How much concrete to order for a slab

Separate quantity from structural design

A volume calculator answers how much concrete fills a defined shape. It does not specify slab thickness, compressive strength, reinforcement, joints or subbase. Those choices depend on loads, soil, exposure and local requirements and may need professional design.

Measure the pour

For a rectangle, multiply finished length × width × thickness. Convert thickness to the same unit first. A 6 m × 4 m slab at 0.12 m is 2.88 m³. Divide irregular work into measurable sections and add them rather than relying on one rough average.

• Measure several excavation depths.
• Subtract large permanent block-outs separately.
• Count repeated pads or footings explicitly.

Allowance and ordering example

An allowance covers uneven subgrade, form movement and handling loss; it should not hide uncertain measurements. At 7%, the 2.88 m³ slab becomes 3.0816 m³. Discuss the supplier's order increments and short-load rules before rounding the purchase quantity.

For bags, use the finished yield printed on the exact product. If one bag yields 0.012 m³, the example requires 257 bags after allowance, rounded up. For a pour of this size, ready-mix may be operationally more practical, but local price, access and labor decide.

Before the truck arrives

Confirm access, discharge method, crew, forms, reinforcement, joints and curing materials. A correct volume does not prevent delays or cold joints. Ready-mix tickets and bag labels should be checked against the specified mix and quantity.

• Never add uncontrolled water to make placement easier.
• Include pump, waiting and minimum-load fees in the budget.
• Plan curing before placement begins.