Methodology

Every number on The Concrete Calculator comes from a transparent, industry-standard formula, not a black box. This page documents exactly how each result is calculated, the constants we use, and the assumptions you should know about before you order.

Core volume formula

All slab-type pours start from volume = length × width × thickness. We convert every dimension to feet first, multiply to get cubic feet, then divide by 27 to get cubic yards (1 yd³ = 27 ft³). Cubic meters use 1 ft³ = 0.0283168 m³. Round shapes such as columns and sonotubes use V = π × r² × height. Stairs, curbs, footings and walls are broken into rectangular prisms and summed.

Constants and yields we use

Concrete weight assumes a density of 150 lb per cubic foot (about 4,050 lb per cubic yard) for normal-weight concrete. Bag yields follow manufacturer averages: a 40 lb bag yields ≈ 0.30 ft³, a 60 lb bag ≈ 0.45 ft³ and an 80 lb bag ≈ 0.60 ft³. Ready-mix truck loads assume a full drum of 10 yd³. Cement, sand and gravel splits use a 1.54 dry-volume factor against the selected mix ratio, and a 94 lb sack of cement is treated as ≈ 1 ft³.

Waste allowance

Spillage, uneven subgrade and over-excavation mean the concrete you place is always a little more than the theoretical volume. We add a waste allowance (default 10%) on top of the raw volume so your order reflects the real world. You can change this figure on any calculator that exposes it.

Rounding

Volumes are shown to two decimals, weights and bag counts are rounded up to the next whole unit because you cannot buy a partial bag, and cost is rounded to the nearest dollar. Bag counts always round up so you are never short on site.

Limitations

Results are planning estimates, not engineering specifications. Actual quantities vary with mix design, moisture, compaction, admixtures and site conditions. For structural work, confirm quantities and strength with your ready-mix supplier or a qualified engineer.

Spotted a number that looks off? Tell us on the contact page and we will review it. See our editorial standards for how we check and update the tools.