = 10 x 10 x (4/12) = 33.33 ft3. cubicYards = 33.33 / 27 = 1.235 yd3. bags = ceiling(33.33 / 0.60) = ceiling(55.56) = 56 bags.

When to use it

Add 5-10% waste for spillage and uneven subgrades — a typical residential pour uses 10%.
For pours larger than about 1 cubic yard (27 ft3), ordering ready-mix by the yard is usually more economical than pre-mix bags.
Slabs under 4 inches thick are generally not suitable for vehicle traffic; driveways typically use 4-6 inches.
Thickness is the single biggest driver of material cost — verify your design depth before ordering.

Related calculators

All results are an engineering estimate — not for custody transfer. Field measurement and certified tables govern.