Concrete Calculator

How to work out how much concrete you need

Every concrete estimate is the same three steps. Work out the volume of the space you are filling in cubic feet, convert that into the unit your supplier sells in, then add a margin because some of the mix always ends up on the ground, in the wheelbarrow or absorbed by an uneven subgrade. The arithmetic is easy to get wrong in one specific place: thickness is measured in inches while length and width are in feet, and multiplying before converting gives an answer twelve times too big.

The formulas behind this calculator are the same ones a site foreman uses. A slab is length x width x (thickness / 12). A footing is length x (width / 12) x (depth / 12), because both the trench width and its depth are usually quoted in inches. A round column is pi x (diameter / 24)² x height, where dividing the diameter by 24 turns inches into a radius in feet in one move. Cubic feet divided by 27 gives cubic yards, the unit every US ready-mix plant quotes in; multiply cubic feet by 0.0283 for cubic metres.

Worked example: a 10 x 10 ft patio, 4 inches thick

Four inches is 0.333 ft, so the raw volume is 10 x 10 x 0.333 = 33.3 cubic feet. Add the standard 10 percent for waste and you are ordering 36.7 cubic feet, which is 1.36 cubic yards or 1.04 cubic metres. In bags that is 36.7 / 0.60 = 62 eighty-pound bags, or 82 sixty-pound bags. Sixty-two bags is roughly 2.5 tons of material to buy, load, unload and mix one or two bags at a time — this is exactly the size of job where people start phoning ready-mix suppliers.

Worked example: a 20 ft footing, 16 in wide and 8 in deep

Convert first: 16 inches is 1.33 ft and 8 inches is 0.67 ft. The volume is 20 x 1.33 x 0.67 = 17.8 cubic feet, or 19.6 cubic feet with waste. That is 0.72 cubic yards and 33 eighty-pound bags. Note how a footing that sounds small in linear feet still eats a third of a truck's minimum load. If your footing steps down a slope, split it into level sections and add the sections together rather than averaging the depth — averaging consistently under-orders on stepped ground.

Worked example: four 12 in columns, 8 ft tall

A 12 inch tube has a radius of 0.5 ft, so each foot of height takes pi x 0.25 = 0.785 cubic feet. Eight feet gives 6.28 cubic feet per column, and four of them come to 25.1 cubic feet. With waste that is 27.7 cubic feet, or 1.02 cubic yards — just over the line where a truck starts to make sense, and 47 eighty-pound bags if you stay with bagged mix. Cardboard forming tubes are sold in fixed diameters, so check the actual inside diameter rather than the nominal size when the column is large.

Bags or a truck? A reality check on bag counts

Bag counts hide the physical work. Forty-five 80 lb bags to the cubic yard means 3,600 lb of dry mix per yard, moved twice: once from pallet to mixer and once from mixer to form. A rented mixer holds about three bags per batch, and each batch takes four to five minutes including loading, so a cubic yard is about fifteen batches and well over an hour of continuous mixing before you count placing and finishing. There is also a quality issue: slow hand mixing risks cold joints where one batch stiffens before the next arrives, which is why anything structural over about half a yard is better poured continuously.

Ordering ready-mix without surprises

Most plants have a minimum load of three to five cubic yards and charge a short-load fee — commonly a flat fee or a per-yard surcharge — for anything under it. Ask for the short-load threshold and fee before you compare prices, because a 1.5 yard order can cost the same as three yards. Ask also about the free unloading window, usually five to seven minutes per yard, after which demurrage charges start; have enough hands and barrows ready before the truck arrives. A standard chute reaches roughly 12 to 18 feet from the back of the truck, so measure the access route and budget for a pump or a line of barrows if the pour is behind a house. Finally, order the volume you calculated rather than shaving it: running short mid-pour leaves a cold joint that no amount of trowelling hides.

What this calculator does not include

The result is the volume of the concrete itself. It does not account for rebar or mesh displacement (negligible at domestic scale), thickened edges and turn-downs around a slab perimeter, or the gravel subbase, which is a separate order. It assumes a level, properly compacted base — an excavation that dips an inch here and there can easily add 10 to 20 percent to a slab and is the single most common reason a pour runs short. Bag yields are manufacturer figures and vary a little by brand and by how wet you mix; treat 0.60 and 0.45 cubic feet as nominal. For anything structural, footing sizes and slab thickness should come from local building codes or an engineer, not from a volume calculator.