Concrete Volume Calculator

The Concrete Volume Calculator turns the dimensions of a slab, footing, column, or wall into the cubic yards, metres, and bag counts to order.

Every concrete take-off starts as a geometry problem. A slab, a strip footing, and a wall are all rectangular boxes, so you multiply length by width by thickness to get the volume. The one trick is units: thickness is usually quoted in inches while length and width are in feet, so a 4-inch slab is 4/12 of a foot thick, not 4 feet. A round column, pier, or Sonotube is a cylinder, so its volume is pi/4 times the diameter squared, times the height.

Concrete is sold two ways, and both use the same volume figure. Ready-mix plants quote in cubic yards, where one cubic yard is 27 cubic feet, while much of the world works in cubic metres of about 35.3 cubic feet each. This tool reports both alongside the raw cubic feet so you can talk to a batch plant or a builders' merchant without converting in your head.

If you do not yet know a footing's size, work out its width and depth from the load first with the footing sizing tool. Round deck piers and column pads are handled the same way by the pier and post footing sizer, and you bring the finished dimensions back here to price the pour.

A clean take-off comes from measuring the actual formed space, not the nominal size on a drawing.

1. Slabs and footings. Measure the length and width in feet and the thickness in inches. For an L-shaped or stepped slab, split it into rectangles, size each one on its own, and add the results. Over-dug ground swallows extra concrete, so check the trench first with the excavation volume tool when the dig is rough.

2. Walls. Enter the wall length and height in feet and the thickness in inches. A 30-foot foundation wall at 8 feet tall and 8 inches thick works out near 6 cubic yards, which almost always comes from a truck.

3. Round columns and piers. Enter the tube diameter as the length and the height as the width. A 12-inch Sonotube is 1 foot in diameter, so a 4-foot pier holds about 3.1 cubic feet.

4. Count the elements and the steel. Set the quantity field to the number of identical piers or footings so the tool totals them in one pass. Rebar and mesh displace a small amount of concrete, but common practice is to ignore that and let the waste allowance cover it. When the pour is reinforced, size the grid with the rebar and mesh tool before you order.

Bagged concrete is sold by weight but bought by volume. Each bag prints the cubic feet it yields once mixed, and the two big brands list the same numbers.

Yields are taken from the Quikrete Concrete Mix (No. 1101) and Sakrete High-Strength Concrete Mix data sheets, both tested to ASTM C387 at 4,000 PSI. The bags-per-yard columns divide 27 cubic feet by each yield, so they land on whole numbers only for a full yard. Real pours rarely land on a whole yard, which is why the calculator rounds each bag count up to the next full bag against your actual order volume.

A standard bagged mix already holds cement, sand, and stone in the right proportions, so you only add water. If you would rather match a named product to a job or batch your own from separate ingredients, the named-product and mix-strength guide maps each bag type to the strength it delivers.

The crossover between bags and a ready-mix truck sits at roughly one cubic yard.

Bagged concrete suits small, spread-out, or hard-to-reach pours. A single 80-lb bag yields 0.60 cubic feet, so a full cubic yard takes 45 bags and two to three hours of mixing. Bags cost more per yard than delivered concrete, but there is no delivery fee, no minimum order, and no truck idling while you place the pour. Fence posts, a handful of deck piers, a small pad, or a repair are textbook bag jobs.

Ready-mix wins once the volume passes about a yard. A truck holds 8 to 10 cubic yards batched to a set strength and places it in minutes rather than hours. The catch is the short-load charge: most plants add a fee on orders under about 5 cubic yards, and the truck needs access within reach of the pour or a pump line. A 20 by 24-foot garage slab at 4 inches needs close to 6 cubic yards, which is firmly truck territory.

Freshly placed concrete then has to cure before it carries load, and cold or hot weather stretches or shrinks that timeline. Check when a pour reaches walk-on and design strength with the curing time estimator before you strip forms or park a car on a new slab.

Why can't I just order the exact volume? Because the formed space is never as clean as the drawing. The subgrade dips, forms bow a little, and some concrete stays behind on the chute and in the wheelbarrow. Ordering the precise figure almost guarantees running short near the end of a pour, and a cold joint from a paused pour is a weak line you cannot easily fix later.

How much waste should I allow? Five to ten percent is the standard range. Use 5 percent for a small, flat, well-formed slab on firm ground, and 10 percent for footings, uneven subgrade, or anything with rough excavation. This tool defaults to 10 percent and lets you dial it between 0 and 20.

Is it better to have too much or too little? Too much, every time. A few leftover bags or a small surplus of ready-mix costs little, while a short pour can mean a second delivery, a second short-load fee, and a cold joint through the middle of the element. Order the volume-with-waste figure the tool shows, not the raw volume.

What about price? This tool sizes quantity, not cost. Concrete prices move with region, season, and fuel, so get a current quote from a local plant or merchant once you know the cubic yards you need.