Concrete Calculator

Concrete is one of the most used construction materials in the world — and one of the most underestimated to calculate. Ordering too little means an emergency trip or a visible seam where old and new concrete meet. Ordering too much wastes money (ready-mix trucks have minimum charges and can't take material back). Getting it right the first time requires accurate measurements and the standard 10% waste buffer.

The basic calculation: multiply length × width × thickness (all in feet) to get cubic feet, then divide by 27 to get cubic yards. Most ready-mix concrete is sold by the cubic yard ($120–$200 per yard plus delivery, with typical minimums of 1 yard). For projects under 1 yard, bagged premix is more practical ($6–$10 per 80 lb bag, each making 0.6 cu ft).

Concrete mix design for common applications: driveways and garage slabs require 4,000 psi concrete. Standard patios and walkways: 3,500 psi. Non-structural slabs: 3,000 psi is adequate. For areas with freeze-thaw cycles, specify air-entrained concrete, which contains tiny air bubbles that relieve pressure from freezing water — preventing cracking and spalling.

Curing properly is as important as the mix. Concrete continues gaining strength for 28 days. The critical period is the first 7 days — keep the surface moist by covering with plastic sheeting or curing compound. Proper curing increases final strength by 30–50% compared to letting it dry quickly in sun and wind.

Reinforcement: most slabs should include rebar (#3 or #4 rebar on 18-inch grids) or fiber reinforcement to control cracking. Without reinforcement, concrete cracks are inevitable — reinforcement holds the pieces together and limits crack width. For footings, rebar size and spacing should follow local building codes and your structural engineer's specification.

Always call 811 (Dig Safe) before digging for concrete footings or posts — this free service locates underground utilities and is legally required in most states.