How Much Concrete Do I Need for a Slab? Yards, Bags & Waste

Estimating concrete is one of the most unforgiving takeoffs in construction: order too little and the pour stops cold; order too much and you pay for yards you cannot return. The math itself is simple volume in cubic yards but the waste allowance and the ready-mix-vs-bags decision are where jobs win or lose money.

The core formula

Measure length and width in feet and thickness in inches. Convert thickness to feet, multiply, then divide by 27 the number of cubic feet in a cubic yard, the unit ready-mix is sold in:

Volume (cu yd) = length × width × (thickness ÷ 12) ÷ 27

A 20 × 15 ft patio at 4 inches thick is 20 × 15 × (4 ÷ 12) ÷ 27 = 3.70 cubic yards. That is the net volume what the finished slab physically contains. You never order the net number.

Add waste before you price

Waste is applied to the net volume first, and the order quantity rounds up from the waste-adjusted number never the other way around. A 10–15% allowance covers spillage, over-excavation, and uneven subgrade that eats concrete. On the patio above, 10% waste takes 3.70 yards to 4.07 yards, which you would order as 4.25 or 4.5 yards depending on your supplier's increments.

• 10% waste formed slabs on well-prepared, compacted subgrade
• 12–15% waste hand-dug footings, irregular shapes, or soft subgrade
• Always round the order quantity up suppliers do not sell fractions of a yard

Bags vs. ready-mix

An 80-lb bag of concrete mix yields about 0.6 cubic feet, so one cubic yard needs roughly 45 bags (60-lb bags need about 60). For the 4.07-yard patio that is over 180 eighty-pound bags clearly a ready-mix truck job. Bags make sense under about half a yard: small pads, post footings, and repairs where the truck's short-load fee would cost more than the sacks.

When a truck does come, remember it carries a full load whether you need it or not. Most suppliers charge a short-load fee under their minimum (often 4–5 yards), so small pours sometimes pencil out better as bags even when the per-yard price looks higher.

Common mistakes

• Forgetting the ÷ 27 and ordering cubic feet as cubic yards 27× too much
• Measuring thickness in inches but forgetting ÷ 12
• Ordering the net number with no waste, then running short mid-pour
• Rounding down the order quantity to 'save' money