Concrete Calculator Buying Guide: Cubic Yards, Bags, Waste, and Delivery

Quick Answer

Measure length, width, and thickness, convert the thickness to feet, multiply the three numbers, divide by 27, then add 5% to 15% waste. Small jobs can be converted to 60 lb or 80 lb bags; larger slabs usually belong with ready-mix delivery.

For the exact quantity, open the Concrete Calculator and enter your real dimensions, coverage rate, waste factor, and local material price.

Search Intent This Answers

People searching for a concrete calculator usually need an order quantity, not a theory lesson. The useful answer is cubic yards for ready-mix, bag count for small pours, and a clear waste allowance before calling a supplier.

Core Formula

Cubic yards = length(ft) x width(ft) x thickness(ft) / 27

What to Check Before Buying

Confirm whether the supplier has a short-load fee or delivery minimum.
Use 10% waste for clean slabs and 15% for uneven subgrade, thick edges, or irregular forms.
Order gravel base, form boards, stakes, expansion joint, and curing supplies before pour day.
Round ready-mix up to the supplier ordering increment instead of ordering the exact decimal.

Common Estimating Mistakes

Using inches as feet without converting thickness.
Forgetting thickened edges, pier pads, or post holes attached to the slab.
Ordering bagged concrete for a pour that is too large to mix consistently by hand.
Skipping subgrade compaction and blaming the calculator when the slab settles.

Related Project Calculators and Guides

Paver Calculator - compare a poured slab with a paver patio and base gravel estimate
Fence Calculator - estimate post concrete when the concrete job is part of a fence project
Concrete Bags for a 10x10 Slab - see a worked bag-count example for a common slab size

Today's Material Estimating Cluster

Use these new QuickMaterialCalc guides together when one project touches more than one trade:

Concrete buying guide for slabs, footings, post holes, and ready-mix orders.
Tile buying guide and Flooring buying guide for interior surface estimates.
Paint buying guide and Drywall buying guide for room remodels.
Lumber buying guide, Brick buying guide, and Block buying guide for framing and masonry planning.
Roofing buying guide, Paver buying guide, Insulation buying guide, and Fence buying guide for exterior and envelope work.

FAQ

How much extra concrete should I order?

Use 5% for simple, well-formed pours and 10% to 15% for uneven ground, irregular shapes, or first-time work.

When is ready-mix better than bags?

Ready-mix is usually better once the job needs dozens of bags. It gives a more consistent mix and saves hours of labor.

Should I round concrete up or down?

Round up. A cold joint from running short is usually more expensive than a small amount of leftover concrete.

Next Step

Run the Concrete Calculator, save the result, then use the related guides above to check the materials that normally get missed on the same job.