How to calculate concrete volume
For a rectangular slab, multiply length by width by thickness after converting the measurements into the same unit system.
volume = length × width × thickness
requiredVolume = volume × (1 + wastePercent / 100)BQ Calculator converts measurements internally to meters before deriving cubic meters, cubic yards and bag counts. The conversions use 1 ft = 0.3048 m, 1 in = 0.0254 m, and 1 m³ = 1.307950619 yd³.
Why contractors add waste allowance
Plans describe an ideal volume. Real placements can use more because of uneven ground, form variation, spillage and measurement tolerance. The waste selector lets you see how a 5%, 10% or 15% allowance affects the order quantity.
Cubic yards vs cubic meters
Ready-mix concrete is commonly sold by cubic yard in the United States and by cubic meter in many metric markets. The result panel shows both so you can verify supplier quotes without doing a separate conversion.
How many bags of concrete do I need?
The bag estimate converts the waste-adjusted volume to cubic feet, divides by an approximate nominal yield, and rounds up with a whole-bag ceiling. No fractional bag is presented as purchasable material.
Ready-mix vs premix
Premix bags can be practical for small repairs and small pads. Ready-mix delivery is generally more practical as volume rises, but real project economics depend on local pricing, delivery minimums, access and labor.
Worked slab example
A 12 ft × 10 ft slab at 4 in thick has a base volume of about 1.48 yd³. Adding 10% waste raises the required order to roughly 1.63 yd³. The live result above recomputes this immediately when you change any dimension.
Frequently asked questions
How do I calculate concrete for a slab?
Multiply slab length by width by thickness after converting every measurement to consistent units. Then add a waste allowance before ordering.
Why add a waste allowance?
Real projects can lose material to uneven subgrade, spillage, over-excavation, form variation and measuring error. A small allowance can reduce the risk of running short.
How many 80-lb bags are in one cubic yard?
Using the approximate 0.60 cubic foot yield assumed by this calculator, one cubic yard of 27 cubic feet requires about 45 80-lb bags. Actual bag yield varies by product, so verify the manufacturer label before purchase.
Does the cost estimate include delivery or labor?
No. The ready-mix cost field multiplies the selected price per cubic yard or cubic meter by the waste-adjusted volume only. It does not add delivery, short-load fees, pumping, taxes or labor.
Should bag counts be rounded up?
Yes. You cannot buy a fraction of a bag, so this calculator always rounds estimated 60-lb and 80-lb bag counts upward.