How to Estimate Concrete: Cubic Yards, Waste and Rebar

Concrete looks simple to estimate: measure the volume and multiply by a price per cubic yard. In practice, concrete scopes include footings, walls, slabs, piers, formwork, reinforcing, finishing, curing, pumping and testing, and each of those needs its own quantity. Here is how to estimate concrete step by step.

Step 1: Calculate concrete volume in cubic yards

Ready-mix concrete is sold by the cubic yard. One cubic yard equals 27 cubic feet. The basic formula is:

Cubic yards = length (ft) × width (ft) × thickness (ft) ÷ 27

The standard concrete volume formula

Always convert inches to feet first. A 4-inch slab is 0.333 ft thick, a 6-inch slab is 0.5 ft, and an 8-inch wall is 0.667 ft.

Example: a slab on grade

A 30 ft × 40 ft slab, 4 inches thick: 30 × 40 × 0.333 = 400 cubic feet. 400 ÷ 27 = 14.8 cubic yards.

Example: continuous footings

A 2 ft wide × 1 ft deep footing running 160 linear feet: 2 × 1 × 160 = 320 cubic feet. 320 ÷ 27 = 11.9 cubic yards.

For walls, multiply length × height × thickness. For round piers, use π × radius² × depth. Measure each element separately so you can price and check them individually.

Step 2: Add a waste factor

Concrete is never placed at exactly the volume on the drawings. Uneven subgrade, form movement, spillage and the concrete left in the truck all add up. Many estimators add about 5% to 10% depending on the element and site conditions, with more on footings dug against earth and less on formed, elevated work. Round up to the nearest practical ordering increment.

In the slab example, 14.8 cubic yards plus 5% is about 15.6 cubic yards, so you would plan to order 16.

Step 3: Estimate rebar

Rebar is usually estimated by weight. Count the bars in each direction, multiply by their length, add laps, and multiply by the weight per foot for the bar size. Common weights are #3 at 0.376 lb/ft, #4 at 0.668 lb/ft and #5 at 1.043 lb/ft.

For the same 30 × 40 slab with #4 bars at 12 inches on center each way: 31 bars × 40 ft = 1,240 ft, plus 41 bars × 30 ft = 1,230 ft, for 2,470 linear feet. 2,470 × 0.668 is about 1,650 pounds before laps and waste. Laps and accessories such as chairs and tie wire are added on top. See our rebar estimating services for detailed bar lists.

Step 4: Don't forget the rest of the concrete scope

Volume is only part of the cost. A complete concrete estimate also includes:

• Formwork in square feet of contact area for footings, walls, columns and slab edges.
• Vapor barrier and base under slabs, in square feet and tons of aggregate.
• Finishing by finish type, such as broom, trowel or polished.
• Joints, sealants and embeds including control joints, expansion joints and anchor bolts.
• Pumping or placing equipment where trucks cannot discharge directly.
• Curing, testing and cold or hot weather protection.
• Excavation and backfill for footings, if it is in your scope.

Common concrete estimating mistakes

• Forgetting to convert inches to feet before dividing by 27.
• Measuring slab area without deducting large openings, or deducting small ones that will be poured through.
• Missing thickened slab edges, pier caps or grade beams shown only in sections.
• Leaving out laps and accessories in the rebar weight.
• Using one waste factor for every element regardless of conditions.

Let NYNMO estimate your concrete

Our estimators produce detailed concrete takeoffs with volumes by element, formwork areas, rebar weights and finishing quantities, all tied to color-coded plans. Most concrete estimates are delivered in 24 to 48 hours. Send us your plans for a free quote.

Frequently asked questions