Concrete Calculator

Why concrete is ordered in cubic yards

A cubic yard is 27 cubic feet, and a standard ready-mix truck carries 8 to 10 of them. Anything much above a cubic yard is cheaper and far easier from a truck than from bags: one cubic yard is 45 bags of 80 lb mix, which is roughly 1.8 tonnes to carry, mix and place before the first batch starts setting.

The waste allowance is not padding

Subgrade is never perfectly flat, forms bow slightly under pressure, and some concrete is lost in the chute and on the ground. Five per cent is the minimum realistic allowance on a well-prepared flat slab; ten is normal; footings in soft or uneven ground can need more. The figure is shown separately above so you can judge it rather than inherit it.

Thickness matters more than area

Volume scales linearly with thickness, so going from 4 inches to 5 adds 25 per cent to the order — a much bigger jump than most people expect from “just an inch”. It is also the single most common cause of an order coming up short: the slab was priced at 4 inches and poured closer to 5 because the subgrade dipped.

Order by the yard, and order slightly long

Ready-mix is sold by the cubic yard, which is 27 cubic feet, and suppliers normally quote a minimum load plus a short-load surcharge below roughly one cubic yard. Running out mid-pour is far more expensive than a small surplus: a cold joint forms where fresh concrete meets concrete that has already begun to set, and that joint is a permanent weakness. Adding five to ten per cent covers subgrade that is not perfectly level, which is where most of the extra volume quietly goes.

Bagged mix is a different calculation. A 60 lb bag yields about 0.45 cubic feet and an 80 lb bag about 0.60, so a modest 4-inch slab of 80 square feet still needs roughly 45 bags of the larger size. Beyond a cubic yard or so, mixing by hand stops being realistic and ready-mix is both cheaper and more consistent. Control joints should be tooled or cut at roughly 24 to 30 times the slab thickness in feet, which puts a 4-inch slab at 8 to 10 feet.

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